Problems of Commercial Hydrofoils 257 
550 passengers was calculated as a function of speed in order to determine the limit from 
a commercial point of view. In doing this the following conditions were assumed: 
Load factor: 60 percent (330 passengers) 
Ticket price per nautical mile (in correspondence 
with the average economy class fares for 
airplanes): 7 cts 
Operating hours per year: 2000 
Range: 500 nautical miles 
Total weight of turbine including gear and 
accessories: 1.55 pounds/hp 
Specific fuel consumption: 0.6 pounds/hp 
Purchasing price of turbine including gear and 
accessories: $42/hp. 
For the fixed costs the same assumptions as in Figs. 2 and 17 were made. The 
specific power requirements shown are based on measurements in various boats of the 
Schertel-Sachsenberg system. It turned out that the power coefficient Ne/A uf given on a 
logarithmic scale over the Froude number V/V2 (t= foil distance) lies on a nearly straight 
line for all hydrofoil boats constructed, so that reliable figures are available. Values of a 
much lower order given in other theoretical treatises, which neglect the fact that after a 
relatively short operating time the foils are no longer hydraulically smooth, are not realistic. 
In Fig. 3 the curves of profitableness and of the boat’s power in consideration of the 
increasing displacement (turbine and fuel weight) are plotted against speed. The number of 
turbines to be installed was based on the Bristol Siddeley Olympus turbine with 22,700 hp 
maximum and 17,500 hp continuous output. Under the assumptions made, the best profit is 
obtained at about 50 knots. With increasing speed earning power reduces and one may con- 
sider 85 knots as the utmost limit at which the boat can still render a profit under otherwise 
favorable conditions. The limit of technical practicability, however, can probably be 
expected at 75 knots at the present stage of the art when observing the requirements of the 
classification committees and the regulations of the London Ship Safety Convention, espe- 
cially with reference to electrical installations, auxiliaries, and safety installations. Apart 
from the fact that a larger engine plant can hardly be properly installed, the amount of fuel 
necessary for greater speeds increases at such a rate that the number of passengers neces- 
sary for obtaining a profit can no longer be maintained. 
Figure 1 shows that 38 knots was chosen as the lower limit of application of the com- 
mercial hydrofoil boat of the given size (160 feet) and 80 knots as the upper limit. Exceed- 
ing this limit under economically acceptable conditions, for the case in question, seems 
possible only if hydrofoil boats with better lift/drag ratios or lighter engines with less fuel 
consumption are developed in future. In the speed range beyond 80 knots the superiority of 
the airplane — as far as power requirement is concerned —increases continuously with the 
speed. 
The diagram also contains the dotted curve of the “flown” values of ground effect 
machines (hovercraft) which operate at an altitude of about 5 percent of the craft’s diameter. 
Because only scant results are available, this curve does not pretend to be correct. Data 
of projects which have not yet been completed or power figures at altitudes of less than 
0.05 diameter are not represented. A comparison between the hydrofoil boat and the hover- 
craft is justified only if the flight altitude of the latter would be great enough that both 
types can manage the same height of waves. However, it can be assumed that the respec- 
tive curve will be considerably improved in the course of development. 
