JET PKOPULSION 407 



necessary to give this discharge increase with A, the limit of efficiency in 

 this direction is soon reached. 



EXAMPLE. 

 If k = 2'4 and if A = 2 square feet, we have efficiency of jet 



4 4 



7= = = -7-^ = 82'5 per cent. 



' 



Suppose the boat to travel at 15 miles per hour, u = 22 feet per second. 



Then efficiency = -^- = '825 

 u + v 



.'. 1-175 u = -825 v 



... v = 1 ' 17 f < * 22 = 31-33 feet per second. 



*OaO 



Q 31-33 X 2 = 62-66 cubic feet per second. 



Assuming the combined efficiency of engine, pump, and jet to be 

 80 X '60 X '825 = '396, we have : 



. 2-4 X 22 _ 117TTTP 

 - 550 X -396 - 



The most noteworthy experiments on hydraulic propulsion have been 

 carried out by the British Government. In 1866 the Admiralty built 

 two almost similar gunboats, the Waterwitch and the Viper, the former 

 being fitted with jet propulsion and the latter with a screw propeller. 

 Their displacements were, Waterwitch 1,161 tons, Viper 1,180 tons. The 

 Waterwitch took water in through a vertical opening amidships, passed it 

 through a 14-foot centrifugal pump, and discharged astern through two 

 24-inch nozzles. When discharging 5*2 tons of water per second with a 

 relative velocity of 29 feet per second, the engines indicated 760 I.H.P., 

 and gave a speed of 9*3 knots (15*71 feet per second) with a jet efficiency 

 of 70 per cent. The Viper, with 696 I.H.P., gave 9'58 knots. This 

 comparison is, however, rather unfair to the Viper, as its speed suffered 

 from the provision of a double bilge keel, and from its slightly fuller run. 



In 1878, the Swedish Government built two torpedo boats, 58 feet long 

 10' 9" beam and with 20 and 21 tons displacement, the heavier 

 machinery of the hydraulic boat necessitating the larger displacement. 

 The latter boat took in water vertically, and with 78 I.H.P. gave a speed 

 of 8-12 knots. The screw boat with 90 I.H.P. gave 10 knots. 



In 1882, the Admiralty had one of a batch of torpedo boats fitted with 

 hydraulic propulsion. 



