604 W. J. Marwood and A. Silverleaf 
The authors’ presentation might have tempted the unwary to believe that the 50-ton boat 
was twice as efficient as the 200-ton boat. It follows therefore, since the authors’ presen- 
tation has very seriously exaggerated the apparent importance of @ value, that the influ- 
ence of other factors will not necessarily have quite the insubordinate influence which the 
authors are led to believe. I am not, of course, suggesting that the authors themselves in 
their specialised tank practice would be at all unwary; but I do feel very strongly that all 
data presentation for general professional consumption should be visually correct, so that 
he who runs may read, and read quickly and accurately. 
\ 
R. N. Newton (Admiralty Experiment Works) 
Any paper which provides data which can be usefully employed by a designer is very 
welcome at any time or place. This paper by Messrs. Marwood and Silverleaf is just such a 
paper and any opportunity to add to the information they present should not be missed, for 
the same reason. 
Experiments on high speed planing forms having been carried out at A.W. for many 
years, with forms of different lengths and coefficients, in calm water and in waves, and with 
various devices for improving performance, I will attempt to take this opportunity to qualify 
some of the statements in the paper and add to them, in a general way, in terms of the ratio 
of length of wave to length of craft and in terms of Froude number. 
Here let me hasten to add that while I incline very much to Prof. Telfer’s opinion that 
we should use more rational performance coefficients and parameters to show the trends 
arising from changes in form and displacement for the present purpose I must adhere to the 
commonly used ones, such as Froude number and length of wave to length of ship ratio. 
First, then, speaking generally, on the effectiveness of steps in the bottom, a large 
number of resistance experiments conducted at A.E.W. between 1935 and 1944 led to the 
general conclusion that although stepped hulls had advantages over unstepped hulls in some 
ways, they also had inherent disadvantages and particularly increase of resistance at low 
speeds. 
Experiments conducted with two, three, and five steps in the bottom, at different 
spacings and positions along the bottom indicated that these gave generally inferior perform- 
ance to a single step on the same form, particularly as regards an increased tendency to por- 
poise and, judged by tests of one form in waves, inferior behaviour in waves. 
There is an optimum position for the step which effects a comparison between reduction 
in resistance above planing speed, increase in resistance below planing speed, and the 
liability to porpoise. The risk of porpoising increases as the step is moved forward. As 
the step is moved aft the speed at which planing starts increases until when the step is 
well aft the craft can hardly be said to be planing at all in the usual sense of the word. 
Secondly as regards resistance in calm water, the position of the center of gravity is 
critical and a small movement has a marked effect. Generally speaking a moderate stern 
trim in still water is an advantage at planing speeds but adds to resistance at lower speeds. 
Too large a dihedral angle amidships adds to resistance due to increased running in- 
cidence and reduced rise. A low dihedral angle is good for calm water performance but 
detracts from behaviour in waves. 
