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IAMSAR Search Patterns Explanation with Sketches.

EXPANDING SQUARE SEARCH:

• Most effective when the location of the search object is known within relatively close limits.
• The commence search point is always the Datum Position.
• To be used by a single ship during a search.
• Often appropriate for vessels or small boats to use when searching for persons in the water or other search objects with little or no leeway.
• Accurate navigation is required.
• The first leg is usually oriented directly into the wind to minimize navigational errors.
• All course alterations are of 90^O.
• Two first two legs will be of same length ‘d’. ‘d’ will depend upon the visibility and the height of eye of the lookouts and the swell and sea height.
  • Legs 3 and 4 will be a length of 2d.
  • Legs 5 and 6 will be a length of 3d.
  • Legs 7 and 8 will be a length of 4d.

SECTOR SEARCH:-

• Most effective when the position of the search object is accurately known and the search area is small.
• Used to search a circular area centered at the datum.
• Can be used by only one craft at a time at a certain location.
• An aircraft and a vessel may be used to perform independent sector searches of the same area.
• A suitable marker may be dropped at the datum and used as a reference point.
• The commence search point is where the ship or aircraft enters the area to be searched.

PARALLEL SWEEP (TRACK) SEARCH:-

• Used to search a large area when the location of the search object
  is uncertain.
• Most effective over water or flat terrain.
• Usually used when a large search area must be divided into
  sub-area for assignment to individual search facilities on-scene at the same
  time.
• The commence search point is in one corner of the sub-area,
  one-half track space inside the rectangle from each of the two sides forming
  the corner.
• Search legs are parallel to each other and to the long sides of
  the sub-area.
• The main legs indicate the direction of drift.

Multiple vessels may be used as shown opposite:

• Parallel sweep: for use by two ships.
• Parallel sweep: for use by three ships.
• Parallel sweep: for use by four ships.
• Parallel sweep: for use by five or more ships.

TRACK LINE SEARCH (TS):-

• Normally used when an aircraft or vessel has disappeared without a
  trace along a known route.
• Often used as initial search effort due to ease of planning and
  implementation.
• Consists of a rapid and reasonably thorough search along intended
  route of the distressed craft.
• Search may be along one side of the track line and return. in the
  opposite direction on the other side (TSR).
• Search may be along the intended track and once on each side, then
  search facility continues on its way and does not return (TSN).
• Aircraft are
  ‘frequently used for TS due to their high speed.

CONTOUR SEARCH (OS):-

• Used around mountains and in valleys when sharp changes in
  elevation make other patterns not practical.
• Search is started from highest peak and goes from top to bottom
  with new search altitude for each circuit.
• Search altitude intervals may be 150 m to 300 m (500 ft to 1,000
  ft).
• The aircraft may make a descending orbit away from the mountain
  before resuming the contour search at the lower altitude.
• The aircraft may spiral downwards around the mountain at a low but
  approximately constant rate of descent when there is not enough room to make a
  circuit opposite to the direction of search.
• If the mountain cannot be circled, successive sweeps at the same
  altitude intervals as listed above should be flown along its side.
• Valleys are searched in circles, moving the centre of the circuit
  one track spacing after each completed circuit.

CO-ORDINATED VESSEL-AIRCRAFT SEARCH PATTERN:-

• Normally used only if there is an OSC present to give direction to and provide communications with the participating craft.
• Creeping line search, co-ordinated (CSC) is often used.
• The aircraft does most of the searching, while the ship steams
  along a course at a speed as directed by the OSC so that the aircraft can use it as a navigational checkpoint.
• The aircraft, as it passes over the ship, can easily make corrections to stay on the track of its search pattern.
• Gives a higher probability of detection than can normally be attained by an aircraft searching alone.
• Ship speed varies according to the speed of the aircraft and the size of the pattern.
• The relationship among the speed of the surface facility, the aircraft’s speed, the track spacing and the length of the search legs is defined by the following equation:

Vs = (5 x Va)/ (L + 5)

Where,

• Vs is the speed of the surface facility in knots;
• S is the track spacing in nautical miles;
• Va is the aircraft’s true air speed (TAS) in knots, and
• L is the length of the aircraft’s search leg in nautical miles.

