Problems of Commercial Hydrofoils 235 
with each other, all of them having—with regard to the Froude number—the same length of 
160 feet, representing vessels of about 300-ton displacement.* It can be seen that hydrofoil 
boats in the interesting speed range between 40 and 60 knots require only about 55 percent 
of the propelling power of planing vessels, which shows that foil-supported craft can be 
designed for comparatively high speeds and that they can operate at such speeds with rea- 
sonable efficiency. This is to say that at high Froude numbers, hydrofoil systems are 
known to function at drag/weight ratios below these of conventional motorboats. In conclu- 
sion, hydrofoil boats must be expected to be comparatively economical within certain size 
and speed ranges where other types of waterborne craft cannot operate effectively. 
To illustrate this last statement we show in Fig. 2 the earning power of the hydrofoil 
boat Type PT 20 in relation to speed. PT 20 represents a type of boat which today is 
operating in regular passenger service in many parts of the world. This particular diagram 
will be referred to in detail later on. It is shown that under the given service conditions an 
interest of more than 50 percent on the invested capital can be achieved. The diagram, 
which is based on actual experiences, reveals that the hydrofoil boat has succeeded in 
| 
| 
= L 
Effective power 
| 
1500 \yearly operat hours 
60%\passeng. load 
10 Ticket price acts/n.m. | ] 
1 
| | 
| 
| 
| 
t + } 
30 325 35 375 40 425 454N 
Fig. 2. Earning power of the PT 20 type 
hydrofoil boat in relation to speed 
*Some of the notations used in the text and figures are listed at the end of the paper. 
