Reverse Spiral Manoeuvre

The reverse spiral manoeuvre is usually used to check the ship's directional stability. The image shows the reverse spiral curve of a directionally stable ship and a marginally stable ship:

The following image shows the reverse spiral curve of a directionally unstable ship:

The degree of instability is usually judged by the slope of the curve at the origin, positive slope means directionally stable ship and negative slope means a directionally unstable ship. The loop width and the loop height give a good presentation of the directional instability of the ship.

The following differences arise between the two ship types:

Merchant

Shallow or deep water.

Conventional rudder type only.

Naval

Dep water only.

Choice of conventional, Becker or Schilling rudders.

For small rudder angles the calculations are done using an autopilot.

The calculations are carried out for port and starboard sides. For each direction, the turning manoeuvre is carried out for rudder angles from 15 to 35 degrees with intervals of 5 degrees.