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sometimes high seastates (see, [1] to [5] for examples). It is of particular interest to note, that depending on the specified service speed and payload capacity all designs mentioned above, except for the patrol vessel [4], have been developed assuming steel as main structural material, therefore avoiding the particular problems related to an aluminum alloy or synthetic material structure. The increased structural weight of a steel catamaran introduces additional requirements into the present hydrodynamic optimization problem.

The demihull form of the addressed twin hull vessels can be assumed, by common design sense, to be slender, thus changing slowly in the longitudinal direction, but else being of arbitrary shape. Thus we should consider herein symmetric or non-symmetric demihull sections, but in general arbitrarily shaped thin or slender hull forms, varying else arbitrarily in both the transverse and in the vertical direction. Therefore we address practically all thinkable displacement CATAMARAN hull forms, including SWATHs1 and Hybrids2.

The present paper is focusing on the calm water performance of Fast Displacement Catamarans and their hull form optimization with respect to least horsepower requirement, assuming the desired vessel's speed and displacement3 known and considering various geometric parameters set by design or by other operational constraints. The employed optimization procedure consists of two basic stages, namely, in the first phase a global procedure leading to the main dimensions and integral form and weight characteristics of the ship, whereas in the second phase a local form optimization is performed leading to the exact geometric characteristics and the final hull form of the vessel under consideration. The overall goal of the above optimization process is to generate, with the least possible computational and experimental effort, seakind catamaran hull forms with low weight and resistance characteristics. From the naval architectural point of view these requirements are contradictory, because a seakind catamaran requires moderate stiffness (low to moderate metacentric height), thus small separation distance between the demihulls, what has the additional positive effect of low structural weight, whereas the low resistance (and especially, for fast catamarans, low wave resistance) requirement, that indirectly calls for reduced machinery and fuel weight, suggests a large separation distance for the demihulls, for avoiding the negative interference effects on resistance, that can easily triple the single demihull's resistance [6]. Therefore, a formal optimization procedure for fast catamarans should be looking for twin hull arrangements exhibiting the least possible separation distance for the demihulls, thus disposing reduced structural weight and moderate stiffness in roll direction, without compromising on the increase of the sum total of the demihulls' single resistance. Instead of, it is expected that through proper shaping of demihulls the interference resistance can be tuned to be, for the speed range of interest, small and even below zero. Because of the multiple parameters involved, it seems very difficult, if not impossible, considering reasonable effort, to address the problem of hydrodynamic optimization of fast displacement catamarans only by systematic model experiments. Therefore, a computer-aided hull form design procedure, as presented herein, seems essential for the concept and initial hull form development, that can be later on verified by a limited number of model tests.

The following paper is intended to explain the details of the above outlined theoretical/numerical methods, especially the essence of our thin, non-symmetric twin-hull wave resistance theory as well as our 3D panel source wave resistance method, including the related algorithms and the validation by model experiments. It should further show the impact of theoretical/numerical hydrodynamic methods on catamaran hull form design within a formal optimization procedure and finally to draw conclusions as to the methodology for the design of optimal Fast Displacement CATAMARAN hull forms.

The rest of the present paper is organized as following. Chapter 2 addresses briefly the overall design methodology and optimization procedure. Chapter 3 describes the essence of the employed hydrodynamic module within the formulated optimization procedure. Chapter 4 includes the presentation and discussion of theoretical results for

1  

SWATH: Small Waterplane Area Twin Hull is synonym to SSC: Semi-Submerged Catamaran (MITSUI's Co. brand-name)

2  

Hybrids: herein understood as a mixture of a conventional displacement catamaran hull form with a SWATH: Medium Waterplane Area Twin Hulls (MWATHs), Fast Displacement Catamarans (FDCs, FBM Marine Ltd brand-name), etc.

3  

It is more correct to assume, instead of the displacement, the payload capacity given by the owner's requirements. However, an optimization with respect to the least horsepower requirement, as suggested herein, assuming the displacement and the speed of operation fixed, leads eventually for fast displacement catamarans to ship designs with maximum payload capacity.



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