Practical Ship Hydrodynamics

4 Practical Ship Hydrodynamics
necessary thanks to CFD developments. Combining CAD (computer-aided
design) to generate new hull shapes in concert with CFD to analyse these
hull shapes allows for rapid design explorations without model testing. CFD
allows the preselection of the most promising design. Then often only one or
two models are actually tested to validate the intended performance features in
the design and to get a power prediction accepted in practice as highly accurate.
As a consequence of this practice, model tests for shipyard customers have
declined considerably since the 1980s. This was partially compensated by more
sophisticated and detailed tests funded from research projects to validate and
calibrate CFD methods.
One of the biggest problems for predicting ship seakeeping is determining
the nature of the sea: how to predict and model it, for both experimental
and computational analyses. Many long-term predictions of the sea require a
Fourier decomposition of the sea and ship responses with an inherent assumption
that the sea and the responses are ‘moderately small’, while the physics
of many seakeeping problems is highly non-linear. Nevertheless, seakeeping
predictions are often considered to be less important or covered by empirical
safety factors where losses of ships are shrugged off as ‘acts of God’, until
they occur so often or involve such spectacular losses of life that safety factors
and other regulations are adjusted to a stricter level. Seakeeping is largely not
understood by shipowners and global ‘sea margins’ of, e.g., 15% to finely
tuned (š1%) power predictions irrespective of the individual design are not
uncommon.
1.2 Model tests – similarity laws
Since the purely numerical treatment of ship hydrodynamics has not yet
reached a completely satisfactory stage, model tests are still essential in the
design process and for validation purposes. The model tests must be performed
such that model and full-scale ships exhibit similar behaviour, i.e. the results
for the model can be transferred to full scale by a proportionality factor. We
indicate in the following the full-scale ship by the index s and the model by
the index m.
We distinguish between
ž geometrical similarity
ž kinematical similarity
ž dynamical similarity
Geometrical similarity means that the ratio of a full-scale ‘length’ (length,
width, draft etc.) Ls to a model-scale ‘length’ Lm is constant, namely the
model scale :
Ls D Ð Lm
Correspondingly we get for areas and volumes: As D 2 Ð Am; rs D 3 Ðrm.
In essence, the model then ‘appears’ to be the same as the full-scale ship.
While this is essential for movie makers, it is not mandatory for naval architects
who want to predict the hydrodynamic performance of a full-scale ship. In fact,