Problems of Commercial Hydrofoils 247 
engine is located in the stern of the hull for emergency operation. Driving a small separate 
propeller, this engine can be used to maneuver the boat in displacement condition. 
Fig. 11. The PT 20 hydrofoil boat 
The general arrangement of the PT 20 type is shown in Fig. 12. The boat is operated 
and the machinery controlled entirely from the bridge which is located above the engine 
room. Forty-five passengers can be accomodated in the forward cabin, while the rear cabin 
is somewhat smaller and can take 30 passengers. 
Together with the struts and a horizontal girder each foil forms a uniform framework 
which, apart from giving static advantages, facilitates the exchange of the foil structure. 
The foils themselves are made from medium steel as shells welded at the seams. The for- 
ward foil can be tilted with narrow limits by means of a hydraulic ram acting on the girder 
across the hull. It is therefore possible to adjust the angle of attack of that foil during 
operation, thus counteracting the effect of larger variations in passenger loads and to ensure 
optimum behavior in seawaves. 
The PT 20 can be considered to be the smallest type of foilborne craft suitable for 
passenger-carrying coastal service. In view of its novel and unconventional characteristics 
the first boat of its class was originally licensed as “experimental” and restricted to opera- 
tion in a 6-mile zone off the coast of Italy. After frequent inspections and supervision of 
the service, the Registro Italiano Navale extended the license in 1958 to operation within a 
20-mile coastal zone and in 1960 within a 50-mile limit. Today the PT 20 as well as the 
PT 50 have been classed by the authorities of several countries. 
Technical data which have an influence on the profitable operation of the type PT 20 
are presented in Fig. 13. The shaft power of the engine was measured during operation by 
means of a torque meter. The propeller was model-tested, thereby taking proper account of 
the influence of shaft inclination. The drag/lift ratio was then derived from net power. This 
