A ship handler must very clearly understand and remember the following in respect of his ship:
1. The details of turning circle
2. Any limitation of the vessel, or main engine in respect of maneuvering.
3. Stopping distance, Crash stop data, etc.
A turning circle maneuver is to be performed to both starboard and port with 35° rudder angle or the maximum design rudder angle permissible at the test speed. The rudder angle is executed following a steady approach with zero yaw rate. The essential information to be obtained from this maneuver is tactical diameter, advance and transfer.
The turning circle
1. Turn Circle: When a vessel alters her course under helm through 360° degrees, she moves on a roughly circular path called a turning circle. The circle will be a path traced out by the pivot point. Some refer to it as the path traced out by the centre of gravity.
2. Advance: The advance is the distance traveled by the centre of gravity along the original course.
3. Transfer: The transfer is the distance traveled by the centre of gravity from the original track measured in direction 90° to original heading.
4. Tactical diameter: It is the transfer for 180°
5. The drift angle: It is the angle between the ship’s fore and aft line and the tangent to the turning circle at any instant. Drift angle gives the correct idea of inward inclination of ship wrt. tangent.
6. Pivot Point: It is the point about which the vessel pivots i.e. the bow swings inwards & stern swings outwards. It is approximately one third of vessel’s length from forward when going ahead. While going astern vessel pivots about a point approximately one quarter of length from astern.
Turning Moment in Turn Circle:
With the ship stopped in the water and the rudder hard to starboard, if an ahead movement is applied, the movement is initially resisted because of the inertia. This results in a pivot point well forward. The turning moment due rudder is maximum. When a steady speed is reached whilst turning, the pivot is at 1/3rd L from the bow.
The relationship and balance between the rudder force and the lateral resistance decides the characteristic of turning circle. 1st 90 degree turn for a freighter will take 2 to 3 minutes. During initial 20 degree, the reduction of speed is not very great. Speed reduction is by 25% in the 1st 90 degree and by 33% in later part. The time taken to turn through a given angle depends on the initial speed and the angle of rudder applied. Usually the faster the speed and the greater the rudder angle the sooner will the turn be completed.
The general principles of ship handling are supportive of various theories, which explain the maneuvering pattern or behavior of a ship. A ship handler however, must establish a relation and comparison of learnt principles with actual behavior of ship. The size / time of turning circle may be affected by the following as compared to the data that is provided:
1. Trim by head or by aft.
2. Speed full or slow.
3. Loaded or ballast.
4. Presence of tidal streams.
5. Effect of smelling the ground or bank suction.
6. Interaction with ship in vicinity.
7. Shallow water effect.
The size of turning circle is the function of drift angle. The value of the drift angle varies considerably in different vessels and in the same vessel under different conditions of speed and helm angle. Following is the data for a certain vessel:
The drift angle increases:
a) With increase in speed for a given ship.
b) With rudder area for a given speed and helm angle.
c) With helm angle for a given ship and speed
Effect of different variables on turning circle:
- The draft or load
The momentum of the ship depends upon the mass of the ship and the speed of the ship. Thus a lighter ship will gain or lose speed, faster than a deeply loaded ship. The largest tanker Knock Nevis covers 5.5 nautical miles before stopping.
The shape of the underwater part of the hull also plays important role. Two tankers of the same displacement, but one with finer lines than the other would be able to travel further after the engines are stopped. She will even start and reach the designed speed faster.
Another factor is the condition of the ship’s bottom and the underwater part of the hull. If the undersides are fouled with marine growth, then there would be a drag and the effect on the startup would not be that affected but the travel distance after the engines are stopped would be much reduced. If the under keel clearance is low then the effect is both ways, i.e the ship will take longer to reach her designed speed from stop. She will travel longer when the engines are stopped. For a given speed turning circle is larger when laden than if she was light. When deeply laden a cargo ship has a much larger turning circle than when lightly laden, and she is more sluggish in answering her rudder.
The effect of speed on tactical diameter will vary from one type of ship to another. Often higher speed may lead to a greater tactical diameter. Modern rudders, on smaller ships, however, are able to operate satisfactorily at higher water speeds and greater angles, and hence the tactical diameter may not vary much with speed. On some ships there is an ‘optimum best speed’ giving the minimum tactical diameter. At higher or lower speeds the tactical diameter is greater. Watch-keeping officers should be fully aware of the effect of speed on the turning qualities of ship.
