Giddings and Wermter 



To avoid the defect that zero-response frequency coincides to the resonant 

 frequency of the tank water, a flared tank was studied. In this case, as shown in 

 Fig. 4, the inertia term changes somewhat and h^ becomes h^ in this case. 

 Since h^ > h^ for normal flare, resonant frequency does not coincide with the 

 zero frequency and becomes nearer to the ship's natural pitching frequency. 

 Therefore the effectiveness of the tank will be improved. 



TANK WITH FLARED WALLS 





Cz) 



^» = \fl7 



^=V-|; 



Figure 4 



However, the amount of the flare will not be chosen arbitrarily. If ducts of 

 certain length i are attached to the openings, the equation of motion will be 

 written as Eq. 3 in Fig. 5. 



In general 



^°=1 (-) 



dx 



is called hydraulic length. The longer and narrower the duct, the longer the 

 hydraulic length and the smaller the resonant frequency. 



Therefore it will be possible to bring the resonant frequency of tank water 

 to equality with the pitching frequency, and to make it quite different from the 

 zero-response frequency. 



A 2m model of a catamaran was provided with fore open tank and tested in 

 waves. The waterplane area of the tank is 5 percent of the total waterplane 

 area. 



806 



