4.5 Tests in regular waves

Tests in regular waves are the standard procedure for determining the “regular waves roll-back angle” Φ _1r . In some cases the direct determination of Φ _1r is not feasible, as, for example, in case of large models having long natural roll period TΦ. In such cases alternative procedures can be used as reported in paragraph 4.6.

4.5.1 Test conditions

The wave steepness (factor ”s”) should be selected from table 4.5.1.

Table 4.5.1 Wave steepness as a function of the full scale natural roll period

Ship roll period T _Φ [s]	Wave steepness s = H / λ
<6	0.100
6	0.100
7	0.098
8	0.093
12	0.065
14	0.053
16	0.044
18	0.038
20	0.032
22	0.028
24	0.025
26	0.023
28	0.021
30	0.020
>30	0.020

4.5.2 Direct measurement procedure

4.5.2.1 Tests in regular waves can be used to directly obtain the “regular waves roll-back angle” Φ _1r • Φ _1r is the peak roll response of the ship in regular waves of steepness according to table 4.5.1. In order to determine Φ _1r , the stationary roll motion amplitude should be measured for a sufficient number of frequencies around the natural roll frequency ω ₀ = 2π/T _Φ . The following minimum set of test points is recommended ω/ω ₀ = 0.8, 0.9, 0.95, 0.975, 1.0, 1.05, 1.2, with ω being the frequency of the regular wave in rad/s. Additional measurements in the proximity of the response peak might be necessary to allow for an accurate determination of Φ _1r especially in case of strong influence of righting lever non-linearity.

4.5.2.2 During the tests the ship model should be positioned to be normal to the direction of the waves (90° heading angle). The heading angle of the model is either:

.1 fixed, with a guide attached to the towing carriage keeping the sway-heave-roll motion free from restraints. An example of experimental arrangement is shown in figure 4.5.2.2. The carriage should trace the drift motion of the model induced by the beam wave’s action. Draught, GM and TΦ should be adjusted taking into account the effect of the guide; or
.2 controlled by guide ropes which are fitted to the model on the centreline at the stem and stern, in a symmetrical fashion and at a vertical height between the waterline and the centre of gravity. These lines can be used to correct the model in yaw while allowing drift and sway, provided the heading during tests does not deviate from beam sea for more than 15°. However, whenever the yaw motion is corrected by means of the ropes, the corresponding part of the measured record should be neglected in the subsequent analysis, unless the effect of correction on the quantities of interest is clearly negligible.

Figure 4.5.2.2 An example of the guide for roll test in beam waves

4.5.2.3 During the tests, care should be taken to use appropriate time windows for the measurements, so that the steady roll amplitude is measured without the influence of reflected waves between the model and the wave maker or the model and the beach.

4.5.2.4 Data to be recorded are model motions in all measured degree of freedom (DOF) and wave elevation.