High Speed Displacement-Type Hulls 607 
In longer and steeper waves around 2 ship lengths and height L/20 and at F;, greater 
than 1.] there is a more significant decrease in both pitch and heave of the order of 30 
percent and 45 percent respectively. 
At these high speeds, however, there is a grave danger of the craft plunging. In fact 
during one experiment with a model at F, = 1.4 the model buried so deeply into the slope 
of the oncoming wave as to cause it to sink. Obviously in such conditions the ship would 
suffer grave damage or loss. 
In following seas the use of a flap or wedge introduces a distinct tendency of the 
model to bury into the slope of the wave —much more so than in ahead seas. For instance, 
in the case mentioned previously the same model, in waves 1.5L using 5-degree flap inci- 
dence, plunged and sank at F, = 1.4, The same occurred also at the same speed in waves 
2.5L. In longer wavelengths the model could only be controlled with difficulty and then 
only up to F, = 1.06, when there was still a tendency to broach-to. 
As a result of close study of films of models in waves and experimental data a general 
conclusion can be drawn. This is that whereas a transom flap can prove of advantage pro- 
pulsively in calm weather, the degree of this advantage being determined by consideration 
of the top speed and displacement, before the flap can be used to improve the motion in 
waves, i.e., to make conditions on board more comfortable, it is essential for the operators 
to be able to recognise the sea conditions at the time, or to be provided with some scien- 
tific indication that the craft might be in danger. There is no doubt that with long experi- 
ence of such craft, judgment can be placed upon the “average” length and height of sea 
which may be running, but this is very different from being able to predict the length and 
height of oncoming waves. For the time being, therefore, until much more research has 
been done into the statistics of wave spectra in a given ocean area, and until such time as 
the spectrum can be indicated with some degree of reliability, the use of transom flaps in 
rough weather cannot be recommended —at least not for craft designed for really high speed, 
say beyond 45 knots. 
In conclusion it is perhaps pertinent to remark, in the light of these general observa- 
tions that any change of form, or device, introduced into a new design, although it may 
improve performance from one aspect, may well detract from it in another aspect, and a 
compromise is very often necessary, and can only be found by careful model experiments. 
K. C. Barnaby (John I. Thornycroft Company, Southampton) 
The authors of this interesting paper referred to the difficulty of finding a suitable 
parameter for expressing their data. It is, of course, obvious that a conventional constant 
such as © is quite unsuitable. Once dynamic lift has taken charge, the wetted surface is 
no longer constant, but must diminish rapidly since the product C,%pSV* must always be 
less than the original displacement weight, by at least the amount of submerged hydrostatic 
buoyancy. Also, the resistance to motion increases with approximately V and very defi- 
nitely not with V?. Thus both the terms A2/3 and V3 become incorrect at high speeds. 
The one factor that cannot change as the speed increases is the actual weight and this 
forms the basis of a suitable parameter. If one plots the ratio R/AV, that is, the total 
resistance expressed in pounds per ton divided by the speed in knots, for a common parent 
form at various displacement ratios and speeds, a family of curves is obtained that, at suf- 
ficiently high speed/length ratios will merge into one line that is nearly independent of 
