47 



stlffener. This method Immediately proved impracticable since the float 

 turned broadside to the stream and was entirely unstable. As a result, sys- 

 tematic tests were made only with the float towed from a shackle welded to 

 the nose as shown in Figure 36. 



RESULTS OP THE NMWTS FLOAT TESTS 



Towing tests were conducted under four conditions of load in the 



manner described in the body of this report 

 the loading conditions correspond- 

 ing to the curves of Figure 8 is 

 shown in Figure 37. Photographs 

 of the float taken during the tests 

 are shown in Figure 38. 



Since at low speeds the 

 float is essentially a cylinder 

 towed transversely to the stream, 

 the flow is such as to produce a 

 Kirm^n vortex street in which ed- 

 dies are shed alternately from 

 either side of the float. The 

 periodic shedding of eddies gives 

 rise to alternating side forces 

 with the result that the float is 

 oscillated with the frequency of 

 the eddy wake (n). The oscilla- 

 tion of the float is indicated to 

 some extent in the photographs of 

 Figure 38 in which the angle made 



The drag of the float under 



12 3 



Speed in knots 



Figure 37 - Drag of NMWTS Float under 

 Various Loading Conditions and 

 Towed from the Forward Towpoint 



by the float with the vertical in the plane transverse to the flow is obvious. 



As a result of the lateral oscillations of the float, the drag shows 

 a cyclic variation about the mean. The results plotted in Figure 37 represent 

 the mean drag as measured on the towing dynamometer, which was operated with a 

 high degree of damping. 



Owing to the high drag of this float and the fact that it rapidly 

 loses lifting capacity because of its negative angle of attack or trim, its 

 usefulness as a towed float is limited to very low speeds. Under the highest 

 loading conditions used in the tests, the float was considered completely 

 overloaded above 3 knots. In view of these conclusions, this type of float 

 is not recommended for towing operations. Furthermore, its use as a moored 

 buoy should be limited to regions in which currents are small. 



