Page:The Steam Turbine.djvu/50

!two-thirds of the disc area of the propeller, so as to present a very large bearing surface on the water, and this expedient effectually prevents its giving way under the force necessary to propel the vessel.

In models, and in vessels of moderate speed, the forces are not sufficient to tear

Table I. Performance of Parsons Turbo-Generators at Different Epochs.
 

Date	Power	Steam per kw. hour	Vacuum (Bar. 30")	Superheat	Steam pressure per sq. inch
	kw.	lb.	in.	Deg. F.	lb.
1885	4	200	0*	0*	60
1888	75	55	0*	0*	100
1892	100	27.00	7	50	100
1900	1250	18.22	28.4	125	130
1902	3000	14.74	27	235	138
1907–1910	5000	13.2	28.8	120	200

* These were non-condensing turbines using saturated steam.

the water asunder, but if the pressure of the atmosphere is removed by an air pump, a model screw will cavitate at a comparatively moderate speed.