MANOEUVRING

When ship proceed in water, bow displaces the water ahead of her. In order to fill that place, the volume of water must return down the sides and under the bottom of the ship. This streamlines of return flow causes drop in pressure and reduction of buoyancy, resulting in the ship dropping vertically in water i.e ship trim fwd & aft.

So the overall decrease in static under keel clearance is called ship’s squat

The factor who affect the squat is speed of the vessel & block coefficient

• Squat is directly proportional to the square of the speed
• Squat is directly proportional to the block coefficient

           Maximum squat in open waters = (Cb x V^2 )/100.

• The pivot point is a point at which ship rotates
• It is a point at which turning effect of the vessel takes effect
• When a vessel is stopped, the pivot point normally lies at the ship’s center of gravity which is generally amidship (assuming the vessel is on even keel and in calm weather)

How does pivot point effect manoeuvring of a vessel in following condition:-

(i) When going ahead

• As the vessel moves forward using engine power, the pivot point will be pushed forward to the forward extreme of the vessel with the momentum of the vessel
• However, due to the resistance faced by the water ahead, the pivot point will be pushed behind and it will come & settle at approximately 25% or a 1/4 of the ship’s length from forward.

(ii) When going astern

• Similarly when the vessel is moving stern, the stern motion will cause the pivot point to move aft & settle at a position approximately 25% or a 1/4 of the ship’s length from the stern.

(iii) When vessel stopped

• The pressure of the water that acts on the bow or at the stern cause a shift in the position of the Pivot point
• In this situation no forces are involved and hence, the ship has a pivot point coinciding with its centre of gravity approximately amidships.

(iv) When anchored

• When a vessel is at anchor, the pivot point moves right forward and effectively settles at a hawse pipe.

Effect of wind on ship handling in following condition:-

(i) When stopped

• The effect of wind on a stopped vessel is different in different vessels. It depends on the exposed windage area, loaded or ballast condition & trim of the vessel

• When the vessel is stopped, and there is beam wind:- When stopped, the pivot point (P) will be at midships. For beam wind cases, in a longitudinal sense, the centre of effort of the wind (W) will also be near midships only. So, it can be said that both W and P are quite close together and therefore do not create a turning influence upon the ship.

(ii) When moving ahead

• When the vessel is moving ahead, and there is beam wind:- When moving ahead, the pivot point (P) will be shifted forward. For beam wind cases, in a longitudinal sense, the centre of effort of the wind (W) will again near midships only.  So, this separation between these two points creates a turning lever, depending on wind strength, the bow will swing into the wind.

(iii) When moving astern

• When the vessel is moving astern, and there is beam wind:- When moving astern, the pivot point (P) will be shifted aft (to a position approximately 1/4 L from the stern). For beam wind cases, in a longitudinal sense, the centre of effort of the wind (W) will again near midships only.  So, this separation between these two points creates a turning lever, depending on wind strength, the stern will swing into the wind.

For headwinds and aft winds, the winds can only constructively or destructively interfere with the vessel’s linear motion. Since the wind force vector is concurrent with the ship’s centerline, there is no turning moment created.

In a narrow channel, when the ship moves near the bank, the bow of a ship is pushed away from the bank which is known as bow cushion effect and the vessel is bodily attracted towards the bank, which occurs at stern is known as bank suction effect.

Since at the bank side, there is a restricted flow of water. As the ship enters the narrow channel where there is restricted flow of water, a high pressure is buildup between the bank and bow of the ship resulting in bow cushion effect & low pressure and increase in velocity of water in narrow channel between the vessel and the bank result in bank suction effect.

Transverse thrust is the tendency of a forward or aft running right handed propeller to move the stern to starboard or port. 

(i) When moving ahead

• The effect of transverse thrust when moving ahead is very less in compare of when moving astern. It is very less noticeable.
• When moving ahead, transverse thrust is directed to starboard and the ship turns to port i.e stern moves to starboard and bow swings to port.

(ii) When moving astern

• Since the water pressure in the starboard will be higher than in the port because the water from the propeller blades is thrown upward in the starboard side and downward in the port side.
• So when moving astern, transverse thrust is directed to port and the ship turns to starboard i.e stern moves to port and bow swings to starboard.

(iii) When turning

• The vessel with a right hand turn propeller, if making a turn to port, will have a small turning circle than when turning to starboard side due to transverse thrust.

Advance:- It is the distance traveled by ship in the direction of original course from the position at which helm order is given to the position at which heading is changed by 90° from its original heading.

Transfer:- It is the distance traveled by ship measured from original course line to the point at where the vessel has altered her course by 90°.

Tactical Diameter:- It is the distance traveled by ship measured from original course line to the point at where the vessel has altered her course by 180°.  When the vessel is trimmed by stern, the tactical diameter of the turn is increased and vice versa.

Drift angle:- This is the angle between the ship’s fore and aft line and the tangent to the turning circle (path on which it is turning). We can also say, it is the angle between the heading and the track.

Where we find it?

In the wheelhouse poster present on the bridge

Stopping distance is the distance which ship will cover after the action is taken to stop engines and till the ship comes to rest.

• It is a term used when ship has to sudden stop in emergency situation
• It is when engine is moving in ahead & is given the order for full astern.
• The rudder is kept amidship.

The ship’s manoeuvring data is displayed in the wheelhouse is known as wheelhouse poster.

The following information is available from the wheelhouse poster:-

• Ship’s general particulars: Includes ship’s name, call sign, gross tonnage, net tonnage, displacement at summer draft, LOA, moulded breadth, summer draft and minimum propeller immersion draft.
• Steering particular: Type of rudder, maximum rudder angle, time from hard over to hard over, minimum speed to maintain course.
• Propulsion particular: Type of main engine, main engine power, type of propeller, manoeuvring speed tables established for loaded & ballast condition with RPM for each movement, critical RPM, maximum and minimum revolution, full ahead to full astern time and stop to full astern time.
• Turning circle at maximum rudder on either side for both loaded & ballast condition and in deep water as well as shallow water.
• Stopping capabilities including track reach and time taken for various ahead speeds.

• Each structure has its own frequency of vibration like engine has certain frequency of vibration, the structure on which it is installed has certain frequency of vibration.
• If the natural frequency of vibration of one becomes same to that of other, it vibrates with high amplitude.
• This can cause damage to structural components.
• So the engines are never run at this RPM for continuous period.

• Dead slow ahead on the engines
• Wheel hard to starboard
• The moment vessel starts to turn, stop the engines
• Dead slow astern & wheel amidships
• Due to transverse thrust, the vessel will continue turning to starboard

Shallow water means depth of water is less compared to the draft of the vessel. The effects on ship handling will be more when depth of water is less than 1.5 times of draft of the ship.

Those effects are as follows:-

• Maximum ship squat increases & ship will have some extra trim due to the squat effect
• Bow wave increases
• The load on the engine increases
• The speed over water reduces
• Stopping distance and stopping time increases
• The effectiveness of rudder helm decreases & ship become more sluggish to manoeuvre
• The turning circle diameter increases
• Rolling, pitching & heaving motion reduces as ship moves from deep water to narrow channel because of cushioning effect
• The ship may start to vibrate suddenly
• The appearance of mud could suddenly show in the water around the ship’s hull.

• This occurs when a ship is near an extremely shallow depth of water, such as shoal
• The ship is likely to take a sudden sheer
• The sheer is first towards the shallow, then violently away from it
• The movement of ship suddenly become sluggish
• These effects are called smelling the ground.