Free Surface Effects tn Hull Propeller Interaction 
wake. One could have, for example, a submarine generating waves 
on the surface when the submarine itself was totally submerged. 
There could then be a wake change depending upon the orbital motion 
of the waves passing the propeller. If there were no surface waves, 
however, there could not be any additional wake, at least not in my 
opinion, 
Finally, I should like to refer to the authors' determination 
of the form effect of their model. They have used the ITTC line with 
its 75 numerical coefficient to determine the form effect deduced by 
the Prohaska method. If one allows for the fact that the low value they 
obtained of about 21/2 per cent, by using the 75 coefficient should 
be changed to refer to the Hughes' two-dimensional basis using a 66 
coefficient, one will find a form effect of 161/2 per cent. On the 
other hand, when one investigates what the equivalent plank would be 
for the model - that is a plank having the same length as their model 
and the same measured surface as their model - one finds the mini- 
mum value to be some 22 1/2 per cent. This throws doubt on the 
Prohaska method which is insecurely based, in my opinion, on the 
unjustifiable assumption that the specific resistance initially varies 
as the fourth power of the Froude number. Older work, and certain- 
ly the very early work of Hovgaard, Taylor and others, appeared to 
produce a certainunanimity inthe finding that the specific resistance 
initially varies as the square of the Froude number. One has just the 
choice between the fourth and the second power and if the Prohaska 
method is used with either of these assumptions one finds that using 
the square relation gives a much smaller form effect than using the 
fourth power of the Froude number. It is suggested therefore that the 
method is not sufficiently acceptable for determining form effect ; and 
when this is also associated with the basic defect of the ITTC line in 
its inability correctly to extrapolate to the ship, I think we have to 
fall back on the only alternative way of correctly determining a form 
effect, which is to have a geosim series for each particular ship. 
Against this it is always held that a geosim series is much too expen- 
sive, and this is certainly true. But I suggest that the cost of truth 
has to be faced and in the general euphoria is soon forgotten, 
I would say, in conclusion, that I have greatly enjoyed this 
paper and strongly recommend it to all who are interested in the sub- 
ject. 
1947 
