Tuesday, April 27, 2010

Watch Keeping

CONTENTS OF BRIDGE PROCEDURES GUIDE ( by ICS )

PART A: Guidance to Masters and navigating officers:
Bridge Organization ( Overview; Bridge resource management and the bridge team; Navigation policy and company procedures)
Passage Planning ( Overview; Notes on passage planning; Notes on passage planning in ocean waters; Notes on passage planning in coastal or restricted waters; Passage planning and pilotage; Passage planning and ship’s routeing; Passage planning and ship reporting systems; Passage planning and vessel traffic services )
Duties of the OOW ( Overview; Watchkeeping; Navigation; Controlling the speed and direction of the ship; Radiocommunications; Pollution prevention; Emergency situations )
Operation and maintenance of Bridge ( General; Radar; Steering gear and the automatic pilot; Compass system; Speed and distance measuring log; Echo sounders; Electronic position fixing systems; IBS; Charts, Ecdis and nautical publications; Radiocommunications; Emergency navigation lights and signaling equipment )
ANNEXES
PART B : Bridge checklists
PART C: Emergency checklists

ENTERING FOG:
Memorize fog characteristics of fog signals that may be heard
Order silence on deck
Close water tight doors
If near land , have anchors ready for emergency
Reduce speed
Start fog signal
Make sure your signal not synchronizing with signal of other ships
Decide if necessary to connect extra boilers
Operate radar
If in soundings, start sounding
If in doubt about ship’s position, alter course at once parallel toor away from coast
Post extra lookout esp. on monkey island and forward
Warn E/R
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SCOPE OF CABLE: (led stand )
Length and draft of ship
Degree of exposure to weather
Depth available
Strength of wind and stream
Type of cable
Amount of swinging room available
Nature of bottom
Duration of stay.
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ANCHORING TERMS
Wind rode: Vessel so described when she is riding head to wind
Tide rode: When she is riding head to tide
Lee tide: Tidal stream which is setting to leeward / downwind. Combined forces of tide and wind act on the ship in this case.
Weather tide: Which is setting to windward/upwind. Surface is choppy and forces of wind and tide act in opposite directions on the ship.
Growing (leading ): The way the cable is leading from the hawse pipe, e.g. cable growing forward if she is leading in the forward direction.
Short stay: A cable is at short stay when it is taut and leading into water close to the vertical
Long stay: When it is taut and leading down into the water close to the horizontal.
Shorten in: To shorten in is to heave some of the cable inboard.
Snub cable: To stop the running cable by the application of brake on the windlass
Come to / brought up / got her cable: These terms are used when vessel is riding to her anchor and cable and the anchor is holding
Veer (Walk back) anchor or cable: To pay out cable / to lower anchor under windlass power
A’cockbill: Situation to describe when anchor is clear of the hawse pipe and is hanging vertically.
Up and down: The cable is up and down when it is leading vertically to the water.
Foul anchor: When anchor is caught in an under water cable , or which has brought old hawsers to the surface with it, or which is fouled by its own cable.
Clear hawse:When both anchors are out and cables
Open hawse: When both anchors are out and the cables lead broad out on their own bows. A vessel lying moored to anchors ahead and astern is at open hawse when she lies across the line of her anchors.
Foul hawse:When both anchors are out and the cables are entwined or crossed.
Clearing anchors:Anchors and cables are cleared away when the securing gear on deck is removed. This includes Devil’s claw, lashings ith turn buckles etc.
Nipped cable:The cable is nipped when an obstruction such as the stem or hawse pipe lip, causes it to change direction abruptly.
Surge cable: To allow cable to run out freely, not using the brake or the windlass motor.
Render cable:The cable is rendered when the brake is applied slackly. So that as weight comes on the cable it is able to run out slowly.

A vessel is said to be riding to two anchors when they are both ahead of her, such as is the case when she has had to let go a second anchor to hold her in bad weather.
She is said to be moored when she has one anchor ahead and the other leading astern to hold her in one position.

PRECAUTIONS BEFORE ANCHORING
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Ensure that: anchoring plan prepared; speed reduced in ample time; direction/strength of current, wind and tidal streams known; sufficient sea room maintained particularly to windward, depth of water, nature of sea bed and scope cable required decided and then:
1. Before using an anchor a competent seafarer should check that the brakes
are securely on and then clear bow stopper or other securing devices.
2. A responsible person should be in charge of the anchoring team , with an
adequate communication system with vessel's bridge.
3. The anchoring party should wear appropriate safety clothing - safety
helmets, safety shoes and goggles as a minimum protection from injury
from dirt, rust particles & debris that may be thrown off during operation
4. Where the anchor is let go from the stowed position, if upon release of the
brake, the anchor does not run, personnel should NOT attempt to shake the
cable, but the brake should be reapplied, the windlass placed in gear, and
the anchor walked out clear prior to release.



OTHER SHIP DRAGGING ANCHOR:
Sound “U” signal on whistle; Inform master and E/R; Call up the ship on VHF; Call anchor stations; Switch on steering motors, ARPA/Radar; Keep record of all happenings; Inform port control of the situation; Pay out extra cable to turn the vessel away from the path approach
ANCHOR WATCH:
Determine and plot position on appropriate chart soon after anchoring; Afterwards check position by all available means to ensure that vessel is maintaining her position; Ensure proper look out is maintained; Ensurte inspection rounds of the vessel are made regularly; Obserne meteorological and tidal conditions and state of sea; Notify master and undertake all measures if ship drags anchor; Ensure that engine readiness is in accordance with master’s instructions; Notify master, if visibility deteriorates; Ensure that the ship exhibits the appropriate lights and shapes and that sound signals are made in accordance with rules; Take measures to protect the environment from pollution by the ship and comply with the applicable pollution regulations

OWN SHIP DRAGGING ANCHOR:
Inform master & E/R; call anchor stations; Switch on steering motors and RADAR/ARPA; Inform other vessels in the vicinity of the situation; Call port control and inform of the situation and ask for assistance, if required; Pay out extra cable or drop second anchor after turning the vessel away from first anchor; After engine ready, pick up anchor and anchor again at suitable place; Make entries in log book.

HOLDING POWER OF ANCHOR: Holding power of anchor varies with length of cable paid out. The shorter the scope more upwards is the pull of the ship on the anchor, and consequently less hold will it have. And longer the scope the more horizontal is the pull and better the anchor will hold, best position being attained when enough cable is out to ensure the pull being quite horizontal with some of the cable along the bottom. One anchor with sufficient scope for this purpose will hold better than two anchors with an insufficient scope.
A vessel anchored in deep water, with a proportionally long scope of cable out, will ride easier in a sea, than when in shallow water under the same circumstances, owing to the catenary of the cable giving more elasticity.
CATENARY: A catenary is the curve which a chain or rope assumes when suspended between two points. The curve of a tow rope, when a ship is towed, also when a ship is at anchor, the curve of the cable between the hawse pipe and the point where it rests on the bottom are examples of catenaries.

Mooring

Advantages

1. The vessel occupies little swinging room, turning almost in her own length about her stern.
2. The scopes can be pre-adjusted for the prevailing strength and wind or stream.
Disadvantages
1. The second anchor or lee anchor lies astern and is of no value to the ship if a headwind increases or if the vessel begins to drag.
2. There is a risk of getting a foul hawse.
3. Considerable time to heave in cables.

CALLING MASTER:
Breakdown of machinery, engine , steering etc.
Restricted visibility
On meeting ice, survivors, derelict or other navigational hazard
Traffic movement causing concern
Difficulty experienced in maintaining course
On encountering heavy weather
Unexpected sighting of land or mark or change of sounding
Failure to sight land, light or mark or to obtain sounding as expected
If any vessel fails tin duty to give way
Barometer falls sharply
When in doubt

Look out

Colregs’72, SOLAS’78, STCW’95, and various other regulations have emphasized on look out duties.
A proper look out by all available means i.e, visual (by eyes), by binoculars, radar, VHF, hearing, all other available means must be maintained at all times whether the vessel is underway or at anchor.
If there is a need, more than one persons should be engaged in look out, like in the case of man overboard or while navigating in restricted visibility, ice, etc.
Look out duties & helmsman duties (steering) are separate, so whenever the seaman/ AB on watch is engaged in hand steering for longer durations & whenever there is traffic around, another person should be called to the bridge for the look out duties.
When keeping look out, following must be taken into account :
a) sunlight conditions/ part of the day/ day or night
b) visibility & weather conditions
c) traffic density
d) proximity of dangers to navigation
e) cargo on deck (high container stacks) further affects look out.
f) structures blinding immediately under or near to the vessel’s bow.
g) back scatter due to bright shore lights
h) bright lights onboard or in the bridge (chartroom) affect the night vision or the eyes of the look out persons setting down into darkness.
i) special attention is necessary when navigating in or near traffic separation schemes or narrow channels.

A constant and continuous all-round look out must be maintained. Specially, in the conditions of restricted visibility, proper and efficient use of the radars must be made, although over reliance on radar can place the vessel into a very distressful situation.
Visually observing the characteristics of lights, they are identified and thus in position fixing or confirming vessel’s whereabouts.
Look out also includes the routine monitoring of ship controls and alarm systems e.g. regularly comparing compasses and the course steered.
Look out includes observing all the external factors including weather, etc. which affect the vessel.
Duties of the look out include the detection of ships or aircrafts in distress, shipwrecked persons, wrecks and debris.
The look out must be able to give full attention to the keeping of a proper look out and no other duties shall be undertaken or assigned which could interfere with the look out duties.
In the pirate infested areas, additional persons must be engaged in the look out duties

Navigation under pilotage

Once the pilot has embarked and has arrived on the bridge, the pilot will join the bridge team. The pilot has a specialized knowledge of navigation in local waters. Depending on local pilotage laws, the Master may delegate the conduct of the ship to the pilot who directs the navigation of the ship in close co-operation with the Master and/or the OOW. It is important that the responsibilities of the pilot and the Master are agreed and clearly understood.
The presence of a pilot does not relieve the Master or the OOW of their duties and obligations for the safety of the ship. Both should be prepared to exercise their right not to proceed to a point where the ship would not be able to maneouvre, or would be in any danger.
The preliminary pilotage passage plan prepared in advance by the ship should be immediately discussed and agreed with the pilot after boarding. There should be sufficient time and sea room to allow this to happen safely. Where lack of time or searoom does not allow the plan to be discussed fully, the bare essentials should be discussed immediately and the rest of the discussion held as soon as it is safe to do so. On a long pilotage passage, it may be appropriate to review and update the plan in stages.

Pilot embarks/ disembarks a vessel from a pier on the onshore side or from a pilot boat on the offshore side walking up/ down the shore gangway or ship’s accommodation ladder, pilot ladder or combination ladder rigged as per the freeboard of the vessel.

