14 



Towing Speed, 0.5 Knot 



Towing Speed, 4.5 Knots 



Towing Speed, 6.0 Knots Towing Speed, 10.0 Knots 



Figure 1 1 - Photographs of TMB 4l -Inch Planing Float Being Towed 



These tests were made under no imposed load. 



which the float may be considered as having emerged. This will be shown in a 

 subsequent discussion of transition from subsurface to surface operation. 



The action of the float at the low speeds and its subsequent rising 

 to the surface can be easily understood from an examination of the forces 

 acting on the float form. At the low speeds, the flow around the float is 

 primarily of the displacement type, and the load-carrying capacity of the 

 float is due only to its buoyancy. However, because of the curvature of the 

 bottom surfaces, local velocities higher than the speed of tow are induced, 

 accompanied by a resultant pressure drop along the bottom. The pressure dis- 

 tribution is then such as to produce a downward component of force on the 

 float and, consequently, an increase in draft (5). For no load and for the 

 loading conditions obtained with the 25-pound weight, this force was not 

 sufficient to submerge the float completely before it developed dynamic lift 



