IN A SINGLE-SCREW CARGO SHIP. 171 



value of over 70 per cent, is only about 60 per cent. In other words, the propulsive efficiencj' 

 is about 20 per cent below what would be olitained from a well-proportioned propeller work- 

 ing under conditions favorable to high efficiency. 



There is an enormous field for improvement here. A little investigation will disclose 

 the fact that it is practically impossible to get satisfactory propeller efficiencies from large, 

 slow vessels unless the revolutions are kept very low and propellers made as large as possible. 

 With reciprocating engines, of course, low revolutions mean a great increase of weight and 

 cost of machinery. It seems to me that here is a great field for the geared engine. If the 

 large, slow^-speed vessel can be fitted with gearing, giving low revolutions adapted to a 

 diameter of propeller limited only by the draught of the vessel, marked economies in power 

 are certain to follow. 



Rear Admiral C. W. Dyson, IT. S. N. (Communicated) : — I have been cognizant 

 for quite a long time of the work being carried on by Commander McEntee along the lines 

 as outlined by him in this paper, but have never heretofore had the opportunity offered me 

 to scrutinize the results obtained by him except in one instance, that of the propeller designed 

 by myself for the "Eagle" boats. 



In this latter case the results obtained in the model tank agreed so closely with the esti- 

 mates of performance as computed by the Bureau of Steam Engineering, and, from indica- 

 tions obtained on the trial of the first of these vessels, so closely with the actual perform- 

 ance as to instill into my mind a great confidence in the accuracy, and a supreme appreciation 

 of the value, of model propeller experiments as now conducted by Commander McEntee. 



Close Agreement of Model Experiments with Estimate of Performance.— Commander 

 McEntee has requested me to analyze the propeller which he used in his experiments and to 

 make an estimate of its performance behind the various hulls which he has used in order to 

 ascertain the degree of agreement between the results obtained by the two methods. 



In order to give a clear understanding it will be necessary for me to give a brief descrip- 

 tion of the method used by me and which has been obtained by many years' study of the per- 

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



The form of propeller blade selected from which to derive the design or performance 

 factors is that of which the projected area is an oval with the greatest circular width at 0.7 

 the radius of the propeller from the center. 



From, these performances a series of basic curves of design has been obtained from 

 which the performance of the piropeller under these basic conditions can be obtained and the 

 performance O'f the propeller under any other conditions of performance derived from this 

 basic performance by the application of suitable factors entailed by the changed conditions. 



The basic conditions' are denoted as follows : — 



I. H. P. = Basic indicated horse-power. 

 S.H.P. = I. H. P. X 0.92 = Basic shaft horse-power. 



P. C. = Basic propulsive coefificient with maximum hull efficiency. (Taken for total pro- 

 jected area ratio.) 

 E. H. P. ^ I. H. P. X P- C. ^ Basic efifective (tow-rope) horse-power. 

 P. A. -=-D. A. = Projected area ratio (outside 0.2 diameter) of 3-bladed basic propeller. 

 4/3 P. A. -f- D. A. = Projected area ratio of 4-bladed screw. 

 2/3 P. A. -^- D. A. := Projected area ratio of 2-bladed screw. 



1 — S = 1 — Basic apparent slip (for 3 blades') and for fullness of after body of vessel. 

 I. T. D. = Basic indicated thrust in pounds per square inch of disc area of the propeller. 



