245 
Problems of Commercial Hydrofoils 
conditions a maximum reduction of lift occurs when approaching a wave crest, and, on the 
other hand, when approaching a wave trough, orbital motions have the tendency to increase 
the foil’s lift. Consequently the phase of the foil travel is shifted under the effect of inertia 
in relation to the wave contour (by about 120 degrees) and moves against it. Under unfavor- 
able conditions the hull may be forced into the crest of the waves, thereby reducing the 
speed of the craft considerably. 
The amplitude of orbital motions reduces at a progressive rate, with increasing submer- 
gence. A fully-submerged foil is therefore exposed to a smaller amplitude than the average 
encountered by a comparable surface-piercing foil. Since, as already explained, the maxi- 
mum lift coefficient will also be usually higher, a fully-submerged foil suffers less from 
orbital motion. Apart from that, the control device can be adapted to compensate the orbital 
influence by corresponding variation of lift. 
Conclusion Regarding Practical Applications 
After having discussed the characteristics of the two basic hydrofoil systems we can 
make the following statements in conclusion of the first part of this paper: 
Boats of the surface-piercing hydrofoil type have sufficiently low drag/lift ratios to 
justify their use for high-speed commercial passenger service. Under equal conditions and 
having equal cruising speed they can be expected to reach a higher top speed than vessels 
provided with fully-submerged foils. Natural stability, simplicity of construction, opera- 
tional reliability, ease of handling and maintenance, and, last but not least, a remarkable 
invulnerability of the foil-system have contributed to the acceptance of hydrofoil boats as a 
means of commercial passenger transportation. It can be taken for granted that the described 
qualities will lead to a preference for this type for use on inland waters, in coastal regions, 
and within protected sea areas. 
Fully submerged foil-systems are in the same drag/lift bracket as the surface-piercing 
type. They possess superior seariding qualities and offer higher riding comfort because of 
their smaller and smoother heave and pitch response to sea waves. However, the complex- 
ity of the height- and stability-control which is needed for this type of boat must be con- 
sidered to constitute a serious drawback of this system. No doubt the electronically 
controlled hydrofoil boats, which have been developed in the U.S., have shown excellent 
riding qualities. In spite of this fact there remains for the traditional shipbuilder the 
unusual conception that stability, naturally inherent to any properly built ship from histori- 
cal times, should now be subjected to the faultless functioning of a number of complicated 
gadgets. In case of a failure of such a control system, there may not remain sufficient time 
to cut off the automatic and to “land” the boat by hand as is possible with an airplane. 
Although an unexpected sudden tilting of a commercial vessel will not necessarily result in 
any catastrophic situation, such an incident is liable to destroy the confidence of passen- 
gers. In consequence the introduction of the fully-submerged hydrofoil system in public 
service still presents problems and meets opposition of orthodox shipowners. 
Therefore the requirement exists to maintain the stability of the fully-submerged foil 
type directly by dynamic forces, similar to the surface-piercing foil, without inserting bulky 
and vulnerable mechanical devices or electronic appliances. Based on ideas and experi- 
ments carried out 15 years ago by the author, a new self-controlling system is in develop- 
ment which —not being dependent on servo power sources and amplifiers etc. — can be 
expected to offer sufficient reliability. In heavy seas the system can perform in the manner 
