Page 85 - Practical Design Ships and Floating Structures

Preliminary optimization approach of ship hull lines is based on design experience such as the method
of parent ship, which can provide so much information not only in design but also in operation.
Secondary optimization approach of ship hull lines is set on model tests such as the series tests of ship
model, which can directly give the result of optimized lines. In above optimization approaches the
main mathematical tool is the regressive analysis based on the least square method. Those approaches
used to play an important role in ship design before 1960's as so many ships have been built with big
batches which are based on the series model tests, such as well known Series 60, BSRA Series, SSPA
Series and so on.
In recent 20 years one may find two important changes: the dimensions of newly built ocean-going
ships were gradually increasing and the techniques of Computational Fluid Dynamics (CFD) were
developing. The former led to the difficulties of applying the methods based on experiences and the
latter gave a space to employ the theoretical methods to optimize ship hull lines. To compare with
model test CFD is provided with excellence of economic aspects and celerity. And the predicted
accuracy by using CFD in ship hydrodynamic performance depends on the level of CFD code. It
should be pointed out that CFD can not always be substituted for model tests, but CFD can concentrate
model test with the least scale. In the ship designing practice some of CFD codes have been
successhlly applied to alternate design of ship hull lines and the final result may be determined by
Experimental Fluid Dynamics (EFD). The fine optimization of ship hull lines is aimed at traditional
optimization method. The optimized objects may not only include dimensions and naked lines of hull
but also include local lines and hulls with appendages.
In the present paper an optimization method of ship hull line design for improving resistance
performance is presented computational example exhibits favorable results. Some of criteria used to
distinguish the ship hull lines are discussed.

2 CLASSIFIED OPTIMIZATION
In the different design stages the optimization approach with different levels can be used for efficiently
designing the ship hull lines.

2.1 Optimization of Hull Dimensions (Level 0)
So many regressive results can be applied to mathematical models when optimization of ship hull
dimensions is made. Such as BSRA Series (Wang and Huang, 1993), Series 60 (Wang, 1980), Series of
Fishing Boats (Wang and Huang, 1977), ships with homogeneous hull and high speed (Wang and Xu,
1996) and so on are frequently used at the preliminary design stage. The objective function is the total
resistance (coefficient) or the residual resistance (coefficient) and the optimized objects include
principal dimensions and coefficients of hull form.
2.2 Optimization of Naked Hull Lines (Level I)

A calculation of thick dimensional boundary layer on ship hull with special velocity profiles and eddy
viscosities (Wang and Long, 1989; Wang and Wan, 1989 and Wang and Long, 1992) and with the
integral numerical method is employed to determine the viscous resistance. In addition, the Michell
integral is used to predict the wave-making resistance. The objective functions are the viscous
resistance (coefficient) and the wave-making resistance (coefficient) and the optimized object is naked
hull lines.