High Speed Displacement-Type Hulls 603 
contention that the round-bilge boat exhibits superior performance (less pitching motions 
and considerably less bow spray) than the hard chine boat. The point that I would like to 
clearly emphasize is that the existence of a hard chine or round bilge per se has no rele- 
vance to its behavior in waves. Heavy bow spray and large pitch motions are primarily de- 
pendent upon the fullness of the bow. For very sharp bow forms (high deadrise forward) the 
rough water behavior will be relatively mild and the development of bow spray very small. 
The reverse is true for a full bow (low deadrise forward). It is to be noted from the authors’ 
paper that their round-bilge boat, which exhibited relatively mild rough water motions was 
indeed designed with large deadrise forward while their poorly performing hard chine boat 
was designed with low deadrise forward. With such design features it is expected in advance 
that the hard chine boat will perform poorly in waves. If the full bow is, for reasons of con- 
struction, necessarily inherent in a hard chine boat then the authors’ general comments on 
the relative merits of each boat are correct. However, if this is not the case, that is, if high 
deadrise forward can be designed into each boat, then general conclusions on rough water 
behavior of round-bilge versus hard chine boats cannot yet be made. 
In reply to the authors’ plea for basic force data on planing boats, I would like to sug- 
gest the many reports prepared by the Davidson Laboratory of the Stevens Institute of 
Technology on this very subject. These basic experimental and analytical studies were 
sponsored at our laboratory by the Office of Naval Research. 
E. V. Telfer (Technical University of Norway) 
All I want to say is once more in connexion with presentation. In this paper the 
authors make the very positive statement that “again there is no clear evidence of any pre- 
dominant form parameter.” Against this I would like to suggest that what the authors really 
mean to say, but clearly do not, is that because they have adopted a basic presentation in 
terms of the displacement of the 100-foot ship, which is of course the Froude @ = L/V¥8 
value in another form, this is clearly in itself the dominant form parameter and any other 
usual form parameter is likely to be of minor importance. Had the authors’ diagrams such as 
Fig. 8 been presented in terms of @ the importance of relative length on constant dis- 
placement would have been more evident and the distribution of the individual spots would 
have more uniformly covered the @ base. 
The issue is, however, not quite as simple as this. It should be noted that while at 
any constant displacement the © values correctly grade the corresponding powers, a com- 
parison of the © value at one displacement does not give a direct comparison of the cor- 
responding power ratio at any other displacement. The authors have made their © com- 
parisons under the condition of constant V/\/Z,, or since L = 100 feet, under the condition 
of constant speed. It follows therefore that to get the relative power ratios P and P2 for 
two different displacements A and Ag, the respective © values must first be multiplied 
by the two-thirds power of their displacements. Alternatively to express the true relative 
power per ton displacement over the displacement range covered the © values firstrequire 
multiplication by @ . When values of © @ are plotted to the displacement base, then 
the comparison is both correct and compatible. The authors’ presentation is neither. For 
example, in Fig. 8 for 200 tons displacement a © value of 4.9 may be indicated, while at 
50 tons the © value is probably only 2.45 at most for the same B/d ratio. The correspond- 
ing © @ comparison is thus 4.9 x 17.1 = 84 against 2.45 x 27.2 = 66.6. The 50-ton boat 
thus requires 79.4 percent of the power per ton displacement needed by the 200-ton boat. 
