ONR Hydrofoil Boat XCH-6 223 
[3] Caster, E.B., “TMB 3-Bladed Supercavitating Propeller Series,” David Taylor Model 
Basin Report 1245, Aug. 1959 
[4] “Study of Hydrofoil Seacraft,” Vol. I, Bethpage, New York: Grumman Aircraft Engineer- 
ing Corp. and Dynamic Developments, Inc., Oct. 1958 
[5] Johnson, V.E., Jr., “Theoretical and Experimental Investigation of Arbitrary Aspect 
Ratio, Supercavitating Hydrofoils Operating Near the Free Water Surface,” NACA RM 
L57116 (NASA, Washington), Dec. 1957 
DISCUSSION 
H. P. Rader (Vosper Limited, Portsmouth) 
With my compliments to Mr. Wennagel and also to Mr. Tulin I should like to make just a 
few remarks concerning the practical application of supercavitating sections. Recently we 
have tested two propellers with supercavitating blade sections of wedge-shaped thickness 
distribution. The efficiency of both propellers under noncavitating conditions was rather 
poor. On analyzing the results according to the method of the equivalent polar curve as 
devised by Prof. Lerbs we found that the drag coefficients of the equivalent blade sections 
were much higher than those of supercavitating sections with quasi-elliptic or elliptic- 
parabolic thickness distribution. In fact the drag coefficients agreed very closely with 
values published in Dr. Hoerner’s excellent book “Fluid Dynamic Drag.” The drag coeffi- 
cient ratios for various thickness-chord ratios are approximately as follows: 
Thickness-chord Ratio Cy Wedge/Cp Ellipse 
2.5 
0.04 
0.06 4.0 
0.08 5.9 
0.10 TE 
When we design supercavitating hydrofoils and propellers we must not forget that the boat 
fitted with the equipment has to pass through the noncavitating speed range. This is par- 
ticularly important for hydrofoil boats which have a pronounced hump in their resistance 
curve at a speed where supercavitating conditions may not be obtainable unless artificial 
ventilation is used. Even with ventilation the drag coefficients of wedge-shaped sections 
would be higher than those of quasi-elliptic or elliptic-parabolic sections under noncavitat- 
ing conditions. I noticed in Mr. Wennagel’s interesting paper (or rather in Fig. 8) that at 
the hump the resistance-displacement ratio of the craft described by him is approximately 
0.26, which is rather high. 
Marshall P. Tulin 
With regard to Mr. Rader’s comments on the efficiency of supercavitating sections with 
blunt bases operating in fully cavitating flow, it is, of course, certainly true that such sec- 
tions are not particularly efficient because of the blunt base, which causes a drag that is 
proportional to the square of the base thickness. In fact, it was with some surprise that the 
