250 H. von Schertel 
ratio is D/ W = 8.6 percent at cruising speed (38 knots) and its minimum is 7.7 percent at a 
speed of 34 knots. The corresponding maximum lift/drag is W/ D = 14.3. 
A special type of PT 20 was designed for the transportation of engineers and workers 
to offshore oil-drilling and pumping stations for oil-producing organizations in tropical 
waters (Fig. 14). In this type, bridge and engine room have been arranged in the foreship in 
order to obtain better vision since tropical waters have an occasional influx of driftwood. 
Fig. 14. A special type of PT 20 for oil-workers in tropical waters 
Tropical conditions were taken into consideration in the design and installment of the 
engine plant, and the propeller has been placed in a specially protected position. Because 
passengers board and leave the boat via the aftship this part has been fitted with particu- 
larly sturdy metal guards. Because these boats are undergoing very rough handling by their 
native crews the requirements with regard to their performance, sturdiness, and maintenance 
are extremely high. Thus, for example, the boats must be able to remain foilborne even 
when sudden full rudder is applied at full speed. Four boats of this special 29-ton type 
have been constructed at the Gusto Shipyard in Schiedam (Holland) for the Shell Petroleum 
Company in Maracaibo. » 
Continued successful and profitable operation with the PT 20 stirred up sufficient 
interest in a larger type of boat to be used in open waters farther away from the coast and 
suitable for interisland services. The prototype of the PT 50 was constructed early in 1958, 
again by the Rodriquez shipyard in Messina (Fig. 15). 
In this 60-ton boat, passengers are placed below the main deck. Thus that deck con- 
tributes considerably to the strength of a hull, allowing structural weight savings. The 
machinery consists of two of the same type of Diesel engines tried out with great success 
during a period of over 4 years in the PT 20 boats. Shafting is simplified as compared with 
the smaller craft by eliminating the V-drive. Both rear and forward foil are rigidly attached 
to the hull, but the lift of the forward foil can be modified by hydraulically operated flaps. 
