Unless stated otherwise, each example assumes a ship on even keel in calm conditions and still water. In this situation no forces are involved and the ship has its centre of gravity, approximately amidships.
Pivot point – stopped
Two forces now come into play. Firstly, the forward momentum of the ship and secondly, longitudinal resistance to the forward momentum, created by the water ahead of the ship. These two forces must ultimately strike a balance and the pivot point moves forward. As a rough guide it can be assumed that at a steady speed the pivot point will be approximately 25% or a ¼ L of the ship’s length from forward.
Pivot point – making headway
The situation is now totally reversed. The momentum of sternway must balance longitudinal resistance, this time created by the water astern of the ship. The pivot point now moves aft and establishes itself approximately 25% or a ¼ L of the ship’s length from the stern.
Although not intended, some publications may give the impression that the pivot point moves right aft with sternway. This is clearly not correct and can sometimes be misleading. It should also be stressed that other factors such as acceleration, shape of hull and speed may all affect the position of the pivot point. The arbitrary figures quoted here, however, are perfectly adequate for a simple and practical working knowledge of the subject.
Pivot point – making sternway
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