Requirements for pilot embarking/ disembarking arrangements :
Diagram –
a) Freeboard less than or equal to 9m : Pilot ladder
b) Freeboard more than 9m : Combination ladder


Pilot embarking/ disembarking is a very critical operation, so has to be supervised by a deck officer. The arrangements need to be fully safe. All the structures and projections on deck, which are on the way from the pilot embarking/ disembarking arrangements, should be well lit during night and marked so as to avoid injuries i.e, Pilot ladder and ship’s deck must be well lit by forward shining overside light..
Approach to a pilot boarding ground and boat coming alongside are very important maneouvres, although done with the Master on the bridge, the OOW must assist accordingly and be very careful during these operations.
Following flags have to be hoisted accordingly -
“G” : I require a pilot.
“H” : I have a pilot onboard.

The safe progress of the ship along the planned tracks should be closely monitored at all times. This will include regularly fixing the position of the ship, particularly after each course alteration, and monitoring underkeel clearance.
Verbal orders from the pilot also need to be checked to confirm that they have been correctly carried out. This will include monitoring both the rudder angle and RPM indicators when helm and engine orders are given.
It is recommended that communication between the pilot and the bridge team is conducted in the English language.
If the Master leaves the bridge, the OOW should always seek clarification from the pilot when in any doubt as of the pilot’s actions or intentions. If a satisfactory explaination is not given, the OOW should notify the Master immediately, taking whatever action is necessary before the Master arrives. Wherever there is any disagreement with decisions of the pilot, the cause of concern should always be made clear to the pilot and an explaination sought.
The OOW should be bear in mind that during pilotage, the ship will need to be properly secured for sea. Excessive use of deck lighting at night may cause visibility interference.

Following is a checklist for navigation under pilotage :
( ) Immediately on arrival on the bridge, has the pilot been informed of the ship’s heading, speed, engine setting and drafts?
( ) Has the pilot been informed of the location of lifesaving appliances provided onboard for his use?

Have details of the proposed passage plan been discussed with the pilot and agreed with the Master, including
( ) radio communications and reporting requirements
( ) bridge watch and crew stand-by arrangements
( ) deployment and use of tugs
( ) berthing/ anchoring arrangements
( ) expected traffic during transit
( ) pilot change-over arrangements, if any
( ) fender requirements

Has a completed Pilot Card been handed to the pilot and has the pilot been referred to the Wheelhouse Poster?
Have the responsibilities within the bridge team for the pilotage been defined and are they clearly understood?
Has the language to be used on the bridge between the ship, the pilot and the store been agreed?
Are the progress of the ship and the execution of orders being monitored by the Master and officer of the watch?
Are the engineroom and ship’s crew being regularly briefed on the progress of the ship during the pilotage?
Are the correct lights, flags and shapes being displayed?
Miscellaneous
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Transverse thrust or Starting bias or screwing effect: When a vessel turns under helm her end skids about her PP. There is also bodily sideslip or skid due to centrifugal force. When a vessel is light her sideslip and skid, become more apparent because of reduced underwater volume. This reduces the grip on water. At high speeds the skidding has a marked effect in reducing headway.
In a right handed propeller vessel’s bow turns to port when going ahead and turns to starboard when going astern because of transverse thrust. If it does not, it could be because of overriding effect of wind and/or current. It may be possible to reduce the effect of wind/current by increasing the RPM.The effect of TT is maximized when water around the propeller is confused so that propeller cannot get a good grip on the water This is achieved by giving short bursts of engine ahead or astern and then stopping in between. On the other hand, to minimize the effect of TT build up or slow down the speed in stages, so as to build up flow of water through propeller and allowing it to have good grip on it.
When a vessel alters course through 360 she moves on roughly circular path called turning circle. Through the T.C.her bow remains inside and stern outside the circle. The circle is the path traced by COG of the vessel. This definition is coined by naval architects. Seamen say that TC is traversed by PP. The two circles are very close and concentric. At any instant during the turn a line drawn from centre of curvature of the path perpendicular to F & A line, meets the latter at a point called PP. This about 1/3rd from F. When vessel moves astern it shifts aft.































The circle does not link up with the original course, due to side slip when helm is first used.
ADVANCE: Distance traveled by COG along the original course
TRANSFER: Distance traveled by COG measured from original course to the point where course has changed by 90
TACTICAL DIAMETER: Transfer for 180
DRIFT ANGLE: At any point, the angle between ship’s F & A line and tangent to the TC at that point.
An advance is 3-4 ship’s lengths. Average tactical diameter for an easily turned ship is about 4 ship’s lengths. Time taken to complete a TC is about 7-8 minutes.
LOADED VESSEL: Greater advance, greater momentum and hence greater damage on collision or grounding because of greater impact. She will be sluggish in gathering way and accordingly in stopping, be least affected by wind because of less windage area (due lesser freeboard) .She will have greater TC than when light and will have a TC least affected by speed. Stopping distance of a loaded ship may be three times the stopping distance when in ballast.
LIGHT VESSEL: She is highly responsive to engine movements and responds to rudder action also promptly.She turns easily at anchor, is more easily brought up to her anchor and moorings. She is easier to pick up speed and also to stop. But she is subject to larger amounts of skid and side slip.She will have a smaller TC for a given peed than if she were deeply loaded.And she will have a TC , the radius of which increases as the speed is increased.
TRIM: PP goes further aft .Steering improves but TC becomes larger because of reduced turning lever. Ship is more easily turned downwind.
TRIMMED BY HEAD: PP goes more forward, therefore smaller TC. Ship does not develop full power, is difficult to turn and once starts swinging difficult to check the swing. She turns more readily into wind. It will be slow to seek the wind with her stern under sternway, and with wind on quarter becomes unmanageable.
LIST: Ship turns more easily towards higher side and in case of twin screw vessel low side engine will be more effective than the other. Low side helm will be necessary to correct this.
INHERENT DYNAMIC STABILITY: A ship is dynamically stable on a straight course if it, after a small disturbance, soon will settle on a new straight course without any corrective rudder. The resultant deviation from the original heading will depend on the degree of inherent stability and on the magnitude and duration of the disturbance.
DIRECTIONAL STABILITY: Also known as course-keeping ability is a measure of the ability of the steered ship to maintain a straight path in a predetermined course direction without excessive oscillations of rudder or heading. In most cases, reasonable course control is possible where there exists inherent dynamic instability of limited magnitude.
Ships which are dynamically stable have better directional stability and can be kept on straight line with the rudder in a neutral position close to midships.. And with a right handed propeller this neutral helm is typically of the order of 1 degree to starboard. Other ships which are dynamically instable , however, can only maintain a straight course by repeated use of rudder control. While some unstability is fully acceptable, large instabilities should be avoided by suitable design of ship proportions and stern shape.
The directional stability improves as speed increases. Course keeping ability also depends on counter rudder timing and on how effectively the rudder can produce a yaw checking moment large enough to prevent excessive heading error amplitudes.
When the ratio of wind speed to ship speed is large, wind has an appreciable effect on ship control. The ship may be unstable in wind from some directions. Waves can also have significant effect on course keeping and maneuvering. It has been shown that for large wave heights a ship may behave quite erratically and, in certain situations, can lose course stability.
A ship that is less than six times her beam long is quite likely to be also shallower drafted than a normal ship for her length, possibly less than one third of its beam, so that it has the advantage of using smaller ports for its size. These ships have become more common in recent years. The trouble is trying to keep them on a steady heading at slow speeds. Especially if they are on an even keel, they seem to have no directional stability and if they start swinging one way or the other, although the rudder is amidships, they keep on swinging in that direction faster and faster, so it needs a lot of helm and may be increased engine speed to stop the swing. The next thing is they are off swinging in the opposite direction and the same problem is repeated all over again. LPG ships come in this category of size and draft. If the ship is fitted with CPP it makes them even worse. Directional stability can be improved by increasing speed or trimming by stern.

BLOCKAGE FACTOR: Ratio of the cross sectional area of the ship to the cross sectional area of water in a channel is known as blockage factor. Squat and shallow water effects increase as BF increases. Excessive speed in shallows can ground a ship in water of sufficient depth to float it at slow speed.

SHALLOW WATERS: In shallow waters water is not easily replaced. Propeller and rudder appear to be working in partial vacuum and steering becomes sluggish. Yaw or sheer is difficult to correct and vibrations set up. Because displaced water is not easily replaced bow and stern waves increase in height. Further the trough which normally exists under the quarter becomes deeper and stern is drawn towards bottom ( squat ). As soon as it becomes apparent that bow and stern waves are higher than normal, speed should be reduced but slowly otherwise stern wave overtake the vessel causing the bow to sheer which in narrow channels can be dangerous. Though there is reduction in UKC there is no apparent change in draft.
SMELLING THE GROUND: When a ship is nearing an extremely shallow depth of water, such as a shoal, bank or a patch she is likely to take a sudden shear, first towards it and then violently away . This is called smelling the ground. All of sudden, it may appear that the movements which were otherwise sluggish have become very lively at that moment. Such types of unexpected sheers can be avoided by moving at slow speeds.
BANK SUCTION AND BANK CUSHION: As the vessel moves through the restricted channel it is possible that she may close one bank. In this event a streamlining or venture effect arises due to the restricted flow of water on one side of the ship. This causes an increase in the velocity of the water on that side, together with a loss of pressure head. The latter manifests itself as a drop in water level at the nearer bank, and a thrust is set up towards it. The greater fullness over the after body of the ship accentuates the thrust, and it thus appears more strongly at the stern than at the bow. The stern moves towards the bank ( bank suction) and bow away from it (bow cushion).
Ships passing close to each other also experience these interaction forces due to venture effect.
CANAL EFFECT: In a canal, narrow channel etc. one side may be shallower than the other. As a result water flow below the hull at that side is restricted, water flows faster causing a drop in pressure head. As a result vessel heels towards that side so as to displace constant volume. This is called canal effect.
Summary of shallow water effect on maneuvering include:
 Bow wave increases.
 Maneuvering becomes sluggish.
 The engine load increases / engine thrust decreases
 The ships speed over water reduces.
 Stopping distance and time increases.
 The turning circle increases to a great extent and speed falls less during turns.
 Rolling and Pitching reduces.
 The ship may start to vibrate.
 Increased directional stability.
Squat



Squat is reduction of UKC resulting ftom bodily sinkage and change of trim which occurs when ship moves through water. Squat is considerably reduced by reducing speed.

