Manoeuvrability and Propulsion of Very Large Tankers 



NOZZLE SHAPE 

 Fig. 22 - Exhaust nozzle 



Q being the volume flow rate through the nozzle. The generated thrust at zero 

 speed amounts to 



For the manoeuvring experiments described in the preceding section, the capa- 

 bilities of an installation supplied with a nozzle exit diameter of D^ = 0.357 m 

 must be seen as representative. 



The plain nozzle system has the drawback that cargo pumps have an unfav- 

 orable head-capacity relationship (high head, low volume flow rate) which re- 

 sults in a large kinetic energy loss in the jet leaving the nozzle. This is the 

 reason why the plain nozzle system has a low thrust per installed pump horse- 

 power of the order of 2-3 kg/hp at zero ship speed compared to 10 kg/hp for a 

 typical tugboat at zero speed. 



The Ejector System 



By placing the exhaust nozzle described above in a tunnel with open ends at 

 both sides of the ship, an ejector system is obtained (Fig, 3). The jet leaving 

 the nozzle mixes with the surrounding water and creates a flow in the tunnel. 

 Since this installation increases the water mass flow rate and reduces the ex- 

 haust velocity a better thrust-horsepower ratio compared with the plain nozzle 

 installation is obtained. From experiments carried out on a special 100-hp 

 ejector test bench at the N.S.M.B., it was found that a thrust increase factor of 

 two over the plain exhaust nozzle can be achieved corresponding to a thrust of 

 4-6 kg/hp for a practical installation at zero ship speed. The main advantage 

 of this system is that a more favorable thrust -horsepower ratio is obtained 

 without sacrificing the basic simplicity and absence of moving parts of the plain 

 nozzle installation. The results of the manoeuvring tests with a nozzle diame- 

 ter D^ = 1.10 m should be seen as typical for the behaviour of such an ejector- 

 driven steering system. 



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