Practical Ship Hydrodynamics

20 Practical Ship Hydrodynamics
full-scale will become available overcoming current uncertainties in correlating
model tests to full-scale predictions. For some projects, it is only
important to ensure that a given installed power will enable the ship to
achieve contract speed. In these cases, CFD is of little interest. However,
CFD should be considered in cases where model test results show problems
or the shipowner is willing to pay a higher price for lower operating costs
(due to improved hull). CFD allows insight in flow details not offered by the
standard model tests. Insight in flow details is especially important in cases
where initial model tests show that power requirements for a given hull are
far more than expected. Here CFD also allows the investigation of the flow
far below the waterline and modifications can be quickly analysed to see if
major improvements are to be expected. The model tests and experience of
a towing tank mainly indicate the potential for improvement; CFD indicates
where and how to improve the design.
ž Cost benefits (How does CFD reduce costs in ship designs?)
While the influence of certain decisions and actions on the turnover can be
estimated only qualitatively, costs can usually be quantified directly. This
explains why management prefers investments with a short payback due to
cost reductions even though there is general consent that cost reductions
alone do not ensure the economic future of a company. However, CFD’s
potential for direct cost reductions is small. CFD is still not accurate enough
to substitute the model test for power prognosis. Therefore, one modeltestis
always performed. In three out of four projects of the Hamburg Ship Model
Basin this was sufficient. It reduces the cost saving potential to the additional
loops in the towing tank which still account for one-third of all tests. In
extreme cases up to 15 additional loops were necessary to achieve the final
hull design. In these cases, CFD could have saved considerable costs. The
average one additional loop will cost about as much as a CFD computation.
Indirect cost savings in other departments are difficult to quantify. Time
benefits of CFD will also affect costs. It is possible to determine 40% to
60% of the total production costs of a ship in the first weeks of design.
Costs for modifications in later stages are higher by order of magnitudes
than those necessary at the conceptual phase. Various decisions concerning
production costs can be made earlier and involve lower risks if CFD is
employed consistently to determine the final hull form at an earlier time.
The benefits discussed so far only cover one-half of a cost-benefit analysis for a
CFD strategy. Understanding the cost structure of CFD is at least as important
and some general management guidelines can be deduced. This requires a
closer look at the work process in CFD. The work process is split into:
ž preprocessing (generation and quality control of grids)
ž computation
ž postprocessing (graphical displays, documentation)
The individual steps sometimes have to be performed several times in iterations.
Cost structures will be discussed separately for each step:
1. Preprocessing
Preprocessing requires staff familiar with the special programs for
grid generation, especially on the hull. This requires at least a basic
understanding of the subsequent CFD computation. Grid generation is best