IAMSAR Vol.III Contents:

The Mobile Facilities volume (Vol.III) is intended to be carried aboard units, aircraft and vessels to help with the performance of a search, rescue, or on-scene coordinator function and with aspects of SAR that pertain to their own emergencies. It contains:

• Section 1 Overview
• Section 2 Rendering
  assistance
• Section 3 On-scene
  co-ordination
• Section 4 On-board
  emergencies

• Appendix A Regulation V/33 of the International Convention for the Safety of Life at Sea, 1974, as amended.
• Appendix B Search action message.
• Appendix C Factors affecting observer effectiveness.
• Appendix D Standard format for search and rescue situation report (SITREP).
• Appendix E SAR briefing and debriefing form.
• Appendix F Own emergency.
• Appendix G Rendering assistance

Carriage of IAMSAR Volume 3 is compulsory for vessels from 1 January 2004

It includes details on search engine to be utilized by both surface and aircraft in single or combined searches, methods of intercept and contains useful advice on preparations and signals for responding to distress scenarios.

A good working knowledge of this volume is essential for all watchkeeping officers.

Immediate actions on receipt of a distress message at sea from another vessel as per IAMSAR :

• Inform the Master.
  • Acknowledge receipt of message and gather the following information from the vessel in distress if possible:
    • position of distressed vessel
      • distressed vessel’s identity, call sign,
        and name
        • number of POBs
        • nature of the distress or casualty
        • type of assistance required
        • number of victims, if any
        • distressed vessel’s course and speed
        • type of vessel, and cargo carried
        • any other pertinent information that might facilitate the rescue
  • Maintain a continuous watch on the following international frequencies, if equipped to
    do so:
    • 500 kHz (radiotelegraphy)
    • 2182 kHz (radiotelephony)
    • 156.8 MHz FM (Channel 16, radiotelephony)
      for vessel distress
    • 121.5 MHz AM (radiotelephony) for aircraft
      distress.

What is Anderson’s Turn and when & where can this be used?

Ans:- One turn (“Single turn, Anderson turn”):-

• Used during SAR situations.
• Fastest recovery method
• Good for ships with tight turning characteristics
• Used most by ships with considerable power
• Very difficult for a single-screw vessel
• Difficult because approach to person is not straight

Purpose of IAMSAR:

The Purpose of IAMSAR Manual is to provide guidance to those who:

• Operate aircraft, vessels or other craft, and who may be called upon to use the facility to support search and rescue (SAR) operations.
• May need to perform on-scene co-ordinator functions for multiple facilities in the vicinity of a distress situation.
• Experience actual or potential emergencies, and may request SAR assistance.

To fulfill the above functions, the manual has been divided into three volumes as follows:

• Volume I – Organisation and Management (for administrations)
• Volume II – Mission Co-ordination (for Rescue Co-ordination Centre {RCC} personnel)
• Volume III – Mobile Facilities (for ships, aircraft, and coastal radio station (CRS) personnel.

Duties & Responsibilities of the On-Scene Co-ordinator (OSC):-

• Co-ordinating operations of all SAR facilities on-scene.
• Receiving the search action plan or rescue plan from the SMC (SAR Mission Co-ordinator) or planning the search or rescue operation, if no plan is otherwise available.
• Modifying the search action or rescue action plan as the situation on-scene dictates, keeping the SMC advised.
• Co-ordinating on-scene communications.
• Monitoring the performance of other participating facilities.
• Ensuring operations are conducted safely, paying particular attention to maintaining safe separations among all facilities both surface and air.
• Making periodic situation reports (SITREP’s) to the SMC. The reports should include but not be limited to:
  • Weather and sea conditions.
  • The results of search to date.
  • Any actions taken
  • Any future plans or recommendations
• Maintaining a detailed record of the operation:
  • On-scene arrival and departure times of SAR facilities, other vessels and aircraft engaged in the operation.
  • Areas searched
  • Track spacing used
  • Sightings and leads reported
  • Actions taken
  • Results obtained
• Advising the SMC to release the facilities no longer required.
• Reporting the number and names of survivors to the SMC.
• Providing the SMC with the names and designations of facilities with survivors on board.
• Reporting which survivors are in each facility.
• Requesting additional SMC assistance, when necessary (e.g. medical evacuation).

The Williamson Turn

1. Note the position of the ship.
2. Put wheel hard over to the side of the casualty.
3. After the ship has altered course by about 60 degrees, put wheel hard over to the other side.
4. When the vessel is 20 degrees short of the reciprocal course, wheel on midship.

The Scharnow Turn

1. Put the rudder over hard toward the person
2. After deviating from the original course by about 240 degrees, shift the rudder hard to the opposite side.
3. When heading about 20 degrees short of the reciprocal course, put the rudder amidships so that vessel turns onto the reciprocal course.

