pitching motions. The experimental data of the light load tests were not analyzed to the 

 extent required for careful conclusions. 



An important effect of fitting antipitching fins was the elimination of slamming in the 

 conditions tested. Slamming of the ship without fins in the heavier load condition occurred 

 in wave lengths near ship length, and at speeds between 10 and 20 knots. The effect of the 

 fins was to maintain a minimum forefoot submergence of about 9 feet, in waves up to about 

 1.2 times the length of the ship. However, fin skimming and emergence did occur at the 

 longer wave lengths. Skimming of the fins apparently caused no visible violence. Emergence, 

 when referred to the uninterrupted wave surface, was limited to some length forward of the 

 trailing edge of the fins. The whole underside of the fins, though, remained wet from the 

 water drawn by the fins during the up-stroke of the motion. The strain records did not indicate 

 any disproportionate increase in bending moment as might occur during slamming after emer- 

 gence. This observation, of course, is not conclusive because many factors, such as relative 

 frequencies, have not been considered. The dihedral fin was observed to re-enter the water 

 in a smoother way, compared with the other fins. 



The effect of forward speed and wave length on pitch reduction is noteworthy. It will 

 be observed that the damping contributed by the fins attains its maximum effectiveness in 

 reducing the amplitude of pitch when the ship operates in the near-synchronous range. At 

 very low and very high frequencies of oscillation (that is, at frequencies of encounter with 

 the waves far removed from the natural frequency of the ship in pitch) the effects of added 

 damping are generally small. 



The fins have little effect on the phase lag of heave after pitch. The small changes 

 of the phase-lag values, however, assume importance in defining the point of minimum motion 

 and also the vertical motion of any point along the length of the ship. Figures 12a through 

 12d of the results show this effect clearly. Although the point of minimum motion changes 

 location with wave conditions and speed, such changes are rather small. For all practical 

 purposes, the apparent pitching axis remains in the vicinity of amidships. The particular 

 fin is seen to cause a comparatively greater shift of the apparent pitching axis mainly in the 

 forward direction. The amplitude of the minimum vertical motion with and without fins is 

 practically identical, except in the longer waves tested. The dominant effect of the fins is to 

 reduce the amplitudes of the vertical motion and acceleration at points away from the apparent 

 pitching axis. This effect cannot be overlooked when installing equipment that has operational 

 acceleration limitations. 



Figure 15 shows that the reduction of the pitching motion by the various configurations 

 was practically the same. Some differences exist in the 600-foot wave length, but they are too 

 small to assume significance. It can be concluded that the plan area of the fin is an impor- 

 tant factor in pitch reduction. However, the effectiveness of antipitching fins can be also 

 improved by designing for a higher vertical drag coefficient. The effect of area on the anti- 

 pitching effectiveness of the fins may be brought out by comparing Fins No. 2, 2s, and 4. 



