The turning effect of a vessel will take effect about the ship’s ‘pivot point’ and this position, with the average design vessel, lies at about the ship’s Centre of Gravity, which is generally nearly amidships (assuming the vessel is on even keel in calm water conditions).
As the ship moves forward under engine power, the pivot point will be caused to move forward with the momentum on the vessel. If the water does not exert resistance on the hull the pivot point would assume a position in the bow region.
However,practically the pivot point moves to a position approximately 0.25 of the ships length (L) from the forward position.
Similarly, if the vessel is moved astern, the stern motion would cause the Pivot Point to move aft and adopt a new position approximately 0.25 of the ship’s length from the right aft position.
The pivot point at anchor
It should be noted that when the vessel goes to anchor the pivot point moves right forward and effectively holds the bow in one position. Any forces acting on the hull, such as from wind or currents, would cause the vessel to move about the hawse pipe position.
Use of the rudder can, however, be employed when at anchor, to provide a ‘sheer’ to the vessel, which could be a useful action to angle the length of the vessel away from localized dangers.