Problems of Commercial Hydrofoils 237 
and it is well-known that aircraft operation over short distances does not pay very well. 
Consequently hydrofoil craft may successfully compete with airplanes over short distances 
because of their lower costs of operation and maintenance and of their point-to-point rather 
than of airport-to-airport performance. Conditions may be such that a passenger choosing 
the hydrofoil boat will arrive in less time at his destination than he would when choosing 
the airplane. Although the airplane develops many times the speed of the hydrofoil boat the 
journey by air involves the trip to and from the air terminals, which are often far away from 
the original point of departure and destination. Over fairly short distances the helicopter 
might be expected to be a serious competitor of hydrofoil craft. However, as shown in Fig. 1, 
it requires rather high powered engines and its operation and maintenance costs are several 
times those of the hydrofoil boat. Thus in many cases the prevailing conditions will favor 
the use of hydrofoil craft instead of airbome means of transportation. 
Sea-Riding Qualities and Passenger Comfort 
Rapid connection between two points becomes meaningless unless the passenger can 
rely on the timetable: in other words transportation must be regular and not be subject to 
frequent interruptions on account of bad weather. Also the movements and accelerations of 
a vessel caused by adverse weather conditions must be considered. If they assume propor- 
tions which affect the passenger’s comfort, speed must be reduced. 
It has already been stated that the problem of rapid transportation on the water cannot 
be solved by planing boats on account of their buffeting in a seaway. At the cruising speeds 
which are here under consideration accelerations of 6 g and more have been measured in 
such vessels, accelerations which may be endured for a short time by the crew of a naval 
craft but which exclude the use of such boats in commercial operation. 
Principally it can be stated that hydrofoil boats are able to maintain a higher speed 
level in a seaway than any other waterbome craft of similar size. Based on practical experi- 
ences, Fig. 4 shows the approximate size of waves in which hydrofoil boats of the Schertel- 
Sachsenberg system can still remain foilborne. The lower curve represents wave amplitudes 
at which the boats can operate at full power; the upper curve shows wave sizes at which a 
somewhat reduced cruising speed can be maintained while still keeping the vessel in foil- 
borne condition. The diagram permits the estimation of the size of hull required for opera- 
tion in waves of known sizes. With a new type of foil system presently being developed the 
limits indicated in the diagram will be considerably widened. 
Riding comfort in any watercraft is of course affected by the extent of its movements in 
a seaway. Pitching, heaving, and rolling motions of hydrofoil boats are generally much 
smaller than those of similar-sized conventional boats. It can also be stated that the ampli- 
tude of these motions as well as the degree of submergence in the waves decrease with the 
increase of speed and of the frequency with which waves are encountered. This phenomenon 
in the performance of hydrofoil boats is due to a reduction of the influence of the waves on 
the vessel’s inertia and of the orbital wave velocity on the foil’s angle of incidence with 
the frequency of wave encounter. On the other hand, however, vertical accelerations 
increase with speed. In accordance with the linear theory they increase also in proportion 
to the slope of the waves. 
Some indication of the extent of motion in a seaway may be gained from Fig. 5, which is 
based on the latest tests undertaken with a 90-foot boat in waves estimated to have reached 
a height of 5 to 6.5 feet and a length of 100 to 150 feet. The diagram depicts the average 
646551 O—62—_17, 
