Manoeuvrability and Propulsion of Very Large Tankers 



Fixed and controllable -pitch impellers are the most common prime moving 

 devices located in transverse thrusters. In recent literature the use of cargo- 

 oil pirnips of tankers as bow thruster devices has been suggested. Such a sys- 

 tem has the drawback that the efficiency is low due to the large kinetic energy 

 losses in the slipstream. These losses can be diminished by using an ejector in 

 which a high-velocity jet with a low mass flow rate can be converted into a 

 lower velocity jet with a higher mass flow. Therefore, the authors suggest to 

 use bow and stern thrusters working according to this ejector principle. Figure 

 3 shows schematically an ejector bow thruster. 



Fig. 3 - Arrangement of ejector bow thruster 



2. RESISTANCE AND PROPULSION 



The trend in the design of tankers has been toward large dimensions at 

 nearly constant speed and therefore high-powered ships. The high required 

 shaft horsepower and the nonuniformity of the flow at the propeller disk may 

 lead to vibration troubles and cavitation-erosion damage on the propeller blades. 



In order to minimize the chance of propeller-induced vibration and cavita- 

 tion damage, the inequality of the flow at the propeller disk must be reduced. 

 Investigations performed by van Manen and Kamps (1), Nitzki (2) and Hadler and 

 Cheng (3) have shown that extreme shapes of afterbody for obtaining a vmiform 

 circumferential velocity field in way of the propeller offer favourable prospects. 



As a consequence of the decreasing propulsive efficiency with increasing 

 shaft horsepower of tankers with conventional screws in addition to propeller 

 cavitation and vibration problems, the application of ducted propellers (1,4) 

 tandem propellers, contrarotating propellers (5) and twin-screw devices (1,5,6) 

 has been suggested. From these propulsion devices, the ducted propeller and 

 the tandem propellers will be less expensive and complicated than contrarotating 



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