Overview

This chapter will cover the following parts of the Ship Trials:

1. Spiral manoeuvre.

2. Zig-zag manoeuvre.

3. Turning circle diameter (TCD) trials.

4. Crash-stop manoeuvres.

Spiral manoeuvre

This work was first presented to the Institute of Naval Construction in Paris by Prof. J. Dieudonn . He started the manoeuvre by having a steady rudder helm of say 15 to Starboard. This was held until the ship's rate of change of heading became steady. The rudder helm was then reduced to 10 to Starboard, which again was held until the ship's steady rate of heading was reached (see Figure 15.1).

This procedure was repeated, passing through zero, onto 15 to Port, before returning to a rudder helm of 15 to Starboard. These points were then plotted. They produced a hysteresis loop within which directionally unstable conditions apply. The propeller rpm are held constant throughout this spiral manoeuvre.

If rudder helm is moved within O-T to Starboard and O-U to Port, the ship will respond capriciously, changing her heading to either side indiscriminately. This makes course keeping very difficult to achieve. The reason for this is due to hydrodynamic imbalance.

For some ships, the distance U-T in Figure 15.1 is zero. If so, the ship is directionally stable.

Figure 15.1: Dieudonn spiral manoeuvre. Propeller revolutions constant throughout.

Zig-zag manoeuvre

These manoeuvres are conducted to provide a measure of a ship's response to rudder movement. The larger the rudder, the quicker will be the response.

Starting with the ship's heading on a straight course, the rudder...