Fieri and Lee 



the resonant wavelengths should be made, with proper supplemental 

 damping the prediction method can be utilized for parametric study 

 of catamaran hull geometry with respect to seaworthiness characteris- 

 tics. 



II - HULL FORM RESISTANCE AND DESIGN PROCEDURE. 



II . 1 - A General Discussion of Catamaran Hull Resistance. 



A SWATHS is a special category of catamarans. First it will 

 be appropriate to discuss a general catamaran. The resistance pro- 

 blem for a catamaran hull configuration is far more complex than that 

 of a monohull. This complexity arises from two interference effects 

 between the demihulls : a surface wave, which is familiar phenome- 

 non, and the flow curvature, induced by the displacement of each 

 demihull. This displacement interference exists even in the absence 

 of a free surface. 



The pressure distribution over a single hull towed alone is 

 different from that when one hull is towed alongside another hull in 

 that the hydrodynamic property is significantly changed. To predict 

 the combined resistance of two ship models towed side -by-side would 

 be rather difficult, even if we knew all the hydrodynamic properties 

 of each model when towed alone. In the first place, the stagnation 

 points would be altered, and the flow on both sides of each hull would 

 no longer be the same. As a result, a crossflow and a side force would 

 be produced. Such a crossflow would increase eddying, and the side 

 force would have additional resistance similar to the induced drag of 

 a lifting body or lifting surface. 



For these reasons, the surface wave of each model is also 

 altered due to the presence of another hull. Indeed, for a catamaran 

 hull configuration, we are not only confronted with the added comple- 

 xity in wavemaking resistance and viscous resistance but also with a 

 new problem of induced drag. Faced with this situation, it seems very 

 tempting to obtain a catamaran model series for prediction of resis- 

 tance. Such a series, however, would be very expensive to accomplish. 

 Furthermore, its usefulness would be very limited because the inter- 

 ference effect depends not only upon the hull distance but also upon the 

 hull geometry. The interference effects obtained from a catamaran 

 model series could not be applied to a catamaran having a hull geome- 

 try that was different from that of the series. A catamaran series 

 would only be useful if it had good resistance performance and if it 

 were possible to confine our catamaran hull designs within the series 

 Then we would be faced with the same problem in designing a catama- 

 ran hull forms with good resistance performance. This is the subject 



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