The Anderson Turn

1. Stop the engines.
2. Put the rudder over toward the person
3. When clear of the person, go all ahead full, still using full rudder.
4. After deviating from the original course by about 240 degrees (about 2/3 of a complete circle), back the engines 2/3 or full.
5. Stop the engines when the target point is 15 degrees off the bow. Ease the rudder and back the engines as required.

List the “immediate actions” and “subsequent actions” that should be taken in a man over board situation:

Immediate Actions – it is very important that all the 6 actions mentioned below must be executed in quick succession.

1. Shout ‘Man overboard on Stbd/port side’ several times. Inform Bridge.
2. Change over to hand steering and put wheel hard over to the side the man has fallen overboard.
3. Release the MOB Marker from the Bridge wing on the side the man has fallen overboard.
4. Press the MOB button on the GPS receiver to mark the position for future reference. Many ECDIS also have this feature.
5. Sound “O” on the ship’s whistle. This way the Master would rush to the Bridge. If not, call the Master. This would also alert all persons on deck.
6. Post a lookout as soon as possible.

Subsequent action – once the above actions are carried out, carry out the following subsequent actions

1. Carry out the Anderson turn or Williamson turn or Scharnow turn whichever is most suitable under the circumstances.
2. Soon after, announce on the PAS “Man overboard on ___ Side. Prepare rescue boats”. Hearing the whistle and the announcement over the PAS, the rescue boat crew will start preparing the rescue boat. Maintain communication with this team.
3. Inform E/R about man overboard and ask them to prepare engines to manoeuvre as soon as possible. Do not use M/E emergency stop as it will not serve any purpose.
4. Send out Urgency signal on VHF Ch.16. This would alert all vessels in the vicinity and they would keep well clear of own vessel and not hinder the rescue activities. If external help is needed, Urgency signal must be upgraded to Distress signal.
5. Keep the man in sight. If not possible, keep the MOB marker in sight. Post additional lookouts for this purpose on Compass deck or other location as appropriate.
6. Reduce speed and manoeuvre the ship close to the windward side of the man, creating a lee, if necessary, for the rescue boat.
7. The rescue boat must now be lowered and the man picked up. If the man is not easily visible from the boat, the Bridge team must guide the rescue boat. Once man is picked up, administer first aid and resuscitation in the boat itself.
8. The lifebuoy should be picked up and the boat hoisted back on board.
9. Urgency signal can now be cancelled and vessel can resume her course.
10. Maintain a record of all events and timing in the Manoeuvring Book. Appropriate entries must be made in the Ship’s logbook.
11. Inform office after everything is under control.
12. The Master must hold an inquiry into the incident and make appropriate entries in the Official Log Book.

Duties of SAR mission co-ordinator:

SAR Mission Co-ordinator (SMC):- Each SAR operation is carried out under the guidance of an SMC. This function exists only for the duration of a specific SAR incident and is normally performed by the RCC chief or a designee. The SMC may have assisting staff.

        The SMC guides a SAR operation until a rescue has been effected or it becomes apparent that further efforts would be of no avail.

        The SMC should be well trained in all SAR processes, be thoroughly families with the applicable SAR plans, and:

• Gather information about distress situations.
• Develop accurate and workable SAR action plans.
• Dispatch and co-ordinate the resources to carry out SAR missions.

SMC Duties Include:-

• Obtaining and evaluating all data on the emergency.
• Ascertaining the type and quantity of emergency equipment carried by the distressed or missing craft.
• Ascertaining prevailing and forecast environmental conditions.
• If necessary, ascertaining movements and locations of vessels and alerting shipping in likely search areas for rescue, lookout and/or radio watch.
• Plotting the areas to search and deciding on methods and facilities to be used.
• Developing the search action plan and rescue action plan as appropriate.
• Co-ordinating the operation with adjacent RCCs when appropriate.
• Arranging briefing and debriefing of SAR personnel.
• Evaluating all reports and modify search action plan as necessary.
• Arranging for refueling of aircraft and, for prolonged search, making arrangements for the accommodation of SAR personnel.
• Arranging for delivery of supplies to sustain survivors.
• Maintaining in chronological order an accurate and up-to-date record.
• Issuing progress reports.
• Recommending to the RCC chief the abandoning or suspending of the search.
• Releasing SAR facilities when assistance is no longer required.
• Notifying accident investigation authorities.
• If applicable, notifying the state of registry of the aircraft or surface craft.
• Preparing a final report.

