780 



REPORT — 1891, 



Generally a hydraulic cylinder and piston will be employed for moving this 

 collar, and the valve for admitting the water to either side of the piston will 

 generally be operated in the engine-room, but may be operated from the bridge. 



5. Action of Scretu Fropellers. By Major R. de Villamil, R.E. 



Resultant action of a screw propeller is similar to a piston with an infinite stroke 

 and velocity ' c' The speed of screw is revolutions multiplied by etlective pitch. 

 Effective pitch = diameter .^pitch ratio. Minimum circumferential velocity, which 



gives a thrust = -J'lijd, 



Most advantageous circumferential velocity = ^^^•- , 



Inert area = ( -, ) • 



where m is pitch ratio. Centre portion of a screw is inert. 



Centre of screw acts as a drag or resistance — hence the ' Thrust deduction factor.' 

 No screw will convert more than 70 per cent, of the power into longitudinal 

 thrust. Thrust of screw depends on revolutions x effective pitch. Methods of 

 improving propellers : — 



1. Adopting a form which feeds itself from the centre. 



2. Forcing water to the centre by ' feeding blades ' on leading side of pro- 

 peller. 



Desgoffe propeller satisfies the first requirement, and shows economy of 25 to 

 30 per cent, in fuel. Feeding blades will reduce or quite eliminate the ' Thrust 

 deduction factor.' 



Tlie differences between theory now proposed and generally accepted theory 

 were considered. 



6. On the Comparative Values of Various Substances used as Non-conducting 

 Coverinr/s for Steam Boilers and Pipes. By W. Hepworth Collins, 

 F.G.S.,'F.G.S., F.B.M.S. 



The author has recently accurately determined the respective non-conducting 

 values of several of the well-known substances and mixtures used as non-conducting 

 material for covering steam boilers and pipes. These results are of much import- 

 ance, more particularly as there does not appear to be any accessible record of an 

 investigation in this country' of a recent or reliable character. 



Table I. 



