Problems of Commercial Hydrofoils 259 
DISCUSSION 
Christofer Hook (Atlantic Hydrofin Corportation, Villeneuve le Roi (S&O), France. 
Baron von Schertel gave the example of phase shift in the case of following waves for 
the surface-piercing system but he neglected to point out that the main advantage of the 
submerged foil is the opportunity it provides for correcting this out-of-phase effect by 
application of an advance or prediction signal to the incidence changes. Ideally, this would 
lead us to a telescopic sensing device whose advance position with respect to the con- 
trolled foil would be adjusted to suit different ratios of hull to wavelength. In practice, a 
simplified compromise in the form of a fixed advance length is perfectly satisfactory since 
only relatively short waves are significant. 
It is also feasible to incorporate into such a sensing device a wave-height measuring 
system, coupled to powerful damping, so that all waves up to a given maximum (correspond- 
ing roughly to the height clearance) may be filtered out, resulting in practically level flight 
over waves up to this size. For larger waves the aforementioned size may be subtracted as 
a constant so that, for example, a wave 6 feet high may be dealt with as if it were only 3 
feet high. 
It is too often assumed that a given hydrofoil system must be restricted to a given 
severity of sea and that beyond this some disaster must follow. The impression given is 
not improved by the fact that the details of the type of disaster are left to our imagination! 
This is, however, somewhat unfair since a similar restriction should also be placed on 
normal boats, which should then also be sold with a “sea severity tag” attached — which is 
not done. A good hydrofoil can perform in seas well beyond the capacity of a crash boat to 
follow, as has been shown in U.S. Navy reports, and when failure to follow is eventually 
arrived at it is not followed by any kind of disaster but merely by a sitdown, and the ship 
can always continue in a half-foilborne condition; in fact, pitching is so severely damped 
by foil action that a light hull fitted with foils will survive where the same hull with foils 
removed will break up. The matter of wave filtering becomes so important at sea that fine 
comparisons of relative lift/drag ratios of the two methods becomes pointless. 
Finally, I think that the purely mechanical method cannot be lightly dismissed as 
bulky and vulnerable since its reliability is unquestionable. Since an increase of C; is 
obtained by incidence instead of by carrying reserve foil area we can offset the gain in 
weight consequent in the elimination of this weight against the added hardware required to 
manipulate the prediction method and we fill find that there is no added weight. On the 
basis of cost, if I may be excused, I may perhaps point out that for a sport boat or 
4-passenger size the cost of the hardware does not exceed $100. I think that all hydrofoil 
men here today should realize that we are very much riding on the wave created 25 years 
ago by the Schertel-Sachsenberg group and that, had it not been for the commercial suc- 
cesses of this group, hydrofoils would never have got going again after the fizzle out that 
followed the Bell experiments in about 1920. 
H. Von Schertel 
Mr. Hook is right when he points out that the description of phase shift for a hydrofoil 
boat travelling in a following sea, given in the paper, refers to the surface-piercing foil 
