152 S. Schuster and H. Schwanecke 
The lift distribution over the span can be seen in Fig. 5, the lift coefficients for the 
complete hydrofoil are shown in Fig. 6, and the components of the suction side as well as 
those of the pressure side and the ratio between these components are shown in Figs. 7 to 
9, where the C; values are related to conditions in great depth (infinite medium). In ap- 
proaching the water surface the lift of the foil running at supercritical speed decreases 
nearly along a parabola with the depth of submergence, especially for h/c < 2, the effect of 
the suction side being diminished and the effect of the pressure side being augmented. This 
change will decrease for increasing angles of attack. It is remarkable that the drag coef- 
ficient is decreasing with the! decrease of depth in spite of the increasing waves (Fig. 10). 
Summing up, the results of these tests with the flat foil were as follows: 
1. Owing to the proximity of the water surface an additional velocity is induced which 
is contrary to the direction of flow. This results in an increase of the lift component on the 
pressure side and in a decrease on the suction side. Due to the suction side’s greater part 
of the total lift in the deep submerged position, a diminution of lift thus results. Owing to 
the minor flow velocities at both sides the profile resistance decreases. 
2. Owing to the curvature of flow along the upper side the effective profile curvature is 
diminished. Thus the angle of zero lift will be shifted toward positive values, and the effec- 
tive angle of attack and thereby the lift coefficient will become less. Relatively this influ- 
ence decreases with increasing angle of attack. 
3. Owing to the continual lift diminution in the range 2 > h > 0 a submergence stabiliz- 
ing seems to be possible even for hydrofoils not piercing the water surface. 
Dihedral Foil 
The measured lift distribution for the dihedral foil can be seen in Fig. 11. The pressure 
distribution over the profile is similar to that for the flat, fully-submerged hydrofoil. The 
joint of the two foil halves at the keel point, as well as that between foil and struts, causes 
an increase of lift on the suction side, and in the first case simultaneously decreases the 
pressure at the lower side. When piercing the surface a diminution of the local lift occurs 
within the region of the surface approach in such a manner as though the results of flat 
parallel submerged foils had been transferred stripwise. 
Figure 12 shows the lift decrease of both foils plotted versus the relative submergence. 
This diagram shows also the test results of Land [4], which were found at substantially 
higher speeds. 
The lift diminution for the complete dihedral hydrofoil caused by the surface effect can 
be found by integration of the local values 
Coun 
within the range of h = 0 to h= yee} The mean of (C,,/C,_,) amounts to approximately 0.5 
for hy.e1= 0.5, to 0.8 for hy...) = 1.5, and to 0.9 for hy.) = 5- 
Summing up, it follows from the tests with the dihedral hydrofoil that: 
