4.4.1
From the table of wave steepness
(table 2.1 of the Guidelines) the assumed wave steepness, s,
for this ship is 0.0383 (1/26.1). Roll amplitude of the model was
measured in beam waves with this steepness. Moreover, the waves with s = 1/40 and 1/60 were also used for the alternative procedures
mentioned later. The maximum wave height was 27.8 cm, which is close
to the maximum limit of the wave generator.
4.4.2
The model was freely drifting
for avoiding effects of guide system and guide ropes were fitted to
the model on the centreline at the stem and stern at a vertical height
around the centre of gravity. The periodic yaw motion was small, but
the change of mean heading angle was controlled by the ropes carefully
by human hands to keep it within 15°. The carriage followed the
mean drift motion of the model. When the heading was corrected by
the ropes with large force, the corresponding part of the measured
record was neglected in the analysis. The effect of correction was
negligible in small waves, however in high waves the correction was
not easy in some cases.
4.4.3
The measured roll responses
were subjected to Fourier analysis to extract the components at encounter
frequencies. The results are shown in figure 4.1 for all wave steepnesses.
Due to the nonlinearity of roll damping, the non-dimensional roll
amplitudes are larger in smaller steepness. On the other hand, due
to the linearity of GZ curve (see figure 2.2) the peak frequencies
do not significantly change even in high waves. The roll peak amplitude
at the required steepness, i.e. s = 1/26.1, was φ
1r = 27.6° from the experiments. According to
formula (2.1) of the Guidelines, the "angle of roll to windward due
to wave action", φ
1, is thus determined
as φ
1 = 0.7φ
1r =
19.3°.
Roll amplitude in beam regular waves (left: in degrees, right: non-dimensional)