The factors that affect the amount of squat are:
• The speed of the vessel
• The block coefficient
• The blockage factor
• The static under keel clearance
• The ‘at rest’ trim of the vessel
• Passing another ship in a river or a canal
• The ship close to the bank of the river

APPROXIMATE FORMULAE

Open Water = Cb * V 2 / 100

Shallow Water = Cb * V 2 / 50

• Full form vessels, Cb greater then 0.70, will squat by the stern. Fine form vessels, cb greater then 0.70, will squat by the stern. This is when the ship is at even keel when dead in the water. When the ship has trim by the head then the squat will increase the trim by the head. If the ship has trim by the stern then the squat will increase the trim by the stern.







squats.
























































INTERACTION: When two ships pass close to each other on roughly parallel courses forces of attraction and repulsion are set up between them. This effect is known as interaction. It will be greatest in shallow water and when the two vessels are moving at high speed in the same direction with little difference of speed between them. In the case of two vessels passing on opposite courses interaction will have little effect, but in overtaking situations the course of one or both the vessels may be effected to an appreciable extent, especially when a larger vessel is overtaking a smaller one.
The maximum distance between two vessels at which interaction may be noticed will vary with the size and speed of the ships and the depth of water. Even in deep waters interaction may be experienced by fast vessels overtaking at close distances. Example is the collision between vessels Queen Mary and Curacao caused at least partly by interaction yet the depth of water was about 120 meters.
When a vessel is moving at high speed there is a region of increased pressure in the water near the bow and stern and region of decreased pressure amidships. This results in interaction --- forces of attraction and repulsion between two vessels. They cannot be computed exactly. Therefore the safest bet is to keep safe distance or reduce speed in narrow waters when passing close to another ship.

Overtaking encounter: Interaction is most likely to prove dangerous when two vessels are involved in an overtaking situation.One possible outcome is that the vessel being overtaken may take a sheer into the path of the other. Another possibility is that when the vessels are abeam of one another the bow of each vessel may turn away from the bow of the other causing the respective sterns to swing towards each other. This may also be accompanied by an overall strong attractive force between the two vessels due to the reduced pressure between the underwater portion of the hulls. There are other possibilities, but the effect of interaction on each vessel during the overtaking manoeuvre will depend on a number of factors including the size of one vessel relative to the other, the smaller of the two vessels feeling the greater effect.
(ii) The head-on encounter: In this situation interaction is less likely to have a dangerous effect as generally the bows of the two vessels will tend to repel each other as they approach. However, this can lead indirectly to a critical situation. It may increase any existing swing and also be complicated by secondary interaction such as bankrejection from the edge of a channel. In all cases it is essential to maximise the distance between the two vessels. The watchkeeper on the larger vessel should bear in mind the effect on adjacent smaller vessels and take necessary care when manoeuvring.
4. INTERACTION IN NARROW CHANNELS
When vessels intend to pass in a narrow channel, whether on the same or opposing courses, it is important that the passing be carried out at a low speed. The speed should be sufficient to maintain control adequately but below maximum for the depth of water so that in an emergency extra power is available to aid the rudder if necessary. If a reduction in speed is required it should be made in good time before the effects of interaction are felt. A low speed will lessen the increase in draught due to squat as well as the sinkage and change of trim caused by interaction itself. Depending upon the dimensions of both the vessel and the channel, speed may have to be restricted. When vessels are approaching each other at this limiting speed interaction effects will be magnified, therefore a further
reduction in speed may be necessary. Those in charge of the handling of small vessels should appreciate that more action may be required on their part when passing large vessels which may be severely limited in the action they can take in a narrow channel. Regardless of the relative size of the vessels involved, an
overtaking vessel should only commence an overtaking manoeuvre after the vessel to be overtaken has agreed to the manoeuvre.
5. MANOEUVRING AT CLOSE QUARTERS
When vessels are manoeuvring at close quarters for operational reasons, the greatest potential danger exists when there is a large difference in size between the two vessels and is most commonly experienced when a vessel is being attended by a tug. A dangerous situation is most likely when the tug, having been manoeuvring alongside the vessel, moves ahead to the bow to pass or take a tow-line. Due to changes in drag effect, especially in shallow water, the tug has first to exert appreciably more ahead power than she would use in open water to maintain the same speed and this effect is strongest when she is off the shoulder. At that point hydrodynamic forces also tend to deflect the tug’s bow away from the vessel and attract her stern; but as she draws ahead the reverse occurs, the stern being strongly repulsed, and the increased drag largely disappears. There is thus a strong tendency to develop a sheer towards the vessel, and unless the helm (which will have been put towards the vessel to counter the previous effect) is immediately reversed and engine revolutions rapidly reduced, the tug may well drive herself under the vessel’s bow. A further effect of interaction arises from the flow around the larger vessel acting on the
underbody of the smaller vessel causing a consequent decrease in effective stability, and thus increasing the likelihood of capsize if the vessels come into contact with each other. Since it has been found that the strength of hydrodynamic interaction varies approximately as the square of the speed, this type of manoeuvre should always be carried out at very slow speed. If vessels of dissimilar size are to work in close company at any higher speeds then it is essential that the smaller one keeps clear of the hazardous area off the other’s bow.
6. STOPPING IN SHALLOW BASINS
A vessel in very shallow water drags a volume of water astern which can be as much as 40% of the displacement. When the vessel stops this entrained water continues moving and when it reaches the vessel’s stern it can produce a strong and unexpected turning moment, causing the vessel to begin to sheer unexpectedly. In such circumstances accompanying tugs towing on a short line may sometimes prove to be ineffective. The reason for this is that the tug’s thrust is reduced or even cancelled by the proximity of the vessel’s hull and small underkeel clearance. This causes the tug’s wash to be laterally deflected reducing or even nullifying the thrust. The resultant force on the hull caused by the hydrodynamic action of the deflected flow may also act opposite to the desired direction.
7. EFFECT ON THE RUDDER
It should be noted that in dealing with an interaction situation the control of the vessel depends on the rudder which in turn depends on the flow of water round it. The effectiveness of the rudder is therefore
reduced if the engine is stopped, and putting the engine astern when a vessel is moving ahead can render the rudder ineffective at a critical time. In many cases a momentary increase of propeller revolutions when going ahead can materially improve control.
8. GENERAL
Situations involving hydrodynamic interaction between vessels vary. In dealing with a particular situation it should be appreciated that when a vessel is moving through the water there is a positive pressure field created at the bow, a smaller positive pressure field at the stern and a negative pressure field amidships. The effects of these pressure fields can be significantly increased where the flow of water round the vessel is influenced by the boundaries of a narrow or shallow channel and by sudden local constrictions (e.g. shoals), by the presence of another vessel or by an increase in vessel speed. An awareness of the nature of the pressure fields round a vessel moving through the water and an appreciation of the effect of speed and the importance of rudder action should enable a vessel handler to
foresee the possibility of an interaction situation arising and to be in a better position to deal with it when it does arise. During passage planning depth contours and channel dimensions should be examined to identify areas where interaction may be experienced.
9. SQUAT
Squat is a serious problem for vessels which have to operate with small under-keel clearances, particularly when in a shallow channel confined by sandbanks or by the sides of a canal or river.
EXAMPLES OF ACCIDENTS CAUSED BY
HYDRODYNAMIC EFFECTS
1. OVERTAKING IN A NARROW CHANNEL
This casualty concerns a fully loaded coaster of 500 GT which was being overtaken by a larger cargo vessel of about 13,500 GT. The channel in the area where the casualty occurred was about 150 metres wide and the lateral distance between the two vessels as the overtaking manoeuvre commenced was about 30 metres. The speeds of the two vessels were initially about 8 and 11 knots respectively. When the stem of the larger vessel was level with the stern of the smaller vessel the speed of the latter vessel was reduced. When the bow of the smaller vessel was level with the midlength point of the larger vessel the bow started to swing towards the larger vessel. The helm of the smaller vessel was put hard to starboard and speed further reduced. The rate of swing to port decreased and the engine was then put to full ahead but a few seconds later the port side of the smaller vessel, in way of the break of the foc’sle head, made contact with the starboard side of the larger vessel. The angle of impact was about 25° and the smaller vessel remained at about this angle to the larger vessel as she first heeled to an angle of about 20° to starboard and shortly afterwards rolled over and capsized, possibly also affected by the large stern wave carried by the larger vessel into which the smaller one entered, beam on, as she dropped back.
2. MANOEUVRING WITH TUGS
The second category is illustrated by a casualty involving a 1,600 GT cargo vessel in ballast and a harbour tug which was to assist her to berth. The mean draughts of the vessel and the tug were 3 and 2 metres respectively. The tug was instructed to make fast on the starboard bow as the vessel was proceeding inwards, and to do this she first paralleled her course and then gradually drew ahead so that
her towing deck was about 6 metres off, abeam of the vessel’s forecastle. The speed of the two vessels was about 4 knots through the water, the vessel manoeuvring at slow speed and the tug, in order to counteract drag, at ¾ speed. As the tow line was being passed the tug took a sheer to port and before this could be countered the two vessels touched, the vessel’s stern striking the tug’s port quarter.The impact was no more than a bump but even so the tug took an immediate starboard list, and within seconds capsized. One man was drowned.
3. STOPPING IN A SHALLOW BASIN been observed at speeds of about 10 knots, but when speed is reduced squat rapidly In the third category a VLCC was nearing an diminishes. It has also been found that oil berth in an enclosed basin which was additional squat due to interaction can occur approached by a narrow channel. The VLCC when two vessels are passing each other. stopped dead in the water off
the berth while tugs made fast fore and aft.
RESULTS OF LABORATORY WORK

(b) The effectiveness of the rudder is reduced in shallow water, and depends very much on adequate propeller speed when going ahead. The minimum revolutions needed to maintain steerage way may therefore be higher than are required in deep water.
(c) However, relatively high speeds in very shallow water must be avoided due to the danger of grounding because of squat. An increase in draught of well over 10% has been observed at speeds of about 10 knots, but when speed is reduced squat rapidly diminishes. It has also been found that additional squat due to interaction can occur when two vessels are passing each other.
An appreciable (d) The transverse thrust of the propeller time after stopping the VLCC began to turn to changes in strength and may even act in the starboard without making any headway. The reverse sense to the normal in shallow water. efforts of the tugs to prevent the swing proved fruitless and the starboard bow of the (e) Vessels may therefore experience quite tanker struck the oil berth, totally marked changes in their manoeuvring demolishing it. characteristics as the depth of water under the keel changes. In particular, when the
under-keel clearance is very small a
marked loss of turning ability is likely.


STEAM RECIPROCATING: Best from response point of view. It can be rapidly stopped / reversed. And full power is available either way in few seconds.
DIESEL ENGINE: It can be started / stopped almost at once and develops power more quickly than steam engine. However it is difficult to start in reverse direction while vessel is making way through water.
TURBINE ENGINE: It is slow to develop power. It needs time to increase RPM and when stopped must be allowed to run down. Therefore turbine engine must be stopped in little advance. Separate turbine provides astern power and has usually 1/3rd of ahead power.

Vessel fitted with inward turning screw has a very much narrower stream and hence better steering qualities than the outward turning propellers. They also have slightly improved speed. These factors are however greatly outweighed by their poor maneuverability at slow speeds.

