Page 127 - Introduction to Naval Architecture

114 SEAKEEP1NG

Because the irregular surface does not repeat itself, or only over a very
long period, a number of test runs are needed to give statistical
accuracy. The number of runs, however, is less than for testing in
regular waves. A third type of model test uses the transient wave
approach. The wavemaker is programmed to generate a sequence of
wave lengths which merge at a certain point along the length of the
tank to provide the wave profile intended. The model is started so as to
meet the wave train at the chosen point at the correct time. The model
then experiences the correct wave spectrum and the resulting motion
can be analysed to give the response operators. This method can be
regarded as a special case of the testing in irregular waves. Whilst in
theory one run would be adequate several runs are usually made to
check repeatability.
The model can be viewed as an analogue computer in which the
functions are determined by the physical characteristics of the model
To give an accurate reproduction of the ship's motion the model must
be ballasted to give the correct displacement, draughts and moments of
inertia. It must be run at the correct representative speed. To do all this
in a relatively small model is difficult particularly when it has to be self-
propelled and carry all the recording equipment. The model cannot be
made too large otherwise a long enough run is not achievable in the
confines of the tank, Telemetering of data ashore can help. Another
approach has been to use a large model in the open sea in an area
where reasonably representative conditions pertain.
Wetness and slamming depend upon the actual time history of wave
height in relation to the ship. Direct model study of such phenomena
can only be made by running the model in a representative wave train
over a longish period. However, tests in regular waves can assist in
slamming investigations by enabling two designs to be compared or by
providing a check on theoretical analyses.
Then there are full scale ship trials. Some full scale data has been
obtained for correlation with theory and model results. Direct
correlation is difficult because of the need to find sea conditions
approximating a long crested sea state during the trial period when the
ship is rigged with all the measuring gear. A lot of useful statistical data,
however, on the long term performance can be obtained from
statistical recorders of motions and strains during the normal service
routine. Such recorders are now fitted in many warships and merchant
ships.

Deducing criteria
It is not always easy to establish exactly what are limiting criteria for
various shipboard operations. They will depend to some extent upon
the ability of the people involved. Thus an experienced helicopter pilot