Practical Ship Hydrodynamics

3.4 Simple design approaches
Resistance and propulsion 83
In early design stages, the power requirements have to be estimated to judge
the weight and volume requirements of the main engine and fuel. As this has
to be done repeatedly in design loops, model tests are no suitable solution
for reasons of time and costs. Instead, simple, largely empirical methods are
employed which only require a few global design parameters. These methods
are discussed in more detail by Schneekluth and Bertram (1998).
The main approaches are:
ž estimate from parent ship, e.g. by admiralty or similar formulae
ž systematical series, e.g. Taylor–Gertler, Series-60, SSPA
ž regression analysis of many ships, e.g. Lap–Keller, Holtrop–Mennen,
Hollenbach
The estimate from a parent ship may give good estimates if the parent ship is
close enough (in geometrical properties and speed parameters) to the design
ship. The admiralty formula is very coarse and not recommended, but an
estimate based on a form factor approach is popular in practice. Here, it is
usually assumed that the parameter cw/F4 n and the form factor remain constant
in the conversion from parent ship to design ship. Such a more or less sophisticated
plus/minus conversion from a parent ship is currently the preferred
choice for a quick estimate.
All of the systematical series and most of the regression analysis approaches
are outdated. They often underestimate the actual resistance of modern ship
hulls. It may come as a surprise that older ships were apparently better in
terms of resistance. There are several explanations:
ž suitability for container stowage plays a larger role in modern ships;
ž modern ships often have a higher propulsive efficiency compensating
partially for the higher resistance;
ž more severe safety regulations, e.g. concerning stability, pose additional
constraints on the hydrodynamic optimization.
Nevertheless, some of the old estimation methods are still popular as they are
easy to program. Thus they are embedded in naval architectural CAD systems
or more recently in design expert systems. However, they are fundamentally
limited to global predictions, as they represent the hull shape by few global
parameters.
3.5 CFD approaches for steady flow
3.5.1 Wave resistance computations
The wave resistance problem considers the steady motion of a ship in initially
smooth water assuming an ideal fluid, i.e. especially neglecting all viscous
effects. The ship will create waves at the freely deformable water surface.
The computations involve far more information than the mere resistance
which is of minor importance in many applications and usually computed
quite inaccurately. But the expression ‘wave resistance problem’ is easier than
‘steady, inviscid straight-ahead course problem’, and thus more popular.