BERTHING: It is not possible to give definite instructions for bringing a vessel alongside a wharf etc. which will apply to all cases. Much will depend on local conditions, tides, currents etc. Following points however apply generally and should be kept in mind:
Slack water is the best time; stronger the current or tide more difficult the operation.
If there is current or tide, it should be stemmed. She should be kept parallel to the wharf, with the bow slightly canted towards it, and gradually brought alongside.
If the wind or tide sets the vessel towards the wharf, lines must be run out to buoys, if available, so as to ease her down to her berth. If no buoy or dolphin is available take the vessel a little ahead of her berth and drop the off shore anchor, and ease her alongside with the cable.
In any case , when a convenient distance from berth, and stem on to the tide, run a line well ahead from the bow, and also breast ropes from bow and stern. Heave in enough of spring to keep her from dropping astern of her berth.
COMING UP A RIVER ON FLOOD TIDE: Have anchors, heaving lines, fenders and mooring ropes ready for use; when a little way past the berth turn the ship round and stem the tide; steam slowly towards the berth with a slight cant while approaching and using engines if necessary; get a good headline ashore and make it fast; when the weight comes on it she will drop alongside; send other lines and ensure she is properly moored.
COMING UP A RIVER ON FLOOD TIDE WITH STRONG ONSHORE WIND: Round her and stem the tide as above; steam slowly into a good Weatherly position ahead of berth; let go offshore anchor; run a good headline ashore and make it fast; ease away anchor cable and gradually allow the weight to come on the head line and she will drop alongside; make judicious use of fenders as required.
COMING UP A RIVER ON EBB TIDE: Steam slowly up towards the berth having just enough way to stem the tide and carry the vessel over the ground as the berth is approached; when abreast of berth give the bow a slight cant towards the wharf and stop engines; send a good headline ashore and make fast, having second one ready; as the weight comes on the headline she will drop alongside with the tide; send other lines and make her fast.
COMING UP A RIVER ON STRONG EBB TIDE: Steam up a little ahead of berth and drop offshore anchor; send good headline; ease her back alongside with the cable and head rope; make prudent use of fenders; if when coming alongside she is inclined to bump her stern on the wharf, it is important to remember that she will answer the helm because of stream flowing past the rudder.
TO TURN SHORT ROUND USING TIDE WHEN COMING UP RIVER ON FLOOD TIDE: If river is fairly straight, the strongest part of the stream will be in the middle; that being the case slow down, sheer in towards the bank on starboard side to bring bow into the slack water and when far enough in go full astern with helm amidships; the strong flood in mid-river will catch her on the starboard quarter and swing her stern round up river, when far enough, rudder to starboard and full speed ahead to straighten her up. Repeat, if necessary.
If the river is narrow use starboard anchor additionally to turn around within a shorter distance.
WHAT ACTION? Your engine fails and there is a sand bank on your port side. Wind and tide are setting towards the bank.
Head into wind and current with whatever headway available and go as far as possible. On losing headway bring up the vessel to both the anchors.

MAN OVERBOARD:
Release lifebuoy with light and smoke signal on the crew member has fallen overboard; Take immediate avoiding action so as not to run over the man overboard; Sound 3 prolonged blasts on the ship’s whistle and repeat as necessary; Post a lookout with binoculars and instructions to maintain a continuous watch on the man overboard; Hoist signal flag “O”; Commence a recovery maneuver, such as Williamson turn; Engage hand steering; Note ship’s position, wind speed and direction and time; Inform master, if not already on bridge; Inform E/R and get engine standby; Muster rescue boat’s crew; Prepare rescue boat for launching; Distribute portable VHF radios for communication; Rig pilot ladder/ nets to assist in the recovery; make ship’s position available to radio room/GMDSS station; Broadcast URGENCY message to ships in the vicinity.
Maneuver the ship to windward of MOB to create lee for rescue boat; make log entries of the events; Spread oil if necessary;Give first aid and treat for hypothermia ( if any) and for shock; Cancel URGENCY signal after recovering MOB.

WILLIAMSON TURN: ( used in ‘immediate action’ situation’)It is reliable when in darkness, poor visibility, bad weather or when time of accident is unknown because vessel can come to her original track very closely . Helm is given hard over to the side of casualty . After deviation from the original course by 60 , rudder is given hard over to the other side. When heading 20 short of opposite course, rudder is put amidships and vessel steadied on reciprocal course. It takes about 5 min more than normal TC. I man can be kept in sight definitely ordinary TC may be preferable. In 4 cases of WT MOB was in water , on average, for 16 minutes.
SINGLE TURN ( 270 Maneuver) ( ANDERSON TURN): ( used in an ‘immediate action situation’)Rudder is put hard over to the side of the casualty. After deviation from the original course by 250, rudder to amidships position and stopping maneuver initiated.
It is the fastest recovery method. It is good for ships with tight TCs and is mostly used by ships with considerable power. It is difficult for single screw vessels because approach to person is not straight.
SCHARNOV TURN: ( not to be used in ‘immediate action’ situation’ ):
Rudder is put hard over. After deviation from the original course by 240 rudder is put hard over to the other side. When heading 20 short of opposite course , rudder is put amidships so that ship will turn to opposite course.
It takes vessel back into her wake. It saves time as less distance is covered. It cannot be carried out effectively unless the time elapsed between occurrence of the incident and the commencement of the maneuver is known.
SINGLE DELAY TURN: Vessel is continued on original course for about 1 minute and then helm put hard over on either side in calm wx or to windward side. Speed is reduced latter half of the turn and straight line made for MOB. The maneuver is quick but has no automatic return feature.


THE DOBLE TURN: The man in the water remains on the same side of the ship throughout this maneuver. Initially, a turn is made under full helm towards the victim and the ship steadied on reciprocal course. A straight run is then made until the man is approximately three points abaft the beam, when another full turn is made using helm towards the man. The ship is again brought round, but this time is steadied on the original course and placed slightly to windward of man. This turn has automatic return feature only if man’s position is known.

GIRTING OF TUG: It is described as heavy listing and eventually capsizing of tug when tug line is leading in beam direction from the tug and is under great tension. It can be caused by:
i) ship turning independently and too quickly away from the tug.( Tug is made fast forward and the ship increases speed and turns to one side, thus overtaking the tug and tug fails to keep up.
ii) Excessive straight line speed with tug made fast. ( Tug made fast aft and turns beam on to tug line as ship increases speed and tug fails to keep up by accelerating)
Girting can be disastrous leading to loss of life and property.
Thus tug master must keep a close eye on ship’s speed. The problem of girding can also be resolved by if towline is bowsed to stern with gobline. Gobline is a length of rope used in a tug to bowse in the tow rope.
PRECAUTIONS: 1) The towing gear should be designed to minimize the overturning moment due to the lead of the towline, commonly called girding.
2) The towing should have a positive means of quick release opearable inder all circumstances. It is recommended that release mechanism should be controlled from the wheelhouse, from the after deck and independently at the hook itself.
3) Openings on the weather deck giving access to spaces below that deck should be fitted with weathertight closing arrangements and kept securely closed during towing operation. This also applies to openings in deckhouses and exposed machinery casings situate on the weather deck. Air pipes should be fitted with automatic means of closure. E/R ventilation should be by means of high coaming ventilator.

SCOURING: Scoring may be defined as removal of silt around the hull of a stranded ship which may be holding the vessel in position. Tugs carry out the scouring using the stream of water from their propeelers and directed as far down as possible to scour away the sea bed silting up the stranded hull. Scouring is employed when a stranded ship cannot move by her own power, by ground tackle or by direct towing. It may be used to make the vessel settle deeper in water and make her afloat, or to dredge a deep channel to seaward, or to dredge cavities beneath the keel. Any small vessel, like a trawler, trimmed suitably by stern can be used for this purpose.Before doing this area should be roughly surveyed and depths subsequentlychecked frequently. It is quite possible that sand scoured may cause a shoal towards seaward, which should be avoided. Otherwise, vessel will strand again, shortly after refloating.

BEACHING: Intentional grounding is termed as beaching. It is done when ship is damaged to such an extent that pumps cannot cope with the rate of flooding.Beaching is then considered to prevent foundering and also to allow possibility of repair and refloating.If foundering is imminent, beaching should be done without wasting time studying the options, but if time is available following factors should be considered in deciding the shore for beaching:
a) Beach should be gently sloping, with bottom of mud, sand and/or gravel and free of rocks
b) It should not be subject to surfing and not exposed to bad weather.
c) It should be free from currents and scouring.
d) Ideally the beach should have some tidal range, so as to enable over side repairs.
e) Beaching should be done at slow speed, at HW or a t falling tide so that the ship settles slowly.
f) After beaching ground tackle should be rigged to prevent her from going further inshore and steady.She should also be secured ashore using stakes & mooring ropes, similarly, to prevent her from coming off and going to seaward unexpectedly.
g) Then sound all tanks, sea bed around the vessel and assess the damage.

TOWING:
PLANNING:
The route to be followed should be planned in advance taking into account factors such as weather, tidal streams and currents, the size, shape and displacement of the tow and navigational hazards to be avoided. Where available, weather routeing service should be used. Careful consideration to be given to the number and effective bollard pull of towing ship/s to be employed.
There should be contingency plan to cover the onset of adverse weather, particularly in respect to arrangements for heaving to or taking shelter.
Where towing operation falls under the jurisdiction of an approving authority, any certificate issued should specify the intended route and indicate any special conditions.
PREPARATION:
Tows should exhibit lights, shapes and, if manned, make appropriate sound signals as per regulations.
Prior to sailing , the water tight integrity of the tow should be confirmed by an inspection of the closing arrangements for all hatches, valves, air pipes and other openings through which water may enter.
The securing arrangements and weather protection for the cargo, equipment and stores carried on the tow should be carefully examined to ensure that they are adequate for the voyage.
When appropriate, the rudder should be secured amidships and measures taken to prevent the propeller shaft from turning.
The tow should be at suitable draft for the intended voyage.The tow should have adequate intact stability in all the loading/ballast conditions to be used during the voyage.
Boarding facilities should be provided so that personnel from the towing ship can board in an emergency.
Every towed ship, whether manned or not, should hold a certificate as required by the International Convention on Load Lines 1966.
To reduce the risk o pollution, the amount of oil carried on the tow should be limited to what is required for the safety of the tow and for its normal operations.

TOWING ARRANGEMENT:
Towing arrangement should be such so as to reduce to minimum any danger to personnel during the towing operation.
The towing arrangement should be suitable for the particular tow and of adequate strength.
The design and arrangement of towing fitting should take into account both normal and emergency conditions.
Sufficient spare equipment to completely re make the towing arrangements should be available.