National & Regional SAR system Organisation:

National and Regional SAR System Organization:- Many States have accepted the obligation to provide aeronautical and maritime SAR co-ordination and services on a 24-hour basis for their territories, territorial seas, and where appropriate, the high seas.

• To carry out these responsibilities, States have established national SAR organizations, or, joined one or more other States to form a regional SAR organization associated with an ocean area or continent.
• A search and rescue region (SRR) is an area of defined dimensions associated with a rescue co-ordination center (RCC) within which SAR services are provided.
  • SRRs help to define who has primary responsibility for coordinating responses to distress situations in every area of the world, but they are not intended to restrict anyone from assisting persons in distress
  • the International Civil Aviation Organization (ICAO) regional air navigation plans (RANPS) depict aeronautical SRRs
  • the International Maritime Organization (IMO) Global SAR Plan depicts maritime SRRS.

Preparations carried out on board en route to render the assistance to the distressed vessel as per IAMSAR:

On-Board Preparations:

• A vessel en route to assist a distressed craft should prepare for possible
• SAR action on scene, including the possible need to recover people
• from survival craft or from the water. See “Recovery of survivors by
• assisting vessels” later in this section.
• Masters of vessels proceeding to assist should assess the risks they may
• encounter on scene, including the risks such as those associated with
• leaking cargo, etc. Information should be sought as necessary from the
• distressed craft and/or from the RCC.

A vessel en route to assist a distressed craft should have the following equipment ready for possible use:

Life-saving and rescue equipment:

• lifeboat
• inflatable liferaft
• lifejackets
• survival suits for the crew
• lifebuoys
• breeches buoys
• portable VHF radios for communication with the ship and boats deployed
• line-throwing apparatus
• buoyant lifelines
• hauling lines
• non-sparking boat hooks or grappling hooks
• hatchets
• rescue baskets
• stretchers
• pilot ladders
• scrambling nets
• copies of the International Code of Signals
• radio equipment operating on MF/HF and/or VHF/UHF and capable of communicating with the RCC and rescue facilities, and with a facility for direction finding (DF)
• supplies and survival equipment, as required
• fire-fighting equipment
• portable ejector pumps
• binoculars
• cameras
• bailers and oars.

Signalling equipment:

• signalling lamps
• searchlights
• torches
• flare pistol with colour-coded signal flares
• buoyant VHF/UHF marker beacons
• floating lights
• smoke generators
• flame and smoke floats
• dye markers
• loud hailers.

Preparations for medical assistance, including:

• stretchers
• blankets
• medical supplies and medicines
• clothing
• food
• shelter.

Miscellaneous equipment:

• If fitted, a gantry crane for hoisting on each side of ship with a cargo net for recovery of survivors.
• Line running from bow to stern at the water’s edge on both sides for boats and craft to secure alongside.
• On the lowest weather deck, pilot ladders and manropes to assist survivors boarding the vessel.
• Vessel’s lifeboats ready for use as a boarding station.
• Line-throwing apparatus ready for making connection with either ship in distress or survival craft.
• Floodlights set in appropriate locations, if recovery at night.

Search & Rescue Co-ordinators (SCs) as per IAMSAR:

SAR Co-ordinators:

• SCs are the top level SAR managers; each State normally will have one or more persons or agencies for whom this designation may be appropriate.
• SCs have the overall responsibility for:
  • establishing, staffing, equipping and managing the SAR system
  • establishing RCCs and rescue sub-centers (RSCs)
  • providing or arranging for SAR facilities
  • coordinating SAR training
  • developing SAR policies.

Search & Rescue Region (SRR) as per IAMSAR:

A search and rescue region (SRR) is an area of defined dimensions associated with a rescue co-ordination center (RCC) within which SAR services are provided.

1. SRRs help to define who has primary responsibility for coordinating responses to distress situations in every area of the world, but they are not intended to restrict anyone from assisting persons in distress
2. The International Civil Aviation Organization (ICAO) regional air navigation plans (RANPS) depict aeronautical SRRs
3. The International Maritime Organization (IMO) Global SAR Plan depicts maritime SRRS.

