19UG.] on The Steam Turhine on Land and at Sea. 831 



separate shafts, arises from the fact that smaller propellers may be 

 used, making higher speeds of rotation admissible, which again acts in 

 lightening and improving the economy of the turbines. 



The second question, that of the propeller, was much more 

 difficult. It was not simply the problem of designing a screw with a 

 moderate slip ratio and a moderate loss by skin-friction of the blades 

 in the water, but it was complicated by cavitation, or the hollowing out 

 of the water and the production of vacuous cavities caused by the 

 force of the blades tearing through the water, a phenomenon first 

 noticed by Sir John Thornycroft and Mr. Sidney Barnaby in 1893, 

 and by them named cavitation. This apparatus shows the phe- 

 nomenon. 



[A small tank was shown, with a model of the screw of a cross- 

 Channel boat or of an Atlantic turbine liner. It was pointed out 

 that it was very difficult to make the screw cavitate, because it was 

 especially designed not to cavitate : it was, however, made to do so 

 in the tank by removing the atmospheric pressure from the surface of 

 the water above the propeller by the air-pump. The removal of the 

 atmospheric pressure, which helped to keep the water solid, enabled 

 cavitation to be induced at a much lower speed of revolution. In the 

 tank there was a head of about 1^ inch of Avater above the topmost 

 blades. If the tank had not been exhausted there would have l)een a 

 head equivalent to 32 feet, plus 1| inch, plus capillary forces, tending 

 to keep the water solid. Tlierefore, instead of 1500 revolutions (the 

 speed of the propeller when serious cavitation was induced) a speed of 

 at least 20,000 revolutions would have been required (because forces 

 that induce cavitation vary as the square of the surface-speeds of the 

 blades).] Serious cavitation causes an inordinate loss of power, chiefly 

 because it disturbs the steam lines around the propeller blades, and 

 it was proved by this experiment how easy it is to put too much work 

 on a screw. There is a limiting thrust that it will bear, and if Ave 

 exceed this thrust it will, so to speak, more or less strip its thread in 

 the water and its efficiency will rapidly fall. The solution of the 

 problem, as regards the screw propeller, has therefore resulted in a 

 modification of the proportions of the ordinary propeller, and has lain 

 in the direction of smaller diameters, wider blades, and a slightly 

 finer pitch-ratio, which three slight changes have combined towards 

 higher angular speeds of the propeller without material loss of 

 efficiency. 



Let us now turn our attention to the economic results of the 

 steam turbine. In the case of large engines and dynamos that are 

 coming generally into use, for the generation of electricity in this and 

 other countries, of a horse-power of 1000 to 12,000 and upwards, the 

 steam turbine with its accompanying dynamo is found to be cheaper 

 in first cost, running expenses, and fuel, than the reciprocating engine 

 and its slow-speed dynamo ; and so much is this the case that it seems 

 possible to generate electricity in colliery districts almost, if not quite, 



