Page 126 - Introduction to Naval Architecture

SEAKEEPING 113

Acquiring seakeeping data

Computations of performance criteria require good data input,
including that for waves, response operators and limitations experi-
enced in ship operations.

Wave data
The sources of wave data were discussed in Chapter 5. The designer
must select that data which is applicable to the design under review.
The data can then be aggregated depending upon where in the world
the ship is to operate and in which seasons of the year.

Response amplitude operators
The designer can call upon theory, model testing and full scale trials.
Fortunately modern ship motion dieories can give good values of
responses for most motions. The most difficult are the prediction of
large angle rolling, due to the important non-linear damping which
acts, and motions in quartering seas. The equations of motion can be
written down fairly easily but the problem is in evaluating the various
coefficients in the equations. Most modern approaches are based on a
method known as strip theory or slender body theory. The basic assumptions
are those of a slender body, linear motion, a rigid and wall-sided hull,
negligible viscous effects apart from roll damping and that the
presence of the hull has no effect upon the waves. The hull is
considered as composed of a number of thin transverse slices or strips.
The flow about each element is assumed to be two-dimensional and the
same as would apply if the body were an infinitely long oscillating
cylinder of that cross section. In spite of what might appear fairly gross
simplification, the theory gives good results in pitch and heave and with
adjustment is giving improved predictions of roll. The same principles
apply to calculating vibration frequencies as discussed in Chapter 11.
To validate new theories or where theory is judged to be not accurate
enough, and for ships of unconventional form, model tests are still
required.
For many years long narrow ship tanks were used to measure motions
in head and following regular waves. Subsequently the wavemakers
were modified to create long crested irregular waves. In the 1950s, as
the analytical tools improved, a number of special seakeeping basins
were built. In these free models could be manoeuvred in short and long
crested wave systems. For motions, the response operators can be
measured directly by tests in regular seas but this involves running a
large number of tests at different speeds in various wavelengths. Using
irregular waves the irregular motions can be analysed to give the
regular components to be compared with the component waves.