Track Spacing:

Most search patterns consist of parallel tracks or sweeps covering a rectangular area. The distance between adjacent tracks is called the track spacing. Recommended uncorrected track spacings for merchant vessels are provided in the table following this discussion. Correction factors based on weather conditions and search object are provided in the table after the track spacing table. Multiplying the uncorrected track spacing (Su) by the appropriate weather correction factor (fw) produces the recommended track spacing (S):

S = Su x fw

Changes in weather, number of assisting craft, etc., may occur, making it prudent to alter the track spacing. The SMC must ensure that all searching ships and aircraft maintain safe separations from one another and accurately follow their assigned search patterns.

Datum as per IAMSAR

It will be necessary to establish a datum, or geographic reference, for the area to be searched. The following factors should be considered:

• reported position and time of the SAR incident
• any supplementary information such as DF bearings or sightings
• time interval between the incident and the arrival of SAR facilities
• estimated surface movements of the distressed craft or survival craft, depending on drift (The two figures following this discussion are used in calculating drift.) The datum position for the search is found as follows:
  • drift has two components: leeway and total water current
  • leeway direction is downwind
  • leeway speed depends on wind speed
  • the observed wind speed when approaching the scene may be used for estimating leeway speed of liferafts by using the graph following this discussion (Persons in the water (PIW) have no leeway while liferaft stability and speed vary with or without drogue or ballast.)
  • total water current may be estimated by using the computed set and drift of vessels at or near the scene
  • drift direction and speed is the vector sum of leeway and total water current
• drift distance is drift speed multiplied by the time interval between the incident time, or time of the last computed datum, and the commence search time
• datum position is found by moving from the incident position, or last computed datum position, the drift distance in the drift direction and plotting the resulting position on a suitable chart.

Q. Describe the various methods of carrying out damage control after collision, to restore the structural integrity of ships temporarily. State the legal aspect to be followed.

Ans:- Damage survey by the vessel hands and emergency measures:

1. Survey collision damage, (later) prepare a rough sketch of the damage and inspect the cargo for damage/confirm content in cargo tanks.
2. Sound tanks and bilges in affected areas.
3. If any flooding, close watertight doors and openings and start discharging operation. If the breakage is small, take waterproofing measures (using blankets, tarpaulins, cement box, wooden plugs, shoring and the like).
4. In serious flooding in E/R, use Bilge Injection system.
5. Isolate cargo, fuel, and ballast pipeline sources to affected areas of vessel.
6. If the breakage is above waterlines, take waterproofing measures against sea spray.
7. If the breakage is on the Fore Peak Tank, adjust speed so that excessive pressure will not be applied to the collision bulkhead.
8. Consider filling opposite end tank to offset list/trim caused by flooding
9. Compare the flooding rate with the discharging capacity.
10. If flooding rate is greater, consider the possibility of loss of buoyancy.
11. If loss of buoyancy is likely to occur, consider where to intentionally strand the vessel.

Confirming details on the other ship: Ships name, type, gross tonnage, Masters name, Ship owner and operator Port of registry, Cargo, last port and next port.

Securing Document for Evidence:-

1. It is often demanded to submit the originals of the following documents to the authorities, so make their copies at an early stage. Consult with the Company before submission).
2. Charts and the deck logbook/bell book
3. Record paper for the course recorder. (Since replacing paper usually produces a time lag of one to two minutes, it is recommended to affix on the paper the time of its removal for easier check of the time in the future).
4. Record paper for the telegraph logger and the bell book.
5. Capture the Voyage Data Recorder data
6. STCW records of working and rest hours of Master, officers and crew on duty
7. Drug & Alcohol Test of involved ships personnel
8. Statement of fact by pilot if on board.

Various Investigations:-

1. Access control as per SSP must be maintained at all times.
2. Render full cooperation in the investigations by lawyers, surveyors, P&I correspondents who are appointed for own vessel.
3. Reject investigation by the opponent surveyor unless permitted by Company. However, surveys on hull and cargo (known as W.P. survey) are permitted but limited to only checking extent of damages and no more.
4. Master should fully cooperate with interviews by a Flag or Coastal State Administration by giving them honest and accurate answers (or such signed statement).
5. NO statements to the media shall be given by the vessels except with company permission.
6. Safest – Politely refer media to Company.

