1974). These two and some other methods
are implemented in the strip theory
computer program SEAWAY by Journée
(1992a), used here to obtain the calculated
data. The advantage of the direct
diffraction calculations is that the
amplitudes and phases of the loads are
obtained without any difficulty, because
the in and out phase parts of the loads are
calculated. The disadvantage is that the
calculation speed of the strip theory
program is slowed down, since the
calculations are complex and the
diffraction problem has to be solved for
each wave direction.
In the classic relative motion theory,
sectional averaged orbital motions of the
water particles are calculated from the
pressure distribution in the undisturbed
waves on the section contour. With these
average orbital accelerations and velocities
and the potential mass and damping
coefficients, the in and out phase parts of
the diffraction loads are calculated. The
advantage of using the relative motion
approach is that the amplitudes and phases
of the loads are obtained from the potential
coefficients, which are independent of the
wave direction. This method delivers a
fairly accurate prediction of the 2-D first
order wave loads for heave of a ship
sailing in bow waves. But in beam waves
the method gives in many cases a poor
prediction of the wave loads for sway,
heave and roll, when compared with
results when using the diffraction theory.
With the Haskind-Newman method, see
Newman (1962), the amplitudes of the
overall wave loads on a cross section of a
ship can be obtained from the potential
damping coefficients, without solving the
diffraction problem itself. This relationship
can be derived analytically from the
radiated energy of the cross section.
However, no information concerning the
phases can be obtained.
In this paper, the phases of the wave loads
with respect to the incoming waves are
obtained from the results of the potential
mass and damping calculations. A perfect