A Survey of Ship Motion Stabilization 



DISCUSSION 



E. Numata 



Stevens Institute of Technology 



Hoboken, New Jersey 



Davidson Laboratory is pleased to have been associated with the USN Bu- 

 reau of Ships and DTMB in experimental model research on anti-pitching fins 

 and passive anti-rolling tanks since 1956. One of the earliest, although subsid- 

 iary, investigations conducted at DL concerned the magnitude of the influence of 

 fixed bow anti-pitching fins on the longitudinal midship bending moment of the 

 COMPASS ISLAND. It was found that the fins had no adverse effect on hull bend- 

 ing moment. 



In connection with stern anti-pitching fins, an analytical study for DTMB at 

 Davidson Laboratory showed that in head seas at wave and ship speed conditions 

 bracketing synchronous pitching motion, the hydrodynamic angle of attack of 

 fixed stern fins is very small. Thus stern fins must be activated to be effective, 

 producing a stabilizing moment and decrease in pitch angle which are propor- 

 tional primarily to their amplitude of oscillation and relatively independent of 

 the pitching amplitude. We found this to be true also in the case of oscillating 

 fins astern of a pump jet propeller on a destroyer escort model tested for East- 

 ern Research Group several years ago. This characteristic of a fixed number 

 of degrees reduction may explain why in the author's Table 7 the percentage 

 pitch reduction decreases as sea state and pitch angle increase. 



In connection with full scale evaluation trials of passive anti-rolling tanks, 

 it seems to me that instead of vainly hoping for ideal wave conditions of unvary- 

 ing severity and direction, it might be better to conduct trials in the calm seas 

 one usually finds when searching for rough water. Rolling excitation could be 

 provided by some form of portable oscillating weight device. Since most naval 

 and oceanographic vessels fitted with passive tanks are of modest size with 

 reasonable metacentric heights, it should not be too great an engineering prob- 

 lem to design and assemble a device whose oscillation frequency can be varied 

 while providing sufficient roll exciting moment to give a static heel of about 2°. 

 Thus a frequency response could be obtained for the ship with and without the 

 passive tanks operating. The omission of sway excitation would be a necessary 

 but not totally undesirable condition. 



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