264 H. Von Schertel 
Mr. Newton emphasized that the range of 300 miles given in the paper for our hydrofoil 
boats does not bear any comparison with fast planing craft. May I draw the attention Mr. 
Newton to the fact that the paper only deals with rather short-distance commercial boats 
with a high passenger load. In the case of a planing boat and a hydrofoil boat of equal 
speed and displacement, the range of the hydrofoil craft will be considerably higher on 
account of the much lower motor power and the associated low fuel consumption. 
A. Hadjidakis (Aquavion Holland N.V.) 
There are many things wherein I fully agree with Baron Von Schertel, for instance: that 
the efficient speed for a hydrofoil craft lies between 40 and 50 knots; that the required 
engine power per ton displacement is about 40 b.h.p.; and that seaworthiness increases with 
size (it would be terrible if we disagreed on this point). Our thinking was very much in the 
same direction. Compare for instance Baron Von Schertel’s Fig. 7 with Fig. 7 in my paper. 
However, there are some points where we differ. In the first place, I wonder why the 
foil incidence of your hydrofoils is such that a lift coefficient of only 0.22 is obtained. Is 
this so chosen to prevent cavitation at full speed or perhaps to get better takeoff character- 
istics, or is there some other reason? We use lift coefficients of up to 0.45 at top speed. 
In the second place, it surprised me that a gas turbine engine gives a lower profit if used 
in the PT 20 type, as is indicated in your Fig. 2. In my opinion it should be contrary. Let 
the specific weights be 4.3 kg/hp for the Diesel engine and 1.3 kg/hp for the gas turbine, 
which gives a difference of 3 kg/hp, or for 1375 hp a weight decrease of say 4 tons. 
Because of double fuel consumption, fuel weight will increase say 1.5 tons. But there is 
still 2.5 tons left, which means at least some 25 extra passengers; that is to say an 
increase of income of one third, as the PT 20 carries 75 passengers normally. Furthermore, 
only a small engine room would be required, so there would be more space left for other 
things. 
H. Von Schertel 
The lift coefficient in the example given is only 0.22 because a speed of 45 knots was 
assumed, so that the lift coefficient must be kept rather low to minimize or avoid ventila- 
tion and cavitation. A certain margin must be left between the lift coefficient at which 
ventilation occurs and the C;, value actually used in order to allow for the orbital motion 
in the waves. 
As to the question of operational economics with turbine propulsion for the type PT 20, 
it has to be taken into consideration that the number of passengers is limited by the avail- 
able space in the boat. The turbine-driven boat will be lighter and require a smaller engine 
power, which however does not compensate for the high initial cost and consumption of 
turbines. In my paper I remarked that economy would improve with the turbine boat if pas- 
senger space could be gained by reducing the engine room size. 
E. V. Telfer (Institute for Shipbuilding, Trondheim) 
As a simple naval architect listening to your deliberations I have been a little puzzled 
as to how to present your thoughts nondimensionally so far as these exciting craft are 
