WAVE RESISTANCE THEORY AND APPLICATION 21 
Fig. 9 is from Guilloton’s work on stream lines. 
Fig. 10 shows resistance curves for two models, 
the calculations being made by the source method. 
The forms were not experimental models but 
were actual ships, of high-speed form and not sym- 
metrical fore and aft. The models were divided 
into ten compartments and the strengths and 
positions of the sources determined directly from 
the plans of the model, the chief point of the work 
being to show that the calculations can be carried 
out in such cases. 
Finally, I reproduce in Fig. 11 a diagram from 
Lunde’s recent paper in which he examined the 
effect of placing sources and sinks off the longi- 
tudinal vertical section. Here the model was of 
destroyer type, but it is unnecessary to enter into 
details of the comparison except to note that some 
improvement was obtained by the space distribu- 
tion of the sources. 
Wave at Surface 
“ee 
Model No. ie (Mr. Wigley) for ae 0.274 
Tangents Sac the Streamlines 
In Dotted Lines,Approximate Traces of 
are treamlines 
Fic. 9 
In some cases, and not only in those cases 
which have been reproduced here, one may sus- 
pect that the agreement with experimental re- 
sults is too good; or perhaps one should say 
rather that the agreement may be deceptive when 
pushed too far in view of certain considerations 
which have been neglected. There are, for in- 
stance, the effects of trim and sinkage at higher 
speeds, of which it is possible to make a rough es- 
timate; but, specially, there is the question of 
the effects of viscosity. 
We talk of comparing calculated wave resist- 
ance with experiment, but there is no such thing 
as an experimentally measured wave resistance; 
for that we must wait for the day when someone 
From Experiment 
From Approximation 
iol | 
= 
CPSs 
0.08 
035 040 045 050 055 060 
f=v/V(gL) 
Fic. 10 
065 070 O75 
invents a frictionless liquid. The only experi- 
mental result is the measured total resistance. We 
may adopt the usual procedure, which has been 
so well justified for most practical purposes, of 
considering the frictional resistance and the wave 
resistance separately, and we use some standard 
method, Froude’s coefficients or some more recent 
formulation, for determining the frictional re- 
sistance. Then we begin to realize, when we re- 
quire greater accuracy, that the ship is not a 
plank and that we should make some allowance for 
the effect of form upon frictional resistance; 
and, as the importance of boundary layer theory 
becomes recognized in ship problems, we find how 
necessary it is to know more of the conditions in 
the boundary layer, the extent of laminar flow, 
turbulent flow, separation and so forth, a matter 
which may be described as a burning question at 
the moment. It may well be the case that some 
of the differences between calculated wave re- 
sistance and so-called experimental values may 
prove to be due to error in estimating the fric- 
tional resistance. No doubt as we push on to 
greater accuracy, we may find it inadequate to 
treat the two parts of the resistance as independ- 
ent; the problem is one, and the two must have 
mutual interaction, the important point being 
whether it is of appreciable magnitude. On the 
one hand, it is obvious that viscosity effects have 
a very considerable direct influence upon the 
wave-making; on the other hand, it seems pos- 
sible that the wave motion may have an appreci- 
able effect upon conditions in the boundary layer 
in special circumstances, as, for instance, the po- 
sitions of crests and troughs in relation to the 
lines of the model. 
571 
