260 H. Von Schertel 
system, and that the behavior of a boat fitted with fully-submerged foils controlled by a 
predicting sensor would be different. I tried to give the advantages of fully-submerged 
foils as far as | could in a paper of the present length. 
L. S. Snell (The De Havilland Engine Company Limited) 
Baron Von Schertel makes certain assumptions in Fig. 17 when comparing the perform- 
ance of the Diesel engine and gas turbine. He goes on to say that “conditions will be 
changed if prices and consumption figures for turbines can be further reduced in the future.” 
It must be remembered that though the specific fuel consumption (S.F.C.) of the Diesel 
engine can be taken as generally constant at about 0.4 lb/b.h.p.-hr, a wide variety of gas 
turbines with equally widely differing performances can be produced. The S.F.C. quoted in 
the paper has already been improved upon, though unfortunately at the expense of cost and 
bulk. Figure Dl shows schematically three engine types recently evaluated for a small 
high-performance craft. That on the right utilizes a simple open cycle and its design owed 
much to aero engine practice. The efficiency of this simple cycle can be improved by rais- 
ing the turbine inlet temperature and increasing the compression ratio, if this can be 
associated with an increase in compressor efficiency. Modification to this cycle is desir- 
able if good economy is the main aim. The center illustration shows the addition of a heat 
exchanger which recovers some of the exhaust heat previously lost, while the remaining 
diagram depicts a quite sophisticated arrangement using three stages of compression with 
inter-coolers and exhaust heat regeneration. 
Figure D2 shows the performance expected from each of these arrangements compared 
with that obtained from the conventional Diesel engine. The component efficiencies and 
maximum cycle temperatures used in these calculations are quite modest and are those 
liable to be encountered in a marine engine rather than an aero engine. Examination of 
these curves indicates a most significant point, that is whereas the Diesel engine has a 
consistently good specific fuel consumption over a very wide load range, the simpler gas 
turbines markedly worsen as the throttle is closed. 
The author said that after using practically full power to get on the hydrofoils, cruise 
then takes place at about 55 percent maximum power. Cruise economy at this point is, 
therefore, the main consideration in the choice of prime mover. ‘Unfortunately the choice of 
a simple open cycle working on high compression ratios and high turbine inlet temperatures 
can be most misleading as, although the S.F'.C. is considerably improved at near 100 per- 
cent full power, at 55 percent the S.F.C. is not materially better. This state of affairs 
might well be more of a problem for the present generation of hydrofoil craft as it would 
appear that as cruising speeds increase so the cruise power will proportionately increase. 
A near ideal solution would be to evolve a variable-mass-flow engine fitted with an 
exhaust heat regenerator. With decreasing throttle, mass flow would similarly decrease, and 
if a near-constant pressure ratio and turbine inlet temperature was employed, the thermal 
ratio of the heat exchanger would be markedly improved and, coupled with reduced ducting 
losses, would give an increasingly better S.F'.C. with closure of the throttle. 
During the course of the analysis previously referred to, account was taken of the 
additional space available not only for the gas turbine compared with the Diesel engine but 
for the varying amounts of space which could be saved with the different gas turbine con- 
figurations and their associated fuel tanks. Surprisingly enough, the higher fuel consumption 
