766 REPORT— 1897. 



heating cun-ent circuits, together with a similar pair from the mercury cups at the 

 ends of the platinum potential leads, to three pairs of mercury cups on the table 

 near the telescope and scale. The readings for magnetising current, heating 

 current, and E.M.F. over the specimen could be very conveniently and quickly 

 obtained by dipping the ends of the galvanometer leads into each pair of cups in 

 turn. 



TUHSDAT, AUGUST 24. 

 The following Papers were read : — 



1. Some Tests on the Variation oj the Constants of Electricity Siq:>2:)ly 

 Meters tvith Temperature and with Currents. By G. W. D. Ricks. 



Roller Bearhigs. By W. B. Marshall. 



3. Analysis of Speed Trials of Ships. 

 By W. G. Walker, M.Inst.M.E., A.M.Inst.C.E. 



Only about 50 per cent, of the indicated horse-power of the engines of a ship is 

 absorbed in actually propelling the vessel, the other half being wasted in the fric- 

 tion of the machinery and the resistance and sUp of the propeller. The indicated 

 horse-power developed by the engines may be divided into the following five con- 

 stituent parts : — 



1. The power necessary to overcome the friction of the unloaded engines. 



2. The power to overcome the friction due to the working load. 



3. The power to overcome the skin friction of the propeller blades. 



4. The power expended in the slip of the propeller. 



5. The power necessary for the propulsion of the vessel. 



The power necessary for the propulsion of the vessel can be subdivided into 

 two parts. 



1. The power required to overcome the skin friction of the ship. 



2. The power due to the formation of waves. 



The author had carried out a series of progressive speed trials on a river steamer 

 60 feet long. The steam pressure necessary to overcome the friction of the un- 

 loaded engine was equal to about 9 lb. per square inch. The friction due to 

 working load was taken at 7h per cent, of the net power, the net power being 

 obtained by subtracting friction of unloaded engine from the total power, the blade 

 friction was taken at -45 lb. per square foot of blade surface when moving in its 

 helical path at a velocity of 10 feet per second, and for other speeds in the ratio of 

 the square of those speeds to the square of 10. If the first three quantities are 

 subtracted from the indicated horse-power there remains a quantity the sum of the 

 power spent in the action and reaction of the propeller ; from this remainder was 

 subtracted the slip, and the final remainder was the power required to propel the 

 vessel. This final power divided by the net power is a measure of the efficiency of 

 the screw. Taking the results for speed of vessels at seven miles per hour, which 

 was the working speed we have, initial friction equals 15 per cent, of the I.H.P., 

 friction of load 6 per cent, of I.H.P., friction of screw .3-4 per cent, of I.H.P., 

 slip of screw 25 per cent, of I.H.P., propulsion 50 per cent, of I.PI.P., skin friction 

 of vessel 27 per cent, of I.H.P., power lost in wave formation 23 per cent, of I.H.P. 

 The (general shape of the efficiency curve of the propeller is almost the same for all 

 screws • beino- zero at zero speed, it rises to a maximum at a certain speed, and 

 afterwards falls oft' with further increase of speed. The object is to design a pro- 

 peller so that its maximum efficiency occurs at the working speed of the vessel. 



