108 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
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 hmiting thrust that it will bear, and if 
we 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 toward 
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 horsepower of 1,000 to 12,000 and upward, 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, 
as cheaply for electro-chemical purposes as it can be produced at 
Niagara and some other large centers of water power. 
The chief items in which saving has resulted as compared with 
the reciprocating engine are: The total capital cost of the station is 
reduced by from 25 per cent to 40 per cent; the reduction in the 
cost of fuel and boilers is between 10 per cent and 30 per cent, and 
the consumption of oil is reduced to one-sixth, while the engine-room 
staff is reduced by 25 per cent to 50 per cent. 
As to the economic results of turbine vessels compared with vessels 
propelled with piston engines, reliable statistics are available. 
In 1897 the 7urbinia was found to have an economy in steam per 
horsepower developed equal to, if not superior to, that of similar 
vessels propelled by reciprocating engines; and later, in 1903, she was 
again tried with modified propellers as now generally used which 
gave a further increase of efficiency of about 10 per cent over the 
1897 trials. 
Tn 1902 the first turbine passenger boat, Aing Hdward, on the Clyde, 
was found to consume about 15 per cent less coal than a similar vessel 
propelled by triple expansion engines and twin screws. 
In the diagram (pl. vt) is shown the principal running expenses 
of the turbine steamer Queen, plying between Dover and Calais, 
compared with three other vessels on the same service. The cost of 
coal, engine-room staff, and oil are shown in terms of the number of 
passengers each vessel is capable of carrying. 
