Giddings and Wermter 



Model tests of anti- rolling fins, either alone or on ship models, have not 

 been reported in the open literature. As more and more model tanks develop 

 the ability to generate random model seas, perhaps greater use of ship model 

 testing to prove out design concepts will result. Factors such as the interaction 

 of bilge keels and stabilizer fins, yaw-heel, and the interaction of a passive tank 

 stabilizer with active fin stabilizers could be examined on model scale. Even 

 without wavemakers, model tests using rotating eccentric weights or other roll 

 moment devices can be of use in examining the hydrodynamics of stabilizers. 



Stabilizing Tanks 



Anti-rolling tanks have had a checkered history. Since Froude's first spec- 

 ulations [29] a great variety of tank installations have been tried, with different 

 degrees of success. Until recent times, the most successful of these were 

 Frahm tanks [30] either cross-connected within the ship, or with port and star- 

 board tanks open to the sea. More recently, passive tanks with free- surfaced 

 cross connections have been successful [31]. Active tank stabilizers have not 

 had a successful past, but the future looks brighter. 



A series of reports by Chadwick [20,32] analyze the dynamics of both active 

 and passive anti-roll tanks. This analysis for passive tanks was extended 

 slightly in Ref. 31. Blagoveschensky [33] presents a simplified analysis for 

 passive tanks open to the sea. Hydronautics, Inc. under the sponsorship of the 

 Bureau of Ships is currently conducting a theoretical study of active anti-roll 

 tanks. This study will once again reanalyze the equations of roll motion as pre- 

 sented by Chadwick to insure that all significant nonlinear terms are properly 

 included. Pumping rate specifications and tank design criteria will be estab- 

 lished and it is hoped that sufficient information will be generated to permit a 

 successful design. 



The recent success of the free surface type passive tanks compared to the 

 narrow acceptance of Frahm tanks is due to several factors. The high internal 

 damping due to wavemaking in free surface tanks makes precision of design less 

 demanding than for Frahm tanks. The tank response curves are flatter, and 

 highly nonlinear in a fashion kind to the designer. The recent trend for ship de- 

 sign to be controlled more by volume than weight has also made it easier for 

 the owner to accept the weight of tank stabilizers. 



Application of theory to describe the action of Frahm tanks was shown to be 

 fairly successful (Chadwick [20]) in that assumption of linear damping within the 

 tank gave fair approximations to the model test results. Little agreement has 

 been found for free surface tanks. The theory developed in Ref. 31 included a 

 provision for equivalent nonlinear tank damping evident from model test results. 

 In addition, the U-tube analogy for computation of the natural frequency of free 

 surface tanks has been shown to be somewhat inaccurate. Reference 58 presents 

 some corrections, based upon basic shallow water wave theory compared with 

 experimental results. 



An additional comparison is presented here. As derived in Refs. 30 and 31, 

 the natural frequency of oscillation of the fluid in a U-tube can be found from 



752 



