608 W. J. Marwood and A. Silverleaf 
displacement ratio and varies only slightly with speed. In general, there will be a slight 
increase with speed, except in the case of round-form hulls, which usually show a slight 
falling off. 
This parameter R/AV is conveniently expressed in the modified form of K = V/ohp/A > 
where K is a constant depending on the type of craft and the static waterline length (or per- 
haps more correctly the beam as being an unaltered dimension). I have examined several 
hundred cases. Those giving R/AV directly have mainly come from tests at Mr. Thornycroft’s 
Fort Steyne Model Basin. The more numerous K values have come from trial results and from 
published data. In no case have I found any wide discrepancy from Table Dl. These latter 
are of course only approximate, but in naval architecture practice it is often more useful to 
have an approximation that can be made in a few minutes than a more elaborate method which 
takes a considerable time. The approximation can at least show whether a proposal is fea- 
sible or quite impossible. 
Table D1 
Values of the Constant K in the Formula* K = V//bhp/A 
for Planing and Semiplaning Types 
Value of K for Various Types of Hull and Limits of Speed/Length Ratio 
Round bottom, 
transom stern, 
V-Chine, 
Stepped Hydrofoil 
very flat aft sigplese 
V/VL = 2.5-3.5| V/VL =2.75-4.5|V/VL = 3.5-6.5 | Up to 60 knots 
3.6 5.3 
3.96 5.5 
4.3 5.7 
4.6 5.9 
4.8 6.1 
*/\ in tons and V in knots. 
Surprise may be expressed at the inclusion of round-bottom craft in the table. It is 
however necessary for such a type to have a transom stern and a very flat stern in order to 
reach speed length ratios of over V/\/[, = 2.5. Under these circumstances, the stern lines 
are not very different from those of a normal V-chine type and a nearly comparable amount 
of dynamic lift can be obtained at high speeds. The increased drag due to the round bilge 
is however reflected in lower K values. It is assumed in all cases that the form is suitable 
for the speed and runs cleanly with a suppressed bow wave that does not lap high up the 
side but is deflected outwards. It is also assumed that the propulsive efficiency is normal, 
say, about 0.6. 
Powering figures stated by Mr Rader, during the meeting, give a K of 4.76 at 40 knots, 
increasing to 4.90 at 55 knots. These were for a 100-ton V-chine hull of about 50 percent 
increased length over the maximum table length. They are thus in line with the table 
values. 