How ISM code helps to tackle on Emergency Situation:-

• Objective of ISM Code is to ensure safety at sea, prevention of human injury or loss of life avoidance of damage to environment especially to marine environment.
• Purpose of ISM Code to provide an international standard for safe management, operation of ships and for pollution prevention.
• Emergency Preparedness: ISM Code ensures the following procedures are implemented
  • The Company should establish procedures to identify, describe and respond to potential emergency shipboard situations. 
  •  The Company should establishes programmes for drills and exercises to prepare for emergency actions.
  • The SMS should provide for measures ensuring that the Company’s organization can respond at any time to hazards, accidents and emergency situations involving its ships.
• Emergency situations included in ISM code:
  • Grounding
    • Flooding in Engine Room
    • Fire
    • Collision
    • Black out
    • Oil Spill
    • Leakage/ overflow during discharge
    • Emergency steering
    • Medical Emergency
    • Abandon ship
    • Piracy attack

Factors to bear in mind while determining Safe Anchorage / Anchor Planning:

1. Position of anchoring defined.
2. Depth of water and amount of cable.
3. State of tide HW/LW, rise of tide.
4. Type of holding ground.
5. Prevailing weather and shelter.
6. Underwater obstructions.
7. Rate of current.
8. Swinging room from surface objects.
9. Length of time vessel intend to stay.
10. Ship’s draft and UKC.
11. Use of 1 or 2 anchors.
12. Proximity of other shipping.
13. Local hazards outfalls etc.
14. Current weather and expected.
15. Position fixing method.
16. Distance from shore by launch.
17. Types of anchors and holding power.
18. Wind direction.
19. Speed of approach.
20. Night or day signals.

Vessel Brought Up during Anchoring:

• A vessel is said to be brought up when her way has stopped and she is riding to
  her anchor, with the anchor holding.
• The
  terms ‘come to’ and ‘got her cable’ are sometimes used to mean the same thing.
• The
  officer in charge of an anchor party will know when the vessel is brought up,
  by the cable rising up from the surface towards the hawse pipe when the brake
  is holding it.
• The
  vessel should then move towards the anchor, causing the cable to drop back and
  make a catenary.

Procedure & Precautions for Anchoring in over 20m Water:

• In water of over 20m the anchor should first be walked back to within say 4 or 5 m from the sea- bed, and let go from there.
• This ensures that the anchor will not damage itself falling a considerable distance on to a hard bottom, and also that the cable will not take charge and run out so rapidly that it becomes extremely difficult to hold it on the brake.
• This practice therefore considerably lengthens the life of the brake linings.

Precautions for anchoring in deep waters (over 100m):

• In a very deep anchoring depths, 100m and over, the entire operation of anchoring should be done under power.
• The gypsy should not be taken out of gear at all, because the heavy weight of cable between sea-bed and hawse pipe will undoubtedly take charge.
• In a wind it is better to approach the anchorage heading upwind.
• The ship is more easily controlled and will make little leeway.
• If the wind cannot be brought ahead, however, the ship can let go the anchor in the usual way and using her engines to relieve stresses on the cable, swing head to wind as she brings to.
• The weather anchor should be used so as to avoid nipping the cable round the stem.
• If the vessel is heading dead into the wind’s eye she should have her head cast off one way or the other before letting to the weather anchor.
• The cast should not be excessive, because the ship will rapidly seek to lie across the wind and develop a sharp swing to leeward.
• Correcting helm and bold use of engines should be used if the case develops into a swing.

Hydrodynamic Interaction between ships on opposite course in a narrow channel:

• On close approach, the combined positive bow pressure zones encourage the respective bows to be repelled and, if speed is excessive, may require vigorous corrective helm.

• When abeam, the combined low pressure zones encourage a suction which, if the vessel’s
  are very close, can cause a violent broadside collision. The bow of each vessel
  now comes under the influence of the other vessel’s stern, which also means the
  rudder of each vessel is being affected by the positive pressure of the other
  vessel’s bow. This causes each vessel to take a sheer towards the other vessel
  and, if in a narrow channel or fairway, for the stern to be cast towards the
  boundaries of that channel of fairway.

• As each vessel draws clear the combined low pressure zones cause the sterns to be drawn towards each other and assist the vessels to regain their original track.  

Pivot point when moving ahead with constant speed:

Pivot when moving ahead with constant speed:- There are two forces to deal with, the forward momentum of the vessel and the longitudinal resistance of the water ahead of the vessel. For a vessel to move at uniform speed, a proportion of the applied power is spent in overcoming the longitudinal resistance of the water. Initially, the pivot point moves right forward, then, as the forward momentum matches the water resistance, the pivot point will move aft to approximately 1/8 of the waterline length from forward. Once the water resistance has been overcome, the pivot point will move further aft & when at a uniform speed, will settle at a position, approximately ¼ of the waterline length from forward.