21 



Towing Speed, 2.5 Knots 



Towing Speed, 6.0 Knots 



Towing Speed, 8.0 Knots 



Figure l8 - Photographs of TMB tfl-Inch Planing Float Being Towed 



These tests were made under loads imposed by the 100-pound weight towed from the forward towpoint. 



An indication of the magnitude of the oscillations that were ob- 

 served near the region in which signs of porpoising appeared can be obtained 

 from Figure 20. In this figure, the curves of variations in drag about the 

 mean drag have been traced directly from the recording tapes on the carriage 

 dynamometer. In the first three records at the top of the figure, no well- 

 defined oscillations in pitch could be observed visually. For the remaining 

 conditions, i.e., loads imposed by the 75-pound weight towed from the after 

 towpoint, oscillations in pitch seemed to be very gentle when observed visu- 

 ally. However, the actual amplitude of the variation grows with speed, al- 

 though the frequency appears to remain fairly constant. These curves probably 

 do not represent accurately the actual modes of porpoising of a free float, in 

 which the equivalent load is built into the float, because during the tests an 

 effective spring action resulted from the curvature of the towing cable to the 

 faired weight. 



