Problems of Commercial Hydrofoils 265 
concerned. At the moment it appears to me that you prefer to think entirely dimensionally 
round each particular problem and in so doing do not necessarily see the picture as a whole. 
For example, in Mr. Wennagel’s paper there is one very interesting diagram (here Fig. - 
_D83B) given in dimensional form which can with advantage be re-presented in nondimensional 
form. Normally in ship work we use a resistance coefficient based on a speed-squared 
relation associated with a relative speed. One such combination is the Froude © and 
presentation; and so long as resistance does vary approximately as speed squared, the cor- 
responding value is substantially constant. When the performance of a fast vessel is so 
presented, however, we see (Fig. D3A) that the speed-squared relation required for non- 
dimensionality is no indication at all of the power of the speed with which the resistance is 
actually varying. Actually, for speeds beyond the maximum , the resistance is more 
nearly varying directly as the speed. In this case it is better to use a nondimensional 
presentation which respects this fact, and this is easily obtained by plotting values of 
© 
72) 
Lia 
N 
“Wve 
e V/aé =(K) 
Fig. D3. Comparisons of dimensional and 
nondimensional presentations 
