What is a turning circle?
A turning circle is the path of the ship when the rudder is put hard over and held. Tactical diameter is the distance travelled to the side of the original course when the heading has changed 180°. Advance is the distance travelled along the original course. These figures are recorded in the wheelhouse poster and the pilot card.
How this estimator works
Tactical diameter ≈ Turning factor × Ship length. Many merchant ships turn in about 3 to 5 lengths in deep water. Shallow water, low speed, a fouled hull or a strong current can make the circle much larger. If speed is entered, time for 180° is estimated from half the circumference at that speed. This is only a planning figure. Use the ship’s official manoeuvring data whenever it is available.
Who should use it
Useful for officers preparing a pilot card or a manoeuvre plan when the poster is not at hand. Never treat this as a substitute for the official turning circle diagram.