High Performance Ships—Promises and Problems 13 
This result confirms our confidence in the use of the extensive airfoil data that are avail- 
able. Model experiments in takeoff drag and available thrust indicated satisfactory margins. 
It was found, however, that very poor flow conditions existed at the intersection of the 
strut, foil, and nacelle. This poor flow was found to be cavitation and not separated flow. 
In analyzing the problem, aircraft technique was borrowed and simple consideration of add- 
ing pressure distributions led to small changes in configuration; the resultant clean flow 
was confirmed by experiment. The PC(H) represents the first hydrofoil for which the hydro- 
dynamic coefficients were obtained experimentally at the David Taylor Model Basin. These 
coefficients are needed for determination of stability and autopilot gains for vertical-plane 
motion. Horizontal-plane and coupling effects are planned to be obtained in a similar 
manner in the near future. 
A few problems remain to be resolved in the subcavitating regime. Probably the major 
one is the determination of the hydroelastic dangers, i.e., flutter and divergence. There is 
no adequate theory for prediction of instability speeds, and practically no experimental work 
in the marine range of interest. The flow problems at the intersections of strut, foil, and 
nacelle need to be codified. Flap effectiveness at low submergences (as determined from 
PC(H) tests) appears to be less than predicted by theory. These problems are all currently 
under study. 
Flying qualities in waves are being examined. In very high speed hydrofoil craft 
(70 to 100 knots), the frequency of encounter with waves will be so high that it will be 
impossible to “contour” the surface without inducing excessive accelerations. This implies 
the necessity for “platforming;” that is, the craft’s trajectory must remain essentially hori- 
zontal. This further implies submerged foils, because surface-piercing systems cannot 
platform. In addition, long struts will be needed to keep the hull clear of the water and yet 
insure continuous good submergences for the foils in rough water. In order to platform, the 
autopilot system must move the control surfaces continuously and at rapid rates in order to 
nullify the wave-induced disturbances. In some following seas it may not be desirable to 
platform and here the control system will have to permit some vertical motion. This sug- 
gests not only a wave-height sensing device, but also inputs to the autopilot from acceler- 
ometers. Roll control will require additional displacement, velocity, and acceleration 
inputs. 
The major problems to be resolved are primarily in the supercavitating regime. Here we 
have theory to guide us, but systematic experimentation on the effect of geometric charac- 
teristics of foils is lacking. There are no corresponding airfoil data, of course, that are 
useful. 
As indicated earlier, the mechanisms of ventilation are not well understood and require 
research starting with the basic physics of the phenomenon. 
A simplified theory of hydroelastic instabilities has been developed, but there are no 
experimental data. This theory, incidentally, indicates a greater likelihood of hydroelastic 
instabilities than for a comparable all-wetted hydrofoil. Also, advances are sorely needed 
in materials in order that the requirements in hydrofoils for very high strength, erosion, and 
corrosion resistance, toughness, good fatigue life, etc., may be met. 
In the supercavitating or superventilating range, takeoff may well be critical. Feasi- 
bility studies have shown that for very high speed craft, takeoff thrust requirements are 
incompatible with top-speed requirements. Means for achieving takeoff at lower speeds 
need special attention. 
646551 O—62——3 