3. Shallow water
The turning circle is larger and loss of speed during turn is less in shallow waters. This is because rudder works through partial vacuum as water is not easily replaced. Firstly, the rudder force now has to overcome a much larger lateral resistance and is therefore considerably less efficient. Secondly, at the bow, because of the reduced under keel clearance, water which should normally pass under the ship is now restricted and there is a buildup of pressure, ahead of the ship. Due to the increased longitudinal resistance the pivot point is pushed backwards. With the reduced rudder thrust and reduced turning lever, the ship rapidly loses the rudder efficiency as compared to deep water.
The increased size of the stern wave is a sure indication of the presence of shallow water. The energy expended in the waves formed by the ship is a loss from the power available to drive her, and therefore in shallow water her speed is reduced. Furthermore, the restricted flow of water past the stern reduces propeller efficiency, which also tends to reduce her speed. Usually, the reduction of speed is proportional to the speed itself. In shallow water, the diameter of a turning circle can become double the original size and can even become larger than double. Though, the ship’s speed with which a turning circle is made, does not have much effect on the diameter of the turning circle.
4. Helm Angle
Other things remaining same, the turning effect of a rudder increases with an increase in the helm angle up to 40° or so.
5. Hull form
A ship of fine underwater form (container ship) will turn in a larger circle than a ship of similar length and draught but of fuller form (tanker). Modern container ships are generally of great length in proportion to beam and thus tend to have large turning circles.
The effect of a list is to hinder a turn in the direction of the list and assist a turn away from it. A list to port decreases the tactical diameter of a ship turning to starboard and vice versa.
A vessel trimmed by stern will have pivot point further aft than if she were on even keel. Resulting in a larger turning circle. Turning circle would be smaller for a vessel down by head. Thus, trim by the stern usually increases the tactical diameter, but helps a ship to keep her course more easily when on a steady course. When trimmed by the head, her turning circle is likely to be decreased; she does not answer her wheel as readily as usual, and once she has started to swing it is more difficult to check her. The effect of trimming is to move the ship’s pivoting point towards the deeper end.
8. Effect of Tidal Stream, Current in Harbour, River etc.
The Master, in addition to knowing his ship’s turning circle well, must be well-versed with the local currents, especially when turning close to a ship that is tied up or anchored or when turning in the vicinity of fixed objects.
The Heel During Turn:
The vessel heels as she turns. The heel while turning is independent of the displacement and would be initiated by the centrifugal acceleration which depends on the speed of ship and radius of turn. The amount of heel is directly influenced by heel lever, which is the vertical distance between the CG and the centre of lateral resistance, which is at the level of centre of buoyancy.
The angle of heel varies:
(1) Directly as the square of the speed of ship;
(2) Inversely with the metacentric height;
(3) Inversely with the radius of the circle.
Small values of the meta-centric height will give higher angle of heel. If the speed is doubled, the angle of heel will be approximately quadrupled, the radius of the circle and the metacentric height remaining constant.
The initial heel when the helm is given, is inwards, because the rudder force is acting at a point below the centre of gravity of the ship. As the ship begins to turn, the centripetal force on the hull overcomes the tendency to heel inwards and causes her to heel outwards. Normally, centre of buoyancy is also the centre of lateral resistance while turning. Position of this being lower than the centre of gravity, a vessel will heel to outer side (radially outwards) during a turn. This outward heel is very noticeable when turning at good speed. If an alarming heel develops, speed should be reduced instantly. An approximate formula to find the heel is:
θ = angle of heel,
V = speed of ship in metres/sec,
R = radius of circle in metre,
GM = metacentric height,
d = draft KG = KG of ship
A diagrammatic comparison of the ships of varying lengths, sizes and types are shown below. Where MV Coneble; a 135m container vessel, has a tactical diameter of 329m in ballast condition with the turning circle completing in 3.9min; the longest ship, Knock Nevis the supertanker, length 458.5 m and 68.9 meters wide, displacement 564,763 dwt, takes 5.5 miles to stop with a turning circle of over 2 miles.
A large passenger cruise ship of length 220 m b 31 m, draft 7.5 m has a full sea speed of approximately 21.7 kn and a maneuvering full ahead speed of 16.7 kn. The vessel’s sea trial data indicates that when the rudders are placed hard over to starboard while traveling at 20.24 kn, the advance distance is 426 m, the tactical diameter is 292 m.
(You may also visit my youtube videos @captsschaudhari.com)