408 HYDRAULICS AND ITS APPLICATIONS 



The details of the boats were as follows : 



The hydraulic boat was fitted with a centrifugal pump 2' 6" diameter, 

 running at 428 revolutions per minute, and discharged astern through 

 two 9-inch outlets. The pump delivered approximately one ton of water 

 per second at a velocity of 37'25 feet per second (as against 21*28 feet per 

 second of the boat), and was fitted with a vertical inlet pipe, provided with 

 a scoop facing forward. 



In each case the efficiency of the pump was approximately 48 per cent. 

 The last-mentioned torpedo boats gave the highest jet efficiency, viz., 

 72'8 per cent. The combined efficiency of jet and pump was then 

 34'9 per cent, as compared with an average efficiency of about 

 65 per cent, in the case of a screw propeller. Taking an efficiency 

 of 80 per cent, for the engine and shafting in each case, though this 

 would be slightly greater in the case of the pump, we have the combined 

 efficiency of propelling mechanism 



Hydraulic. Screw 



27*9 



52-0 



The low efficiency of the centrifugal pump has, in the past, prevented 

 the adoption of the system of jet propulsion on any large scale. As a 

 propeller, the jet itself is more efficient than the screw, and there would 

 appear to be no valid reason why, with the more efficient modern types 

 of pump, the combination should not be made as efficient as the screw 

 propeller. 



It should be noted that the system works equally well with the 

 discharge either above or below water level, except that in the former 

 case energy is wasted in lifting the water to the level of the discharge 

 orifice. The advantages of the system are 



(1) No racing of propellers in rough weather ; 



(2) No under water obstacles to become entangled in wreckage ; 



(3) Ease of control from some central station on the bridge ; 



(4) Great facilities for manoeuvring. 



