244 H. von Schertel 
Lit ratio L Nt . 
x=) 
Rien aa wl, ; 
b Wi 
o=|span Wh 
IL 
S 
n 
“OS Ra? gus IQ”) Plas May = ais) get eas 
Submergence (h- Yo, 
Fig. 10. Wing characteristic of a surface-piercing V-foil 
its flaps. The respective control device must be designed in such a way that it reacts to a 
change of submergence, corresponding to the same value of Z as for surface-piercing foils. 
The fully-submerged hydrofoil, however, has a significant advantage over the surface- 
piercing type inasmuch as the value of Z can be changed at will whenever desirable and 
inasmuch as a damping or filtering function can be introduced in such a manner that the foil 
does not react to short waves and follows the contours of longer waves only insofar as to 
keep the hull clear from the water surface. Furthermore, lift is being gradually modified in 
conformity with the signal, whereas in the case of the surface-piercing foil, lift-variation 
occurs more or less suddenly with an impact when travelling in waves. Consequently boats 
with fully-submerged foils give better riding comfort. In rough sea conditions it is possible 
to differentiate submergence against time and to modify the foil angle reaction accordingly. 
The control device may also respond to accelerations; pitch and rolling angles can be 
reduced by means of a gyroscope stabilizer. Such control is difficult to materialize for sur- 
face-piercing foils because of their high autostability. 
The second characteristic of a foil in a seaway is its reaction to orbital motion 
indicated by 
W = (dCL/da)/CL, = 
: a-Ay 
where dC,/dqa = lift-curve slope. A high value of VW means that a foil has a tendency to 
react strongly to orbital velocites in waves. To make a foil less susceptible to these 
motions, it would be desirable to make the lift-curve slope as small as possible and the 
design-lift coefficient C7, as high as possible. Since the lift-curve slope is primarily a 
function of aspect ratio, its reduction is bound to increase the drag, and since, on the other 
hand, the lift coefficient C;, is limited by considerations of cavitation and ventilation, the 
value of this parameter W cannot be reduced effectively. 
Rigid hydrofoils are, therefore, always subjected to the influence of orbital wave 
motions. This fact is without practical consequence when running against the sea, but the 
influence of orbital motion is a very undesirable reality in a following sea. In such 
