Practical Ship Hydrodynamics

Ship seakeeping 101
1. No recording of actual seaway during trial; instead measurements of seaway
over many years such that, e.g., the expected maximum values during the
lifetime of the ship can be extrapolated from the recorded distribution of
long-term measured values (long-term measurement). The random variation
of the actual seastate encountered by the ship introduces considerable
inaccuracies for the predicted extreme values even if several years of
measurements are available.
2. Computation of the seaway from the ship motions based on computed or
model-test measured response amplitude operators for the motions. This
allows only a rather rough estimate of the seaway. In following seas this
method is hardly applicable. Nevertheless, averaging over, e.g., 10 to 100
half-hour measurements yields usually good estimates for the correlation
of loads and seaway (medium-term measurement) for example.
3. Parallel measurement of the seaway. Options are:
– Using seastate measuring buoys (brought by the ship).
– Performing the sea trials near a stationary seaway measuring installation.
– Measuring the ship motions (by accelerometers) and the relative motion
between water and ship (by pressure measurements at the hull or water
level measurements using a special radar device); based on these data
indirect determination of the absolute motion of the water surface is
possible.
– Measuring the wave spectrum (energy distribution over frequency and
propagation direction) by evaluating radar signals reflected by the waves.
– Computation or estimation of the seaway from the wind field before and
during the experiments.
– Estimation of significant wave height and period from ‘experienced’
seamen. This common practice is to be rejected as far too inaccurate:
the correlation coefficient between measured (actual) and estimated wave
period is typically