3.4 Drifting tests

 The heeling moment M_water due to drift with respect to O is obtained by means of towing tank tests. An example of experimental set-up is shown in figure 3.4.1. Model tests should be carried out in compliance with the following:

• .1 the ship model should be attached to a guidance system, which allows the model’s free sinkage (an example of experimental arrangement is shown in figure 3.4.1);

• .2 towing direction is to be at right angle to the longitudinal direction of the model (heading angle 90°);

• .3 the towing speed should ensure that the measured drift horizontal force is equal to F_wind scaled with the appropriate scaling law. F_wind should be calculated by equation (3.1.4) using the measured drag coefficient in paragraph 3.3 and the assumed wind speed as prescribed in paragraph 2.3; and

• .4 tests should be carried out in upright condition, and at some heeling angles with appropriate increment to lee and wind side covering a sufficient range of heeling angles to the satisfaction of the Administration.

Figure 3.4.1 Example of an arrangement for drifting tests

 As an alternative simplified procedure, the moment M_water due to drift can be considered as given by a force equal and opposite to F_wind (Φ) (as following from paragraph 3.3.2 or 3.3.3) acting at a depth below waterline equal to 0.5 d (where d is the ship draught in upright position).