thrust, when there is any, is objectionable and more or less 

 avoidable ; whereas in the screw the end thrust is desirable and 

 is a matter of the greatest importance, its determination mask- 

 ing altogether the question of the determination of the rotary 

 force. The third reason is, that, in order to provide, if possible, 

 convenient rules for the construction of propeller blades, and 

 partly in consequence of the rough theory of screw and water 

 conceived as bolt and nut, the pitch of the screw has been dragged 

 in, neck and crop, into all sorts of matters with which it has 

 only a very indirect concern. The purpose of this Note involves 

 the determination, approximately, of the rotary force, in screw 

 propellers, and the consequent inference of the end thrust, which 

 is not separately determinable at all ; neither is there any novelty 

 involved in conceiving the thrust of the shaft, and turning 

 moment of the engine, to be in some way connected together, 

 neither has this doctrine been found to be in discordance with 

 the facts of experience. 



Let Fig. i, then, represent in plan, the stream lines of a ship's 

 body, and propeller ; the lines ASA' being the boundaries of a 

 tube of flow. If, for simplicity, and as is usually done in the 

 accepted way of treating of the thrust of a propeller, it be 

 assumed for the present that the velocity of flow is the same at 

 every point of the screw disc at SS, then in accordance with the 

 accepted laws of mechanics and the usual and accepted doctrine 

 of the thrust of propellers, if the velocity at 55 be v, and be 

 greater than K, the speed of the ship ; and if Q be the quantity 

 of water (expressed as the weight of so many cubic feet for 

 instance) flowing per second through the screw disc, the thrust is 



Q ■ lbs., and as this is produced by a flow of Q lbs. it 



is equal to lbs. per lb. of water flowing through the screw 



disc per second. So far the ordinary theory in effect states, 

 that, conceiving the part of the tube of flow ASA' which lies 

 to the right of SS as the nozzle of a jet propeller, which contains 

 the ship, like a fixed obstruction inside the nozzle, the propelling 



