Problems of Commercial Hydrofoils 255 
Figure 17 presents an analysis of actual commercial experience for the type PT 20 
(assuming 2 boats in service) based on prevailing European conditions and a price of 6 
cts/kg for Diesel oil and 53 cts/kg for lubricating oil. The graph indicates the yearly net 
return to be expected over ticket price (cents per nautical mile) multiplied by the load fac- 
tor (number of passengers per available seats) . As a function of three parameters the graph 
thus enables anticipation of whether commercial application of hydrofoil boats would be 
profitable under given local conditions. Assuming a fare of 8 cents per nautical mile, a 
load factor of 0.5, and 1500 operating hours per year the graph shows a yearly net profit of 
slightly more than 30 percent of the invested capital. 
& 70 D 
Pied th 
NENe 
a 
Soe | 
15 25 } ho GS del es ; 6 65 (ct) 
ticket price [ct /nm] x passeng.load factor 
Fig. 17. Commercial experience for the PT 20 
The diagram is based on a cruising speed of 36 knots. In order to determine the influ- 
ence of speed on profits and also the economical speed of the vessel at which the most 
favorable relation between costs and revenues is obtained the curve depicted in Fig. 17 was 
computed under the following suppositions: 
Load factor: 60 percent 
Fare: 8 cts/naut mi 
specific weight: 4.3 kg/hp 
Diesel engine < specific fuel consumption: 166 g/hp/hr 
price of engine: $26/hp 
specific weight: 1.3 kg/hp 
Turbine specific fuel consumption: 320 g/hp/hr 
price of engine: $44/hp. 
The engine weights include gears and all auxiliaries as well as ducts, pipings, etc. The 
highest profit is obtained with either Diesel or turbine propulsion at a cruising speed of 40 
knots. Although the engine output at that speed is nearly 60 percent higher than at 30knots 
the remarkable profit is obtained by 33 percent higher revenues in accordance with the equal 
increase in passenger-miles. On account of the higher price of the engine and its higher 
