174 VARIATIONS OF SHAFT HORSE-POWER, ETC., 



experiments with one propeller only, and that was chosen of the dimensions ordinarily found 

 in practice with this type of vessel. Such propellers ordinarily make 85 to 90 revolutions 

 per minute at a speed of 11 knots, the pitch ratio being about 0.9. 



By reducing" the reyolutions per minute and increasing the diameter, the propulsive efH- 

 ciency can be increased. I estimate that, with the same pitch ratio as used in the experi- 

 ments, to increase the prismatic coefficient from 0.65 to 0.715 — that is, by 10 per cent — it 

 would be necessary to increase the diameter from 16.7 feet to 26 feet, and decrease the revo- 

 lutions per minute at 11 knots from 86 to 44, with a corresponding decrease in slip from 

 about 39 per cent to about 22 per cent. 



As the full-load draught of the vessel is 26 feet, it is apparent that a diameter as large 

 as 26 feet could hardly be used. If the largest diameter that it would be practicable to use 

 is taken as 24 feet, the revolutions per minute would be 49, the slip 25 per cent, and the pro- 

 pulsive coefficient about 70. This would correspond to an increase of propulsive coeffi- 

 cient of something less than 8 per cent of the maximum found in the experiments. 



There is another point about the matter which would require further experiment to de- 

 termine, that is, as the propeller diameter is increased the propeller at the top' of the disc would 

 be working in water which has relatively a much greater wake, and it is probable that the 

 hull efficiency would be somewhat less than that obtained with the smaller propeller. 



In the foregoing discussion it has been assumed, of course, that there is no change in 

 pitch ratio. It is possible that a reduction in pitch ratio and an increase in diameter might 

 permit higher revolutions per minute without much sacrifice in efficiency. 



With reference to the discussion by Admiral Dyson, it is extremely interesting. As is 

 stated by him, his method of propeller analysis and design is based on data covering the per- 

 formances of actual propellers driving actual ships over carefully measured courses. It is 

 naturally gratifying to find that estimates of shaft horse-power as obtained by self-propulsion 

 tests in the Model Basin agree so closely with estimates based on data obtained from the trials 

 of actual ships. The two methods are entirely independent, so that the agreement between 

 the results obtained by each naturally confirms the confidence in the reliability of the two 

 methods. The agreement between the estimates made by Admiral Dyson and the results 

 obtained in the Model Basin in the case of the best model — that is, No. 2133 — which is 

 nearest to the normal form, is remarkable. 



The statement as to the reason for the difiference between the revolutions obtained by the 

 two methods is due to the standard forms of propeller blades used, particularly to the differ- 

 ence in thickness of the blades, the Taylor form used being considerably thinner than the 

 Dyson standard; but even at that the variation in revolutions is not great. 



Answering Mr. Robertson's query as to the apparent and true slip, these were both 

 measured with reference to the pitch of the blade face, the apparent slip being the difiference 

 between the pitch speed of the propeller, that is, the pitch in feet multplied by the revolutions 

 per minute, and the speed of the ship in feet per minute. The true slip is measured in a. 

 similar manner, but the speed of the ship is reduced by the amount of the wake. 



The President: — I am quite sure that you will agree with me that a vote of thanks 

 should be presented to each of these gentlemen, Professor Sadler and Commander McEntee. 

 AH those in favor of a vote of thanks to these gentlemen say "Aye"; contrary-minded, "No." 

 It is unanimously carried. 



Paper No. 11, "Recent Developments in Shipyard Plants," by Commander Sidney 

 M. Henry, C. C, U. S. N., will now be read. 