PASSING THE TOW LINE:
Maneuver close to tow and pass line directly.
By using life boat OR by gently drifting on to other ship.
By LTA or streaming a lifebuoy

TYPE OF TOW LINE: Will depend on following:
Strength of tow line; ability to take up jerks; ability to facilitate good securing; ability to provide a good lead, good angle, less chafing; strength of leading points; length of tow line, equipment for handling tow line.
Anchor chain combine with towing line and mooring hawsers can be used.
SECURING TOW LINE:
Take turns on many bitts; turns on each bitt to be lashed; should be capable of being released in emergency;feasibility to adjust the length of line; feasibility to monitor/adjust catenary.

TOWING IN BAD WEATHER: In bad weather better arrangement is obtained by shackling the steel towline to the cable of the vessel to be towed, and veering out a good long scope, after which the cable is secured aboard in the same manner as if the vessel was at anchor. The weight of the cable will cause it to form a catenary, which will prevent sudden jerks.
Length of the towline is the most important point. The longer it is the more uniform will be the tension during the time of towing, the aim being as far as possible to secure a steady strain and to avoid slackening and consequent sudden tightening. Therefore sufficient scope should be paid out without hesitation.

ADVANTAGES OF PASSAGE PLANNING:
Problem areas identified; OOW less likely to overlook important data; data is appraised more effectively using checklists; less time is needed for position fixing and a better lookout may be maintained; additional personnel advised in good time; contingencies involving departure from the plan are covered; inadvertent deviations from the plan are readily apparent; rest periods can be planned more effectively.
Furthermore, the existence of a well tried system for planning a passage allows a major deviation to be dealt with quickly and efficiently.

Ship’s Routeing: The purpose of ship’s routeing is to improve the safety of navigation in converging areas and in areas where the density of traffic is great or where the freedom of movement of shipping is inhibited by restricted sea room, the existence of obstructions to navigation, limited depths or unfavouable meteorological conditions.The criteria and principles applicable to all roueing measures have been set out in the General Provisions on Ship’s Routeing, which form part of IMO Resolution A 572(14) as amended. Routeing schemes and the General Provisions on Ship’s Routeing are kept under continuous review by IMO and amendments are made when required.


ROUTEING TERMS:
TSS: Routeing measure aimed at the separation of opposing streams of traffic by establishing traffic lanes.
TSZ: Means to separate traffic lanes in which ships are proceeding in opposite or nearly opposite directions in order to separate lanes from adjacent sea areas.
TRAFFIC LANE: Areas within defined limits in which one way traffic flows established. Natural obstructions incuding those forming separation zones , may constitute a boundary.
ROUNDABIUT: A separation point or circular zone and circular traffic lane within defined limits.
INSHORE TRAFFIC ZONE: Designated sea area between landward boundary of TSS and adjacent coast.
RECOMMENDED TRACK: A route of undefined width for convenience of ships in transit, which is often marked by centre line buoys
PRECAUTIONAR AREA: A precautionary area is defined in the IMO principles of ship’s roueing as a routeing measure comprising an area within definite limits where ships must navigate with particular caution and within which direction of flow of traffic may be recommended.
AREAS TO BE AVOIDD: An area in which either navigation is hazardous or it is exceptionally important to avoid casualties and which should be avoided by all ships or certain class of ships.
DWR: A route which has been accurately surveyed for clearance of sea bottom and submerged obstructions. It is primarily intended for use by vessels which are restricted because of their draft. Through traffic not restricted by draft should not, if practicable, use this route.DWR may form part of a TSS. Such DWRs are also covered by Rule 10. There are also some DWRs which are not part of TSS. Rule 10 does not apply to such routes.DWRs who\ich do not form part of TSS may be intended for use by one-way or two-way traffic, as indicated by arrows on chart. Vessels using two way DWR should keep to the starboard side of the route.


When a wind blows against a VLCC, she will be affected by a force, which acts almost in the opposite direction to the relative wind direction with a magnitude proportional to the windage area and square of the wind velocity:

F = kAV2
K (constant) = 0,52*10-4 for a beam wind

The wind area naturally depends on the conditions of loading

Having the knowledge of the magnitude of the wind force and how it affects your ship is of great importance during harbor maneuvering.
Calculating whether the available tugs have sufficient power to hold the ship against a cross wind or to move the ship against a cross wind.
Calculating whether the thrusters have the necessary power to maneuver the ship safely under the prevailing wind conditions.

Position of the pivot point shifts forward
This changes the previous balance

With the wind on the beam:














A turning lever between P and W is created & depending on the wind strength, the ship will swing the bow into the wind



The final position the ship will lie in depends on the resultant of:

• Wind Force
• Water flow force


With the wind abeam it will be seen that the wind and current forces act with a moment which makes the ship turn a little up into the wind until the moment of the two forces is zero

AUTOPILOT

AUTOPILOTS:
This is intended to remove the need for the operator to assess the correct control settings. In effect the autopilot ‘learns’ the vessel’s handling characteristics in calm weather and then, when the weather deteriorate, can distinguish between those errors in heading due to the weather and those due to vessel’s normal handling characteristics. It has been found that in heavy weather the vessel’s head deviates very little ‘relative to the water’.T4 the adaptive autopilot concerns itself primarily with those errors in heading resulting from normal handling characteristics, so reducing the work and hence drag of the rudder.
For example in Sperry model auto tuning software is fitted. The software performs dynamic tuning of the autopilot during a modest series of initial maneuvers, then automatically continues to retune the tuning throughout the voyage in response to the ship’s behavior. System also maintains two different tunings, one for light and one for loaded condition.
Limitations of auto pilot: Auto pilot should not be used under following circumstances:
(WASTV MNS)
In rough weather
During large alterations of courses
At slow speeds
In heavy traffic areas
In conditions of reduced visibility
While maneuvering the vessel
In narrow channels and confined waters.
In shallow waters
CONTROL UNIT:
1)PROPORTIONAL CONTROL: Output of the controller is proportional to the offcourse error from the course to steer ( deviation)
Controller ouput = constant (Kp) x Deviation
2) INTEGRAL CONTROL: Output of the controller is proportional to the summation of all instantaneous values of error ( deviation ) for as long as error persists.
Controller output = constant ( Ki ) x  error x Time
3) DERIVATIVE CONTROL: Output of the controller is proportional to the rate of change of error ( deviation ).
Controller output = constant ( KD ) x change of error / time

THREE TERM CONTROLLER ACTION: Output signal is proportional to the deviation, persists as long as deviation persists and also depends on rate of change of deviation i.e.
Proportional + Integral + Derivative
CONTROLS:
WEATHER SETTING CONTROL: When steering in heavy weather with wind and sea at an angle to the vessel’s heading, there is a tendency for the vessel’s head to be turned in a particular direction. The effect of this can be offset by maintaining some permanent value of rudder angle; this angle is set using ‘weather helm’ after a period of trial and error.
SYNCHRONISATION CONTROL: Temporairly disconnects gyro repeater from main gyro for sync of heading. Required for sync and when gyro switched off and restarted.
SPEED: Usually from log and manual if log fails
COURSE SELECTOR KNOB: For setting course to be steered.
DIMMER: For illumination of panel
AUTO/FOLLOW UP/ NON FOLLOW UP : For choosing steering mode
RUDDER LIMIT: Purpose is to prevent a maneuver more radical than is compatible with:
Speed loss
Comfort on ship
Safety of ship
Rudder angle greater than 15 to 20 deg do not improve course alteration but result in excessive speed loss.This control limits the number of degrees of helm that can be applied by auto pilot computer in any mode. Limits are : 5, 10, 15 and 20 degrees.
OFF COURSE ALARM: It gives alarm if ship deviates from set course by a pre determined limit. Setting depends on Wx condition, open/coastal waters.


RUDDER CONTROL: This is proportional controller which transmits a signal which is proportional to course error
Controller ouput = constant (Kp) x Deviation
The ratio can be changed by settings ( i.e. the ratio between instantaneous heading error and rudder command)
Also called rudder multiplier
Control knob alters the ratio of output.
Higher setting -- Larger rudder angle ( results in overcorrecting – overshooting )
Lower setting – Less rudder angle ( Long time to return to set Co – Sluggish )
Therefore optimum setting required.

COUNTER RUDDER: This is Derivative control.
Purpose is to apply a relatively greater amount of helm at the beginning of a course alteration to get the ship turning. Once the ship is turning, just enough helm is applied in order to keep her coming around. When new heading is approached, opposite helm is applied to stop the swing. As the ship settles on new heading and the yaw rate disappears, the helm is removed.
Produces an output when course of vessel is changing.
Depends on rate of change of course:
Controller output = constant ( KD ) x change of error / time
Determines amount of counter rudder to steady the ship on set course.
Keeps over shoot to minimum.
Greater the ship’s inertia, greater the setting required. If ship has good dynamic stability, relatively small settings of counter rudder will be sufficient. If the ship is unstable, higher settings will be required.
Depends on ship’s characteristics, loaded/ballast conditions and rate of turn.
Too high setting will bring the ship to set Co slowly
Too low setting allows overshoot
As counter rudder settings increase , counter rudder increases.
KD – Counter rudder time constant ( Calibration done at sea trial to set KD )

PERMANENT HELM: This is integral controller.( In NFU this control is out of action)
When ship has known imbalance to one side, requiring a certain amount of bias helm ( e.g. TT of propeller or extreme case of trawlers working their trawl over one side) manual setting of the approximate bias speed up the effect of the AUTOMATIC PERMANENT HELM calculator, because it started off nearer to its target.
Whether the control setting is estimated correctly or left at zero has no effect on the final steering accuracy but only in the time it takes to reach this heading accuracy.
If not used as described above , the permanent helm should be left at ZERO and the automatic permanent helm will function normally.

Produces output as long a course error persists
Used when beam winds; couple formed causing ship to turn into wind.
Rudder position required to counteract is permanent helm.
Continuous control calibrated from 20 (P) to 20 (S).



Changing over from Hand Steering to Auto Steering :
Before changing over from hand steering to auto steering, the settings on the auto pilot panel must be adjusted for weather and traffic conditions.
The vessel must be made steady on the course on which she has to be set on auto steering.

Changing over to emergency steering system :
When the steering panel gives an alarm, it must be read carefully to see as to what has gone out of order, operation must be changed-over to the other/ alterative steering gear/ motor or transmission system/ telemotor, engineroom must be informed immediately.
If the Auto-pilot gives an alarm or the off-course alarm goes off, adjust the settings on the Auto-pilot panel accordingly.
If the Auto-pilot fails, change-over to hand steering.
If the Follow-up system doesn’t work (the feedback leg of the steering gears doesn’t function properly), change-over to Non-Follow-Up mode.
If the steering transmission systems or telemotors stop working, emergency steering has to be performed by trick-wheel arrangement or solenoids after bringing the rudder mid-ships.
Further, if the steering hydraulic or electric motors also stop working, rudder will have to turned by some mechanical arrangement like chains and blocks, this is not possible in case of large rudders (large ships). As the last resort, Jury rudder is used, which means some arrangement/ structural changes, which overside work as an alternative rudder arrangement e.g. wooden planks on the stern turned/ rotated like a rudder.
Changing over (handing over/ taking over) of a watch between the OOWs whether at sea or at anchor is done in compliance with the ISM checklists onboard which, in general, include the following :

Use of the Automatic Pilot

4.- (1) The master shall ensure that an automatic pilot, where fitted, shall not be used in area of high traffic density, in conditions of restricted visibility nor in any other hazardous navigational situation unless it is possible to establish manual control of the ship's steering within 30 seconds.

