The determination of accurate results when making turning trials is difficult; at least it appears to be difficult to obtain identical results from sister ships or from the same ship when tried twice under apparently identical conditions. It is commonly assumed that after beginning to turn, the track of a ship for the first 180° begins as a spiral and becomes a circle, and that if no change of helm angle or engine conditions is made, the track is circular by the time 180° have been traversed, the diameter of this circular track being called the final diameter and being, of course, somewhat smaller than the tactical diameter corresponding to the first 180° of turn.
There is a very simple method of determining the final diameter of a ship when turning in a true circle, requiring no observations of a distant object or other observations of heading, and with all observations made from the ship herself. The underlying assumptions are two in number, as follows:
1. That a floating pole or buoy about which the ship turns, remains in the same place while the ship turns.
2. That the ship after a time does settle down to turn in a true circle.