Ship Handling

Ship Handling

Written by: 2/off John Anthony Dana   On 06th August 2019

Factors affect Manoeuvering Characteristic:
1. The Ship Deadweight
The Larger the ship:
*Greater inertia
*larger ship will be more difficult to stop

2. Draught and Trim
Shallow water effects turning ability

3. Speed and Rudder angle
* Slow speed requires larger rudder angles to initiate turn and too increase rate of turn
* Also larger rudder angles required to arrest turns
* More speed, more water acting on rudder therefore more sensitive to rudder angles

4. Directional Stability
* Vessels ability to hold course against external influences
* Fine lined vessel better than a blunt ended tanker
* Good directional stability rudder amidship vessel should assume a straight course
* Poor directional stability will continue to turn even though rudder is amidship
* In any case steering is less effective when speed is reduced

5. The Effect of Wind
* Vessels ability to hold course against external influences
* Fine lined vessel better than a blunt ended tanker
* Good directional stability rudder amidship vessel should assume a straight course
* Poor directional stability will continue to turn even though rudder is amidship
* In any case steering is less effective when speed is reduced
* When vessel is light a gentle breeze has the same effect as a laden vessel in a gale
* When vessel is stopped she adopts a position such that the wind is roughly on the beam

6. Effect of Current
Currents Effect all ship’s equally with regard to handling qualities regardless of trim or loading
*Anchored or berthed rudder effective if there is a flow of water past it
*When berthing a vessel should stem the current
* Pivot Points
Imaginary point on the ship’s centerline about which the ship pivots. The pivot point is the point, through which the immediate axis of turn passes and which has no rate of leeway.

Turning Circle:
The path described by a ship’s pivot point as it executes a 360° turn.

Advance
When a ship is turning the distance travelled by centre of gravity along the original course.
Transfer
Distance travelled by COG measured from the original track to the point where the vessel has altered her course by 90 degrees.

Tactical Diameter
It is the transfer for 180 degree.

Stopping Distance
It is the minimum distance that a vessel requires to come to rest over the ground.

Inertia Stop
Distance covered by the vessel when the command is given from Full Ahead to Stop keeping the rudder mid-ship.

Crash Stop
Distance covered by the vessel when the command is given from Full Ahead to Stop keeping the rudder mid-ship.
Distance covered by the vessel when the command is given from Full Ahead to Full Astern keeping the rudder mid-ship.

Head Reach
Distance along the direction of the course that it covers from the moment the full astern commands is given until ahead speed changes sign

Track Reach
Distance along the vessel track that it covers from the moment astern command is given until ahead speed changes sign.

Drift Angle
Angle between ship’s fore and aft line and the tangent to the turning circle.

Manoverboard Manoeuvre :
1. WILLIAMSON TURN
– Rudder hard over to the side of casualty.
– After deviation of 60 degree, rudder hard over to the other side.
– When heading 20 degree short of reciprocal course, put the wheel on midship.
* Advantage – simplicity, good in reduced visibility, accuracy of bringing the vsl on the reciprocal course is maximum.
* Disadvantage – slow procedure, takes ship to great distance from the man where sight may be lost.

2. SINGLE TURN (Anderson Turn)
– Rudder hard over to the side of casualty.
– After deviation by 250 degree, rudder to midship and stop manoeuvering to be initiated.
* Advantage – fastest recovery method, good for ships with light turning characteristics
* Disadvantage – very difficult for single propeller ship, difficult as approach to person is not straight.

3. SCHARNOW TURN
– Rudder hard over to the side of casualty.
– After deviation by 240 degree, rudder hard over to the side of casualty.
– When heading 20 degree short of opposite course, bring the rudder to mid-ship.

SHALLOW WATER EFFECT

1. SQUAT
Squat is the decrease in under-keel water, that is, the difference between her under-keel clearance when making way and when stopped over the water.
*Explanation – Bernoulli’s theorem states that in any moving fluid, the sum of the potential energy, the kinetic energy and the pressure energy is a constant.
As the vessel makes way through the water, she leaves behind a hollow in the water flows aft, its kinetic energy increases.
According to Bernoulli’s theorem, when the kinetic energy of the water increases, it’s pressure energy must reduces.
Since the ship is supported by the pressure energy of the water, as the pressure energy has reduced, the ship sinks to a longer draft.
In addition to the bodily sinkage that occurs, the ship also trims by the head or by the stern.

2. SMELLING THE GROUND
● Occurs when a ship is nearing an extremely shallow depth of water, such as a shoal.
● The ship likely to take a sudden sheer.
● The sheer is first towards the shallow, then violently away from it.
● The movements of a sluggish ship may suddenly become astonishingly lively.
● These effects are called smelling the ground.

3. BOW CUSHION AND BANK SUCTION EFFECT
● Occurs in narrow channels near proximities of banks.
● There is a tendency for the bow of a ship to be pushed away from the bank, called bow cushion.
● The ship moves bodily towards the bank, which appears at the stern, called bank suction.
● Caused by the restricted flow of water on the bank’s side.
● Velocity of water to the bank increases and pressure reduces.
● Results in drop of water level towards the bank.
● As a result, a thrust is set up towards bank.
● A vessel approaching to the bank will have to apply helm to the bank and reduce speed to prevent the sheer from developing.

4. CANAL EFFECT
● Water level drops towards a bank.
● Vessel heels towards bank to displace constant volume.
● Varies as the square of speed.
● Corrective helm to be applied.
Conclusion:
Ship handling is both a science and an art. Science because it requires knowledge of various forces acting on the ship. Art because it requires the skills of an experienced navigator to use these forces in his favour.
We may learn the science part from the various ship handling courses. But the art can only be learned from experience.

Ship handling cannot be learned in few days or few months. Apart from learning the theory of Ship handling, we also need to practice it on board to be master of it.
Ship handling need to be learned right from the time on junior ranks. We need to observe how Masters and pilots handles the ship.