



BASIC DEADRISE 

 0° 



STRAIGHT V 



HORIZ. CHINE FLARE 



VERT. CHINE STRIPS 



8 12 



TRIM, T, DEG 



16 



20 



Figure 7 7. Planing efficiency for various cross sections, l„./b = 3.0. 



Longitudinal curvature. — In the case of a planing surface with longitudinal 

 curvature, the effective trim and the induced drag are not simply defined. A correla- 

 tion of model resistance data to indicate curvature effects on planing efficiency is 

 shown in Figure 12. Here the maximum lift-drag ratio obtained at the optimum trim 

 is plotted against length-beam ratio for a rectangular flat plate (ref. [18]) and plates 

 with curvature. The maximum obtainable ratio for the flat plate approaches a value of 

 8 at low length-beam ratios (high aspect ratios) and falls off to less than 6 at length- 

 beam ratios of practical interest for heavily-loaded skis. Corresponding unpublished 

 data for a longitudinally convex surface, which is commonly found in practice, indicates 

 a large penalty on planing efficiency that increases with length-beam ratio. The brief 

 data of Sottorf (ref. [22]), on the other hand, has indicated an improvement in maxi- 

 mum lift-drag ratio attainable with increasing degrees of longitudinal concavity. Prac- 

 tically, concave surfaces have had adverse stability characteristics which have dis- 

 couraged their use on seaplane hulls. The available evidence presented, however, 

 indicates not only the marked inferiority of convex planing elements, but the possibility 

 of exceeding the efficiency of the flat plate by introducing camber as for airfoils, 

 particularly for the very high length-beam ratios of current interest. 



Shallow-water. — It has been observed many times by service pilots that shallow 

 water seems to improve seaplane take-off performance. As yet, a theoretical solution 

 of this effect has not been made, although it should be possible by available methods. 

 Its magnitude has been evaluated, however, by model tests in a tank with a variable- 

 depth bottom (ref. [23]). This brief investigation indicated that lift, drag, and trimming- 

 moment coefficients about the trailing edge increase with decreasing bottom clearance 

 at clearances less than one beam, and significant changes in the wake occur. 



A condition of practical interest is the case of a hydro-ski airplane approaching 

 a constant-slope ramp as shown in Figure 13. Here it may be assumed that the water- 



195 



