,) 
W. R. Hopkins on Screw-Propellors. 61 
and the centre of gravity do not coincide. No machinery in re- 
volving works well under these circumstances. 
ut the most important disturbing cause is the following. 
The propellor blades of a vessel on leaving port are set in mo- 
tion in a plane at right angles to the vessel's keel. The ap 0c 
lor blades tend to “persist” im this plane, and the greater their 
Momentum the greater their resistance to: any cause tending to 
aw them from this plane. But the motion of the vessel is a 
Constant disturbing cause, and in resisting the motion of the vessel 
the revolving propellor presses with great force on the bearings. 
-uppose, as in some vessels, the propellor (blades and hub) to 
weigh fifteen tons, Propellors of ,this size have their centres of 
oscillation moved at the rate of thirty-six feet per second when 
in full action. We have then a weight of fifteen tons moving at 
thirty-six feet per second, to be deflected from its line of action 
Whenever the vessel rises or falls. The wear caused by this ac- 
tion has been attempted to be overcome by putting wooden lin- 
mgs in bearings; how far successfully has yet to be shewn. 
It would undoubtedly be better to remove the cause than to 
Temedy the effects, It seems to the writer that the cause may 
easily removed by simply so arranging the propellor blades 
(or the frame in which they are mounted), that the propellor 
blades can keep in the original plane of rotation however the 
vessel may move in a sea way. The plans for effecting this are 
not easily explained without drawings. But means of so ar- 
“anging the propellor blades that they will keep vertical how- 
ever the vessel may move will occur to most persons acquainted 
with machinery. 