(2) Before entering any area of high traffic density, and whenever visibility is likely to become restricted or some other hazardous navigational situation is likely to arise, the master shall arrange, where practicable, for the officer of the watch to have available without delay the services of a qualified helmsman who shall be ready at all times to take over the manual steering.

(3) The change-over form automatic to manual steering and vice versa shall be made by, or under the supervision of, the officer of the watch, or, if there is no such officer, the master.

(4) The master shall ensure that the manual steering gear is tested (a) after continuous use of the automatic pilot for 24 hours and (b) before entering any areas where navigation demands special caution.


Operations of Steering Gear

5. In areas where navigation demands special caution, the master shall ensure that the ship shall have more than one steering gear power unit in operation when such units are available and capable of simultaneous operation.


Steering Gear - Testing and Drills

6.-(1) The master shall, within 12 hours before departure of the ship, cause the steering gear to be checked and tested so as to ensure that it is working satisfactorily:

Provided that in the case of ships regularly making more than one voyage a week to or from the same port a check and test of the steering gear need only be made once in that week unless a part of the steering gear or its control system has been dismantled or change since the last test.


1764 MERCHANT SHIPPING

The test procedure shall include, where applicable, the operation of the following:

(a) the main steering gear;
(b) the auxiliary steering gear;
(c) the remote steering gear control systems
(d) the steering positions located on the navigating bridge
(e) the emergency power supply
(f) the rudder angle indicators in relation to the actual position of the rudder
(g) the remote steering gear control system power failure alarms
(h) the steering gear power unit failure alarms; and
(i) the automatic isolating arrangements and other automatic equipment required for steering gear.

(2) The checks and tests shall include:

(a) the full movement of the rudder according to the required capabilities of the steering gear;
(b) a visual inspection of the steering gear and its connecting linkage; and
(c) the operation of the means of communication between the navigating bridge and the steering gear compartment.

(3) The owner shall provide simple operating instructions, with a block diagram showing the changeover procedures, for the remote steering gear control systems and steering gear power units, and the master shall ensure that they are permanently displayed on the navigating bridge and in the steering gear compartment.

(4) A person shall not supervise the operation or maintenance of the steering gear unless that person is familiar with the operation of the steering systems fitted on the ship, and, where applicable, with the procedures for changing form one system to the other.

(5) In addition to the routine checks and tests prescribed in paragraphs (1) and (2) of this regulation, the master shall ensure that emergency steering gear drills which practise emergency steering gear procedures take place at least once every three months. These drills shall include, where applicable, use of direct control form within the steering gear compartment, the communications procedure with the navigating bridge and the operation of alternative power supplies.

(6) (a) The date time and place that the said routine checks and tests are carried out and the date and details of emergency steering drills carried out shall be recorded by the master in the official logbook.

(b) In ships not required to keep an official logbook, a record of each check, test and drill shall be made by the master and be retained on board for a period of six months and be available for inspection on demand by a superintendent, proper officer or surveyor of ships.

Integarted Bridge System

An IBS is a combination system which is interconnected to allow the centralized monitoring of sensor information and control of a number of operations such as passage executions, communications, machinery control, safety and security. IBS is basically a combination of INS and other shipboard systems. It reduces work load, provides cetralised control and black box for later analysis.
IBS enhances speed of command by providing more time for decision making and action.

BENEFITS OF IBS:
Navigation; Anti-collision;; Steering; Machinery control; Logging; Voyage recording; Safety and recording.
There is no single standard IBS design for ships nor is IBS mandatory. Classification societies do offer optional class notations for ships; the NAV1 class for LR, the W1-OC class from DNV are examples of class notations for IBS arrangements designed to support periodic one man bridge operations.
Factors including the design of the bridge , the type of equipment that is fitted and the layout of that equipment on the bridge will determine the extent to which the IBS design allows certain bridge functions to be automated.
WORKSATIONS BRIDGE DESIGN AND LAYOUT: Centralized control and monitoring requires a work station design approach. At the main operating position on the bridge, referred to variously as the work station for navigation and traffic surveillance/ maneuvering, the navigation work station or the conning position, the OOW should be able to undertake all his primary duties unassisted with efficiency and safety. The design should also allow two bridge team members to work unhindered side by side.
Bridge design and layout of work stations, together with equipment and instrumentation at those work stations is an important part of IBS design. The IBS design should ensure that the failure of one sub system does not cause the failure of another and any failure is immediately brought to the notice of OOW

IBS EQUIPMENT: To permit centralized control and monitoring of navigational functions on the bridge, the following system will be required:

1. Navigation Management System: The NMS provides the mechanism of planning, executing and monitoring the passage plan and will therefore provide the link between the charts on which the voyage has been planned, the position fixing systems, the log, gyro and auto pilot. An ECDIS will typically function as the NMS within an IBS, supported by dedicated route planning terminal to allow route planning activities to be undertaken while on passage and without interfering with the OOW.
2. ALARM SYSTEM: The IBS has an alarm system to warn the OOW if potentially dangerous situations could arise. Failure of the OOW to acknowledge alarms --- usually within 30 s --- will transfer the alarm to remote alarm units in cabins, offices and messes to call for back-up assistance. The main navigational sensors, in particular the radar which provides traffic alarms, the gyro and auto pilot which provide course related alarms and position fixing systems which provide position related alarms, need to be connected to alarm system. ECDIS, the steering gear, power distribution panels etc may also be connected. Including in the alarm system should be a watch safety or fitness alarm to monitor the alertness of the OOW.An interval timer for setting alarm intervals of upto 12 mins should be part of the system. A number of alarm acknowledgement points, each with a pre-warning alarm to give the OOW notice that the alarm is about to be activated should be available around the bridge. As with the failure of the OOW to acknowledge a navigation alarm, if the fitness time interval expires, an alarm should sound away from the bridge.
3. CONNING DISPLAY: The display should be available at the conning position to
show information summaries of the important navigational sensors on passage and
while docking. He display also provides the OOW with a central place to monitor sensors
and compare actual settings with those ordered. IBS AND THE AUTOMATION OF NAVIGATION FUNCTIONS:
The process of planning a passage through to its execution and monitoring the progress of the ship against the plan is one bridge operation that can be safely automated as long as certain principles and disciplines are followed:
1. the plan needs to be thoroughly prepared on the charts.
2. the details of the plan and particular the waypoints, need to be carefully prepared on or transferred to the navigation system.
3. the position of ship needs to be safely calculated and quality monitored by the navigation system.
4. if the position of ship is accurate and reliable and the passage plan has been safely entered the XTE deviations off track as calculated by navigation system and transmitted to the autopilot will be accurate, and allow the autopilot to control ship’s direction safely and automatically.
USING IBS: Where fitted , clear guidance on IBS operations should be contained in the shipboard operational procedures manual. In particular, advice on when to commence and when to suspend automatic track keeping should be provided. Over reliance on automatic system, coupled with the OOW paying too little attention to visual navigational and watch keeping techniques, can be dangerous.

As per IMO IBS should include (any two) the following:
Passage Execution
Communications
Machinery Control
Loading, discharging and cargo control
Safety and security

INS:
Similar to evolutionary principle in living beings, marine navigation systems are becoming increasingly complex and sophisticated. Both by design and function, shore based and ship based navigation systems are no longer individual equipment components operating independently. The trend is gravitating towards data fusion,
functional synergy and modular integration.
Unlike dedicated ECDIS display INS displays are task oriented composite presentations that enable the mariner to configure the display for an operational situation by selecting specific chart, radar, ARPA and AIS information that is required for the task at hand.
Introduction of ECDIS, availability of accurate position fixing system ( GPS ) and plotting aids (ARPA) and automatic programmable ship’s steering have made the process of INS easier and faster.
INS based on modular concepts which allows a flexible change of a set of navigational facilities depending on ship’s type and area of operation of the ship will have the three modules forming the integral part of the INS:

NAVIGATIONAL MODULE: This module will take care of voyage planning, position fixing on a continuous basis and ARPA plotting functions. This module will perform the following tasks:
Specifying ship’s route and sailing mode
DR by inputs from gyro and log
Position and speed by signals from GPS/DGPS
Position fixing by inputs from terrestrial based radio navigation system
Position and velocity determination using fixed Radar markers
Automatic selection of radio navigation aids on the basis of accuracy criteria
Record and reproduction of cartographic information on a display
Data logging in log book format
Storage of navigational parameters for a given time
Warnings in case of exceeding the prescribed depth, route cross track, approaching critical points including alteration of course etc. and
Automatic correction of charts

ANTI COLLISION MODULE: This module works out the best possible action to be taken in case of close quarters situations on the basis of the following:
Collision regulations
Sea room availability
Trial maneuver
Desired CPA

STEERING CONTROL MODULE: This module is dedicated to steering the ship and will perform the following functions:
Steering a given course, taking into account the ship dynamics, trim etc. and external conditions.
Automatic course alteration in accordance with planned track and
Alterations for collision avoidance and subsequent track keeping.





The basic difference between IBS and INS is that INS is a combination of navigational data and systems interconnected to enhance safe and efficient movement of the ship, whereas IBS interconnects various other systems along with the INS to increase the efficiency in overall management of the ship. In this sense the INS is specific in nature while IBS has a more generalized approach.

