SHIPHANDLING (Turn Circle, Shallows)

Q. In your plan to approach a pilot station, why you should not include full astern?
The control over ship is lost during astern propulsion. This is because rudder is not usable. The inclusion of full astern in the plan would mean the approach was done at high speed and it was necessary to put the engines to full astern. A delay or problem in going astern would result in a accident. Thus, the plan has not considered the contingencies. Full astern may be kept in reserve for an extreme emergency.

Q. What are the problems in respect of taking a 3600 turn say during a crossing situation? While taking a 3600 turn the following factors must be considered:

  1. The size of the turn may be much bigger than the one displayed in the wheelhouse poster, due shallow waters.
  2. In situations of tidal stream, it will get elongated in a the down stream direction.
  3. During the 3600 turn orientation is likely to be affected. The ARPA may loose it completely.
  4. The big ships may take 12 to 15 minutes to complete a circle. In this period, new ships may appear in vicinity, which were unexpected or untracked.
  5. Close monitoring of vessel until past and clear is not possible.
  6. Commercial angle and ETA related concern though, of less importance is still valid.

Q. What is turning circle of a ship? What are its various components?
Turning circle of a ship is the path traced out by her CG.
Advance: Advance of a ship is the distance traveled by her CG along the original course.
Transfer: The transfer is the distance traveled by the centre of gravity measured from the original track to the point where the vessel’s course is 90° off the original course.
Tactical diameter: It is the transfer of ship for 180°.
The drift angle: It is angle between the ship’s fore and aft line and the tangent to the turning circle at any point.

Q. What is drift angle? Does it depend on speed?
The drift angle is the angle between the ship’s head and the tangent to the turning circle at any given moment. Thus it is amount by which the heading is turned inwards of a circle at any time.
The drift angle increases:

  • with increase in speed when the helm angle and rudder area are constant;
  • with rudder area and helm angle, speed remaining constant

Q. What is the effect of being in loaded condition on maneuvering characteristics and turning circle?
A deeply laden vessel:

  • has a higher momentum than when in light condition and hence will carries her way longer;
  • causes greater damage under impact than if she were light;
  • is directionally more stable and  is slower to answer her helm than if she was light;
  • is sluggish in gathering way;
  • is less affected by wind; and
  • has a larger turning circle for a given speed than if she were light.

Q. What is the behavior in light condition?
In light condition vessel:

  • the vessel is easily stopped;
  • causes less damage under impact than if she were loaded;
  • will quickly respond to helm;
  • is more lively to engine movements;
  • influence of wind is more due higher windage area than if she was loaded; and
  • has a smaller turning circle for a given speed than if she was at load draft.

Q. What is effect of list on turning circle?
A listed ship will turn more readily towards high side and hence will have a smaller turning circle on that side.

Q. What is the effect of trim?
In case of a vessel trimmed by stern:

  • pivot point shifts further aft than if she were on even keel;
  • turning circle will be larger than if she was on even keel;.
  • effect on power development and steerage may be favourable; and
  • though going into winds is a known fact but it can be said that turning downwind is easier than turning upwind because of an increased distance between centre of windage and pivot point.

Q. What is the effect of shallows?
Water which the ship displaces is not so easily replaced by other water. The rudder is said to work in partial vacuum.
As a result:

  • Vessel takes longer to answer her helm.
  • Response to engine moments is delayed.
  • Vibrations may set up within ship due changed virtual geometry of the hull wrt engine vibrations.
  • It becomes very difficult to correct a yaw or sheer.
  • Steering becomes erratic.
  • Bow wave and stern wave formed due advancing of ship increase in height, thoug there is a relief from ocean waves.
  • Increased drafts can be observed.
  • Drop in speed is considerable. 
  • Mud may be shown up.
  • Roll pitch and heave reduces. 

Q. What is the effect of shallows on heel while turning?
Turn circle is larger in shallow water. Heel while turning is intensified in shallows.

Heel is directly proportional to square of speed and inversely proportional to the radius and the metacentric height. The heel is directly proportional to the lever formed between COB and COG.
Thus, \sin \theta =\dfrac{V^{2}}{gRGM}\times GM
Although, it may mathematically show a decrease in heel dure the change of parameters like,V, R and GM in shallow waters but because of reduced UKC and hence a higher build up of water on port side while turning to starboard, the rudder force now has to overcome a much larger lateral resistance and is therefore considerably less efficient. At the bow, because of the reduced under keel clearance, water which would normally pass under the ship is now restricted and there is a build up of pressure, both ahead of the ship and on the port bow. Due to this restriction on port side offered to water passing under the ship, there is loss of pressure under the ship. Ventuary effect causes sinkage on port side and hence the increased heeling.

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