van Manen, Oosterveld and Witte 



A further increase of thruster efficiency is to be expected from a two-stage 

 ejector design (Fig. 23). By using the exhaust of a small primary ejector as the 

 nozzle for a big secondary ejector, the mass flow is increased further. It is ex- 

 pected that a thrust of 6-8 kb/hp can be attained at zero ship speed with a cargo- 

 pump-driven system of this configuration. Since the exhaust velocity of the pri- 

 mary nozzle is of the order of 30-40 m/sec, cavitation in the primary ejector 

 may create problems. 



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fZLl.J_S^, 



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^^ 



V////////T7 



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THRUST VECTOR 



Fig. 23 - Arrangement of two-stage ejector system 



5. CONCLUSIONS 



The main conclusions of this feasibility study are: 



The extremely cigar -shaped afterbody is favourable from a viewpoint of 

 resistance. This is contrary to cigar-shaped sterns investigated up till now. 

 The EHP of the tanker with the extremely cigar-shaped afterbody is about 1 

 percent less in the loaded condition and 5 percent less in the light condition, 

 compared to the conventionally shaped tanker. 



The extremely cigar-shaped stern with shrouded propeller having a large 

 hub-to-diameter ratio, as suggested in the introduction, offers a means of im- 

 proving the propulsive efficiency for large tankers in addition to minimizing 

 vibration and cavitation problems. The Hogner-type stern with nozzle No. 1 

 requires about 6 percent less SHP in the loaded condition and 9 percent less 

 SHP in the light condition than the conventionally shaped stern. This improve- 

 ment is of the same magnitude of an extreme bulbous bow, which leads to a 15 

 percent SHP reduction in the light condition. 



The experiments show a definite advantage of the lateral thrust arrange- 

 ment over the conventional rudder for ship speeds up to 5 knots. Tests using 



268 



