6.4.1
The PIT technique has successfully
been applied to the experimental data used in the previous sections
for the application of the Three Steps Procedure.
6.4.2
It can be concluded that, for
the ship under analysis, a pure quadratic model for damping, together
with a pure linear model for the restoring term is sufficient, for
practical purposes, to predict the roll peak φ
1r at
the steepness required by the alternative assessment of Weather Criterion.
6.4.3
It is however important to
underline that for ships having significant nonlinear 
curves, it is necessary to introduce a nonlinear correction
in the restoring term in order to account for the bending of the response
curve and the corresponding peak frequency shift. It is in addition
noted, from the experience gained from this exercise, that an additional
test in the range of low forcing frequencies (say ω =
0.75 • ω
0 ) could help in the fitting
of the effective wave slope, allowing to take into account a frequency
dependence of this coefficient. This latter frequency dependence could
be important when the bending of the response curve is significant.
6.4.4
As an additional note, it can
be said that the application of different tentative models in the
PIT allows for an assessment of the likely level of uncertainty inherent
in the extrapolation.
6.4.5
In the case under analysis,
the level of uncertainty is of the order of ± 2°, however
this figure strongly depends on the actual analysed case.
6.4.6
The value of the effective
wave slope obtained through the PIT (about 0.85 on average) is slightly
different from the value obtained through the application of the Three
steps procedure (r = 0.759). This difference can be readily
explained by recalling that, in the Three steps procedure, the damping
is evaluated from the roll decays tests, while the effective wave
slope is evaluated from the roll tests in beam waves, using the previously
obtained damping coefficient. In the PIT approach, on the contrary,
both the damping and the effective wave slope are determined from
the same experimental data in beam waves, for this reason the final
outcomes could differ in terms of single components. The final predictions
of the angle φ
1r given by the PIT technique
and by the Three steps procedure are however very close: the two alternative
procedures can be then considered, for this particular case, as equivalent
from a practical point of view.