WHAT IS ECDIS

Clear waters ahead for ECDIS
Regulation 19 of the new SOLAS chapter allows an electronic chart display and information system (ECDIS) to be accepted as meeting the chart carriage requirements of the regulation. The regulation requires all ships, irrespective of size, to carry nautical charts and nautical publications to plan and display the ship’s route for the intended voyage and to plot and monitor positions throughout the voyage. But the ship must also carry back up arrangements if electronic charts are used either fully or partially. Performance standards for electronic charts were adopted in 1995, by resolution A.817(19)), which was amended in 1996 by resolution HSC.64 (67) to reflect back-up arrangements in case of ECDIS failure. Additional amendments were made in 1998 by resolution MSC 86 (70) to permit operation of ECDIS in RCDS mode

Electronic Chart is a generic term used to describe a form of the digital representation of paper chart. An electronic chart can be Raster or Vector chart. They are the next generation of navigational charts.
ENC: An ENC is a presentation of hydrographic and navigational information in digital form.ENC is vector data which is composed of points, lines and features with detailed attributes. They are similar to CAD drawings.From the relationships between these (points, lines and areas) and ship’s features(speed, draft, course) it is possible for an ECDIS system to intelligently detect and alarm the navigator of possible dangerous situations. ENCs are the official vector based ECs designed to meet the relevant chart carriage requirements of SOLAS convention.ENC data can only be issued by, or on authority of, govt. authoorised HOs.Each country is responsible for production of its own ENCs and systematically updating them with all safety- critical information.When displayed with certain parameters and via a type approved ECDIS, ENCs fully satisfy SOLAS chart carriage requirements and thus can be used primary means of navigation.
RNC: RNCs use raster data to reproduce paper charts in an electronic format.Raster representations consist of thousands of tiny clour dots (pixels) that together make a flat digital image.Every pixel is geographically referenced enabling accurate real-time(continually updated) display of vessel position when the chart display system is linked to GPS.Additional user defined information such as route plans and shoal areas can be overlaid on an RNC to provide automatic links to other onboard systems(e.g. warning alarms) but unlike ENCs charted features cannot be selectively displayed or queried.RNC data , itself will not trigger automatic alarms(e.g. anti grounding).However ,some alarms can be generated by the RCDS(RC display system) from user inserted information.These can include: (DISC)
Danger areas, Isolated dangers, Ship safety contour lines and Clearing lines.
ARCS charts are provided on CD-ROM allowing their use in a wide range of equipment, from full integrated bridge systems to stand alone PCs.Worldwide coverage is held on 10 regional CDs and 1 CD for small-scale charts.

ECS and ECDIS: ECS encompasses any electronic system that uses digital chart data.It could be a laptop, PC, PDA or fully integrated IBS. The chart data can be vector or raster. ECDIS is approved type ECS complying with stringent IHO criteria and using official ENC data. IMO requires that:
• ENCs can be used for primary navigation in place of paper charts provided there is a suitable backup( e.g. another ECDIS with a separate power supply, or an appropriate folio of up to date charts.
• RNCs, when used in conjunction with ENCs ( to fill the gaps in coverage) can be used for primary navigation together with appropriate folio of up to date charts.
However this will be up to the Administration to decide what appropriate folio of charts is.
P.S. for ECs were adopted in 1995, which was amended in 1996 to reflect back-up arrangements in case of ECDIS failure.Additional amendments were made in 1998 to permit operation of ECDIS in Raster mode.


In addition to complying with standards of IHO, IMO etc. ECDIS should also meet the specifications laid down by classification society.
Briefly ECDIS must offer following characteristics:
a) Reading ENC data(S57: transfer standard ) and transferring it to SENC
b) Presentation of objects and their characteristics according to presentation library(S52: chart content and display standard)
c) Anti grounding function
d) Warning of obstructions to shipping
e) Updating
f) Various types of chart work must be possible like,entering fixed position,route planning, entering notes and observations,soundings and measuring of distances.

Thus ECDIS does not merely mean presentation of chart in electronic form, it is also an information system.With ECDIS, for the first time, a navigator knows where his ship is and not where his ship was. No single individual invented ECDIS.The currently valid standards have been developed from contributions made by companies, institutions and organizations in many countries.ECDIS offers the same range of features as paper charts and facilitates simpler, faster and above all, more accurate work on the basis of constantly updated data.

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Components of ECDIS: hardware, software and data
Hardware: of ECDIS is generally a computer with graphics capability, a high performance PC or a graphics workstation installed in a console linked with other items of ship’s equipment , like course from gyro, speed from log and ROT from ROTI.Key features are the links with posn sensors of the ship ( decca, loran, etc. ) and in particular with the GPS, supplying via the NMEA interface a constant stream of highly precise posn data(NMEA: national Marine Electronics Association; NMEA0183 Standard for Interfacing Marine Electronic Devices, data record).Even radar pictures can be superimposed, either as raw data from a rasterscan radar or as syntheticARPA.The hardware of an ECDIS must meet the standards of the IEEE ( Institute of Electrical and Electronic Engineers ).

Software: is what makes the computer an ECDIS. It consists of user interface and the so called ECDIS kernel, the software that makes it possible to read the data and display a chart. This software is also called “function library”. Inaddition to the chart picture, the user interface shows buttons and keys for operating the nautical chart.

Data: The official original data (S57 data) of the ENC is usually supplied on CD-ROM or, in case of updates, via digital telephone or satellite communication system.This original data is also called ENC.The chart database is organized incells that cover the entire earth’s surfacewithout overlapping.
The data in the System Electronic Nautical Chart (SENC) is generated from the original of the ENC.The ENC has to be kept unaltered in order to be able to reconstruct the SENC data if this is unintentionally damaged or destroyed.The S57 dta represents a specific kind of attributed vector data.
In addition S57 data should be encapsulated according to the ISO 8211 standard.This guarantees that data from the various national HOs are compatible with one another and can be read by each ECDIS.

S57 however does not contain any information about the symbolization.This standard is given in another IHO publication: S52.It is also called presentation library(PRESLIB)Hence the ECDIS manufacturer must install the presentation library in his unit with the definitions of the symbolization instructions and the colors to be used in an absolute manner irrespective of the color monitor.
Chart data comes without geographic projection, only with coordinates.Ecdis must then provide also geographic projection for a cartographic presenatation.For shipping, it means usually the mercator or sometimes gnomonic projections.

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Correcting electronic charts: Updating is a fundamental requirement for all chart systems.In electronic charts updating can be completed in seconds, in many cases automatically.A useful virtue of electronic updating is that, because official electronic chart databases can contain information from other sources such as Light Lists and Sailing Directions, systems can be arranged so that corrections are made to all relevant references at once.
Full designation ECDIS require that system must be type approved, must display official ENCs issued by national HOs and that they must be kept uptodate.
For full ECDIS compliance the chart database must be updatable automatically or semi-automatically.IHO guidelines indicate that the onboard chart system should be capable of being interfaced with an Inmarsat sitcom station or an EGC SafetyNet-capable receiver for direct transfer of ENC updates.Such equipment combinations can provide fully automatic updating.
For semi-automatic updating the ECDIS should be capable of receiving chart updates in standard IHO format via a telephone network or on 3.5in high density diskettes.This updates must then be fed into the system;s computer.
The above describes the standardization of the data side of ECDIS, or to be precise, of the hydrographic data.There are indeed other chart objects that certainly must be specifiedin order to make ECDIS an adequate navigational tool.A wide range of “objects” are entered into the chart by the captain and the navigators before and during the voyage of the ship.These include leg lines and waypoints, notes, observations, fixed positions and commands.To ensure that this is also possible with the electronic medium of ECDIS, the IMO has developed an ECDIS P.S.This determines how an ECDIS has to work so that it can serve as an adequate replacement for the paper nautical chart.in accordance with the International for SOLAS, 1974. IMOhas produced a catalogue of user-defined objects which has been published under the title “ Mariner’s Navigational Objects” as an appendix to S52.The supplier of ECDIS has to ensure that these objects can be generated, edited and deleted as required. In addition ECDIS should also comply specifications of functionality laid down by classification society.
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ADVANTAGES OF ECDIS OVER PAPER CHARTS: PAWCIP CALDET
Position fixing is real time and is recorded at regular intervals without involvement of OOW.
Alarms can be activated to attract attention of OOW by setting safety parameters whose violation will activate alarms. Can be used as anti- grounding tool.
Weather and forecast data and ice data can be superimposed.
Continuous monitoring of ship’s position wrt planned track and available sea room
When interfaced with ARPA targets acquired by ARPA can also be similarly monitored.
Passage planning can be done on ECDIS itself without referring to various publications as most information available on ECDIS itself.
Charts can be corrected by floppy/ CD
Anchoring can be planned precisely
Log keeping is electronic and easy and voyage playback is possible.
Discrepancy between any two position fixing systems can be easily identified by ECDIS
More accurate ETA can be worked out
With trial manoeuvre being done on ECDIS availability of sea room on new course can easily be checked.

COMPARISON OF VECTOR AND RASTER CHARTS: ccllasssiicc wrrmzd

Customization not possible
Chart appears cluttered
Chart stored in single layer and information cannot be displayed selectively
Since chart is in single layer no possible information can be lost
Alarms of safety depths, TSS etc not possible
Seamless charts not possible
Same symbols and colours as that of paper charts ( symbols clours as per IHO, S 57 )
Scale of chart cannot be changed
Information can only be added
Interrogation for more information is not possible
GPS; RADAR/ARPA information, chart corrections can be overlaid ( Has also same facility )
Cheaper and simple to produce
Worldwide coverage possible
Sensible rotation to any angle like H-up not possible
Display regeneration takes time
Memory requirement is higher
Zooming facility not possible
Direct copy of paper chart. (Computer generated chart)

ERRORS OF ECDIS:
Errors of interpretation or human errors:
Ignoring scale of display ; Uncritical acceptance of own ship’s position; Ignoring difference between True North and Gyro North; Confusion of different type of vectorts, display mode and/or reference system.
Errors of Displayed data:
Source error; An electronic chart can be as good as the source of original data; Object size error; The items on chart are not drawn to scale; Position shift ( due different datums between GPS and chart); reference shift ( a difference in the matching during superimposing of the two displays).
RISK OF OVERRELIANCE ON ECDIS:
ECDIS is only a navigational aid; Limitations of ECDIS and its sensors make over reliance on it dangerous; periodical checks and lookout must be performed and basic principles o watchkeeping observed.


The publishers of official data for the ENCs are the National Hydrographic Offices of the marine nations, as is the case for the paper nautical charts.The IHO committee for ENC database has developed a model for distribution network for ENC services.ThisWorld ENC Database (WEND) model proposes a number of Regional ENC Co-ordinating Centers (RENCs). Each ENC producing nation would send its data to the local RENC that is responsible for storing and distributing chart data and updates to on ship users.

NIMA: National Imagery and Mapping Agency
Topology: study of geometrical properties unaffected by changes in sizes and shapes.
S-57: The official international standard for the programming of software for electronic navigation.
SENC: The database for software manufacturers to display ENC. This is the proprietry internal system format used by the ECDIS and ECS manufacturers for operational use.
ECDIS: It is a complete hardware and software system which conforms to the IMO S-57 requirements. To be called an ECDIS, the system must undergo testing and assessment by authorized IMO representatives.
NOAA: national Oceanic and Administration Administration

GPS

GPS: It stands for global positioning system and is a satellite based navigation system. It is works on the principle of timing and ranging using that fact that signals transmitted by satellites travel at known speed of 3x108 m/s.Time taken by signal to travel is measured to obtain distance of satellite from the earh.This way each satellite gives positions sphere which on interception with earth’s surface gives position circle. Two such circles give 2- D and three such circles give 3-D positions. An additional satellite measurement is taken by GPS receiver to account for receiver clock bias and thus correct all satellite “pseudo ranges” to correct ranges and make the position free of errors. This is needed because receiver clock is not very accurate like the expensive atomic clocks fitted on satellites. It is more accurate than any other existing system, provides worldwide coverage and is unaffected by weather conditions and man made or electrical interferences.
GPS signal consists of pseudo random code, ephemeris and almanac. Each satellite transmits two codes i.e. P (precision) code (gives 10 m accuracy) and C/A (coarse acquisition) code (gives 100 m accuracy). C/A code is for civilian users and P code is reserved for US military and its allies.
These codes are modulated by phase modulation technique on two carrier frequencies:
L1 = 1575.42 MHz (consists of both P and C/A codes)
L2 = 1227.60 MHz (consists of only P code)

The C/A code is made up of 0s and 1s called chip, having a frequency of 1.023 MBits/sec or 1.023 Bits/µ sec or 1023 Bits/msec.
Entire sequence is repeated ever 1 msec. i.e. after every 1023 Bits or every 3x 105 m
Thus each Bit (chip) is given by: 300000 / 1023 = 293 m = 1.023 µ sec
In case of P code the chip frequency is 10.23 MBits/sec, the duration of each Bit becomes 0.1023 µ sec or, in terms of distance 29.3 meters.

