178 S. Schuster and H. Schwanecke 
side, being higher than calculated, cannot be represented by these simple formulas. But 
since these influences compensate each other for the most part, the calculated total lift 
nearly equals the measured lift. For calculating the total lift, therefore, this method may be 
used as an approximation. 
INTRODUCTION OF THE MAIN PARAMETERS 
Laying out foils for hydrofoil boats needs not only information about the decrease of lift 
due to the free surface but also about the influence of the aspect ratio and eventually of the 
dihedral angle. The above-mentioned measurements proved that for supercritical flow above 
the foils the spanwise distribution of lift is only slightly dependent on the depth h. For su- 
percritical flow, therefore, it is possible to consider the influences of the surface and of the 
aspect ratio separately without being greatly mistaken. Moreover this gives the possibility 
of calculating the lift decrease of dihedral foils. 
The main parameters will be introduced now. The influence of the finite aspect ratio 
will be covered by the relation found by Weinig [9] by means of the cascade theory: 
dC 
—L = K, OP 6 
da (17) 
The factor K, is shown in Fig. 29. The aspect ratio of a dihedral foil has to be taken as 
the wetted aspect ratio, i.e., 
03 
| | | 
die K | 
Ez =) 12K (ace. to Weinig [9]) 
k = #4 tanh (4) 
+ 
Ka 
8 10 
A — 
Fig. 29. Dependence of the lift gradient and the increase 
of the lift gradient on the aspect ratio 
