ON HYDROFOILS RUNNING NEAR A FREE SURFACE 
S. Schuster and H. Schwanecke 
Versuchanstalt fiir Wasserbau und Shiffbau 
Berlin Towing Tank 
For the steady state of flow the results of pressure distribution measure- 
ments dependent on speed, depth of submergence, angle of attack, and 
roll angle—both for a flat and for a dihedral hydrofcil—are presented, as 
well as the results of six-component measurements for two dihedral foils. 
Below the Froude depth number 1, related to the depth of submergence, 
the flow and hence the pressure distribution changes according to the 
shallow water laws; above this figure no significant speed dependency 
exists. Here the pressure distribution is influenced only by the distance 
between the hydrofoil and the surface in relation to its profile length. For 
this situation, which is normal for hydrofoil boats under service condi- 
tions, simple theoretical rates for calculating the hydrodynamic forces are 
given, as well as the results of these calculations in the case of a 
parallel submerged hydrofoil of great span. By analyzing the separate in- 
fluences the relation to results of wind tunnel tests can be set up. By 
means of the results from theoretical and experimental examinations, rela- 
tions are derived for the vertical and the lateral stability both of flat and 
dihedral hydrofoils as well as for the influence of sideslip motion. 
INTRODUCTION 
One of the most important suppositions for the design of hydrofoil boats is the precise 
knowledge of the flow forces occurring on the foils under given service conditions. There 
is no doubt that many conceptions are transferable from aerodynamic research but some es- 
sential properties like lift distribution and all values of driving performance and running be- 
havior connected herewith will be more or less influenced by the proximity of the water sur- 
faced. During the last twenty years this problem has been treated very often, but further 
treatment, based on new experiments in the Versuchsanstalt fur Wasserbau und Schiffbau 
(the Berlin Towing Tank), may be expected to clarify and complete the picture. During 
these examinations the pressure distribution both for completely submerged hydrofoils and 
for those piercing the water surface, the deformation of the surface, and the flow forces have 
been measured as well as theoretically treated, all angles being varied. 
PRESSURE DISTRIBUTION MEASUREMENT 
Two brass hydrofoil models with special borings have been used for measuring the pres- 
sure distribution, one of them being a flat hydrofoil and the other a dihedral hydrofoil (V- 
form), both idealized with circle segment profiles of constant length without distortion (Fig. 
1). On the starboard side the flat foil was fitted with 68 borings and the dihedral foil with 
147 
