Uram and Numata 



FOLLOWING SEA 



OVERTAKING SEA 

 SHIP SPEED (KNOTS) 



AHEAD SEA 



Fig. 21 - Heaving motion in regular 1.0 L waves 



seas, the ship "locked in" with the wave pattern and experienced no pitch or heav- 

 ing motions. It is unfortunate, or fortunate, depending upon how one views the 

 situation, that the model natural frequency and wave -exciting frequency, as well 

 as the model speed and wave celerity correspondence occurred roughly at the 

 same operating condition. In irregular seas, this condition can be expected to 

 occur, but would be of importance only if the wave with celerity correspondence 

 is that wave having a major contribution to the ship excitation. 



Figures 19 and 20 indicate that the pitch response of the Hydrofoil Semi- 

 Submarine is indeed critical, as theory predicts, in following seas in the veloc- 

 ity range between 10 and 25 knots. The pitch amplitude response is somewhat 

 larger than the destroyer, but only slightly larger than the Large Bulb Ship and 

 the Semi-Submerged Ship in their respective critical ranges. In 2.0 L waves, 

 the Hydrofoil Semi-Submarine in its critical region has a substantially higher 

 pitch response to the wave system than any of the other three ships. However, 

 it must be pointed out that, whereas in surface vessels very little can be done 

 to control or alleviate the situation because of their inherent very large longi- 

 tudinal metacentric height and large wave exciting moments, such is not the 

 case for the Hydrofoil Semi-Submarine. The very small longitudinal metacentric 



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