In addition each satellite transmits 2 types of orbit data Almanac and Ephemeris. Almanac data contains the health and approximate location of each satellite in the system. A GPS receiver gathers Almanac data from any available satellite; using information from the Almanac, the receiver then determines which set of satellites will give the best geometries for position fix. And then the receiver obtains Ephemeris from those satellites.
The receiver then assesses the transmission time and signal quality from each satellite and multiplies the difference in transmission time by speed of light to arrive at estimated satellite’s distance.
Ephemeris data contains the precise orbital parameters of each satellite.

GPS signal is extremely resistant to conditions that disturb other electronic navigation systems. In general GPS is not affected by:
Weather conditions
Passing ships
Onboard electronic installations
Onshore electronic installations
Onboard engine ignition
Portable radio receivers

INITILISATION OF GPS: Go to set up and feed:
1. Coordinate system for posn display ( UTM: Universal Transverse Mercator, Lat/Long etc. )
2. Elevation/Antenna Height: Select 2D or 3D mode. When in 2D mode, the receiver computes position only and to maximize accuracy in this mode you should enter the elevation/antenna height. In 3D, the receiver computes position and elevation.
3. Lat and Long of the vessel: Enter your approximate position. It will greatly shorten the time to fix the first position otherwise it will take 20 min to give first fix.
4. Time : Select UTC or local time
5. Velocity Averaging: Velocity averaging can be used to minimize the effects of SA on velocity output. The settings are off, 20 s, 60 s.
6. Compass Reference: Select magnetic or True north.
7. Chart Datum: Select the required datum , preferably WGS 84
8. Last Fix Interval: Sets the rate at which positions are automatically saved.Settings are: off, 10, 20, 30 min.
9. Plot Setup: Adjusts the track orientation and track history on the plot screen. For track select 0.1, 0.5, 1.0, 2.0 miles. For orientation choose between Head up and North Up.
10. Alarms: Select any or all of the following alarms: Anchor alarm; Arrival alarm; cross track alarm.
11. Light intensity and contrast settings.



GPS SEGMENTS: The GPS system has 3 segments:
GROUND BASED SEGMENT: It consists of 4 land based monitoring stations (Kwazalein, Hawaii, Ascension Island and Diego Garcia). It also has master control station and upload station located at Colorado Springs in USA. The monitoring stations track the satellites, obtain the data from these satellites and pass the information to master control station. After receiving the data from the monitoring stations, the master control station does the necessary computation to predict the future path and position of all the satellites. The master control station also determines the error of the atomic clocks in all the satellites. The updated data are fed to the upload station, which in turn transmits the same data to each satellite three times a day.
The master control station can selectively degrade satellite data.This degradation is known as Selective Availability and it can cause position errors of 100 meters.

SPACE SEGMENT: It consists of 24 satellites evenly placed in 6 different orbits. They move at the speed of 3.9 km/sec at a height of 20,200 km above earth’s surface. Each orbit is inclined at an angle of 55 deg to the equinoctial and the angle between 2 successive orbits is 60 deg. The satellites are arranged in such a manner that at any time signals can be received from at least 4 satellites having elevation of more than 9.5 degrees.
If any satellite emits erroneous data or is otherwise not operating properly, a ground station marks it “unhealthy”. The affected satellite broadcasts its status to the GPS receiver, which is programmed to ignore an unhealthy satellite and use the next best satellite.
USER SEGMENT: It consists of receiver with built in computer, display unit and antenna. The receiver locks on to one satellite and from this satellite it obtains the almanac of all the other satellites and thereby selects the four most suitable satellites for position fixing. The fix obtained is displayed on the display unit along with other information like COG, SOG and UTC.
Each satellite transmits a navigational message of 30 s in the form of 50 Bits/s data frame. This data, which is different for each satellite is supplied by MCS and is divided into 5 sub-frames. Each sub-frame commences with telemetry word (TLM) containing satellite status followed by hand over word (HOW) data for acquiring P code from C/A code. The 1st sub frame contains data relating to satellite correction. The 2nd and 3rd sub frames contain the satellite ephemeris defining the position of the satellite. The 4th sub frame passes alphanumeric data to the user and will only be used upload station has a need to pass specific message. The 5th sub frame gives the almanac of all other satellites which includes data on satellite health and identity codes.

ERRORS IN GPS POSITIONS: 1. Satellite clock error: this is caused by error in satellite’s clock wrt GPS time. This is monitored by ground based segment and any error in the Satellite clock forms part of the 30s navigation message. Though satellite clocks are highly accurate, they are not perfect. Clock errors can cause positional errors of about 1.5m.
2. Use clock error: It is caused by user clock not being synchronized with the satellite clock. Because of this range measurement will not be correct and is known as pseudo range. This error is eliminated within the receiver by extra satellite measurement (three for 2D and four for 3D positions).Extra measurement is used to find the extra unknown variable i.e. the error between user and satellite clock.
3. GDOP: The lower he value of GDOP, higher the accuracy of the fix obtained. The value of GDOP is indicated on the display. Since the receiver knows he position of all the satellites through 30s navigational message, it is programmed to select the best available satellites considering their elevation and geometry.
4. Multi path Error: This is caused signal coming to receiver after being reflected from other obstructions in addition to reaching the receiver directly. The two signals thus received simultaneously cause distortion of signal and in turn affect the range measurement. This problem is resolved by suitable siting of antenna. Multipath errors are difficult to detect but good quality sophisticated receivers can minimize the problem using intricate signal rejection techniques.
5. Ionospheric and troposphere error: Radio waves transmitted by satellites pass through different layers of atmosphere and space and undergoes change in its velocity which in turn affects the range measurement and hence the fix. Dual frequency receivers are able to receive both the frequencies and calculate the correction and do the compensation within the receiver. Ionosphere [position errors can be about 5m whereas troposphere conditions can cause rerrors up to 1 m.
6. Deviation of satellite from predicted path: The satellites are monitored and their paths predicted by ground based segment. Between two consecutive monitoring there may be small deviations from their respective paths. These deviations hen translate into errors in fixes. These errors can be up to 2.5 m.



DOP: A dimensionless number that takes into account the contribution of relative satellite geometry to errors in position determination. Positions tagged with a higher DOP value generally constitute poorer measurement results than with lower DOP.Thus DOP is a measure of the quality of GPS data received from the satellites. The main factors affecting DOP are the number of satellites being tracked and where these satellites are positioned in the sky. There are 5 types of DOPs:
HDOP: It is a measure of the accuracy of 2 dimensional positions. Higher HDOP values can be caused by satellites at high elevations.
VDOP: It is a measure of how well satellites are arranged to give vertical position .Higher VDOP means less certainty and can be caused if satellites have low elevations.
PDOP: is a measure of overall certainty in GPS position with TDOP not included in the estimated certainty.
PDOP = square root of (HDOP square + VDOP square)
A perfect PDOP implies one satellite directly overhead and 3 satellites positioned at 120 degrees intervals just above the horizon.PDOP less than 4 gives best accuracy. PDOP 4 -8 means acceptable accuracy and PDOP greater than 8 implies poor accuracy.
TDOP: is measure of certainty of how well GPS measures time.
GDOP: is overall certainty of position as well as time measurement i.e. including TDOP as well.
GDOP = square root of ( PDOP square + TDOP square )

The position accuracy = DOP x Measurement Precision
E.g. if DOP is 5 and M.P. is 1 meter, then
P.A. = 5 x 1 = 5 meters


Geodetic Datum: Also called horizontal datum. It is a reference for specifying positions on earth’s surface. Each reference (datum) is associated with a particular reference spheroid. Positions referenced to different datum can differ by several hundred meters.
WGS 84 stands for World Geodetic Datum (system) 1984.All GPS positions are wrt WGS 84.If chart is not drawn to the same datum, position from GPS has to be reduced to the datum of the chart by the difference known as “datum shift”. This is given on charts under the title “satellite derived positions”. An example is also given showing how to apply datum shift to GPS positions before plotting them on the chart.Eventually all charts will be referenced to WGS 84 datum.But this is a lengthy process and one that can proceed when the relationships between existing surveys and WGS 84 datum have been established.Any differences in positions obtained between GPS and other source should be reported by mariner on Form H102b ( Form for Recording GPS Observation and Corresponding Chart Positions )
Most GPS receivers now have the facility to perform transformation of positions from WGS84 Datum to variety of local horizontal datums.The general parameters used in the software may differ from those used by the Hydrographic Office resulting in the possibility that positions may not agree with the chart, even if the horizontal datum is stated to be the same.It is therefore recommended that GPS be kept referenced to WGS84 datum and apply datum shift indicated on chart.
COLD START: The ability of a GPS Rx to start providing posn without the assistance of any almanac info stored in its memory.










Spheroid: It is a mathematically regular surface resembling a slightly flattened sphere, defined by the length of its axes and used to approximate the “geoid” in geodetic computations.
Geoid: An imaginary surface which is everywhere perpendicular to the plumb line and which on average coincides with MSL in open oceans. Its shape approximates to that of a spheroid but it’s irregular because of uneven distribution of earth’s mass.
Almanac: A set of parameters included in GPS satellite message that is used by receiver to predict the appropriate location satellite.

Pseudo random code: It is a fundamental part of GPS.Physically it is just a very complicated digital code i.e. a complicated sequence of “on” and “off” pulses. The signal is so complicated that it looks like random electrical noise. Hence the name “pseudo random”.
The complexity serves two purposes.
It makes sure that receiver doesn’t accidentally sync up to some other signal. The pattern is so complex that it is unlikely that a stray signal will have exactly the same shape.
Since each satellite has its own PR code this complexity also ensures that receiver won’t accidentally pick up some other satellite’s signal. So all satellites can use same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system. In fact PR code gives DoD a way to control the system.

Scale of charts: At a scale of 1:25000 plotting precision of 0.2 mm means an accuracy of about 5 meters on ground.