Hydrodynamics of High-Speed Hydrofoils 135 
control of motions, becomes the task that must be successfully accomplished by the flaps, 
black boxes, etc. 
CAVITATION SUPPRESSION 
As boat speeds increase beyond 40 knots the danger of cavitation occurrence on foils, 
vertical struts, and propulsion nacelles rapidly increases. Because of the erosive effects 
of cavitation it does not seem attractive to design for operation with small regions of local 
cavitation occurring on the underwater structure, nor does it seem really feasible to allow 
extensive cavitation short of supercavitation because of the buffeting that very often accom- 
panies such “transcavitating” flows [20]. The designer must thus strive to suppress cavi- 
tation, a process which as earlier mentioned leads to underwater structures which become 
increasingly thin. 
A great deal has been written about the estimation of minimum pressure coefficients on 
hydrodynamic bodies, and I will here only add several small straws to the camel’s back. 
The first concerns the point that it would seem to us generally dangerous in design, even in 
the earliest stages, to consider the inception properties of components by themselvesrather 
than in combination. This might seem to be a rather obvious remark, but nevertheless there 
has been considerable discussion of the inception properties of components, and very little 
on combined configurations. In Fig. 12 are shown theoretical inception speeds for a modi- 
fied spheroidal nacelle and a NASA 16 series strut both separately and in combination, as 
a function of pod length-diameter ratio. The strut maximum thickness was assumed to be 
1/6 of the nacelle diameter and its length 67% of the nacelle length; since such a nacelle 
may at times operate in practice right up to the free surface, the calculations were made for 
zero depth although the relieving effect of the free surface was not taken into account. 
These calculations were made for illustrative purposes only, and although thought to be 
realistic, do not necessarily represent optimum configurations; further they neglect the 
influence of the propeller and horizontal foils should they be called for, and the additional 
influence of pitch and yaw. Even so the relation between component and configuration 
inception speeds is indicated. 
In considering the inception speeds of components it would seem very important that 
the effect of wave motions be taken into account, for the minimum pressure coefficients of 
high-speed foil and strut sections are very sensitive to incidence and yaw. Shown as Fig. 
13 is an estimation of the probability that local cavitation will occur on a hydrofoil wing of 
aspect ratio 4, with Series 16 sections of 4-percent thickness and a design lift coefficient 
of 0.15, while operating at 5-foot depth in the average North Atlantic environment described 
in Fig. 3; it was assumed that cavitation occurs during motions involving fluctuating angles 
of attack for values of rms angle of attack equal to steady inception angles, and this is 
believed to be a conservative assumption. It is observed that this wing, which could accord- 
ing to theory make 70 knots in calm water, will cavitate over 50 percent of the time during 
year-round operation at speeds as low as 50 knots, and over 75 percent of the time at 60 
knots. These predictions of average performance do not, of course, show how inception 
speed varies with sea state, so in Fig. 14 is presented (solid line from the left) a curve of 
inception speed versus sea states (and corresponding wind speed) for the wing described 
above, assuming again that the rms angle of attack corresponds to the steady inception 
angle. The dashed curves are based on the slightly different assumptions that the average 
of one-third highest and maximum angles of attack in the seaway correspond to the steady 
inception angle. It is of interest to know how much faster than inception speed a wing may 
be driven before the cavity grows long enough to cause serious buffeting, and the right-hand 
