90 THE APPLICATION OF DYSON’S METHOD 
DISCUSSION. 
Tue Presip—Ent:—Gentlemen, Paper No. 3, entitled “The Application of Dyson’s 
Method to Propellers of Ocean-Going Merchant Vessels,’ by Edwin A. Stevens, Jr., Mem- 
ber, is now open for discussion. 
Mr. Witiiam W. Situ, Member:—On page 85, the author states that 20 to 30 per 
cent should be added to the ideal effective horse-power to obtain the effective horse-power 
at sea. I think this is a very good average allowance, but sea conditions vary widely, and 
the power might be considerably more or less. With such widely varying conditions, no 
method of estimating can predict the performance very closely for all conditions. 
Referring to the last paragraph on page 89, it seems to me that a valuable feature of the 
Dyson method is that it can be used with data of this character, which, I regret to say, we 
have to use very often, especially for merchant vessels. 
The author appears to be justified in his conclusion that the Dyson method is quite ac- 
curate for merchant type vessels. The trial results check very well indeed with the estimated 
performance. I would suppose from the nature of these trials that there would be appreciable 
errors of measurements, etc., which no doubt accounts for some of the differences. 
These analyses are further proof of the reliability and accuracy of the Dyson method. 
As far as I know, it is the only completely organized practical method for designing and 
analysing propellers. Also, it is the only method where the estimated performance checks 
closely with the trial results. Further it is the only method where size, speed and cavitation 
are fully provided for. 
Propeller estimates to be reliable must be based on standard data from actual trials. 
Model tests alone can never be conclusive without the proof of trial analyses of this kind. 
Model data would increase greatly in value if verified in this manner. 
Trial analyses of this kind are of great practical value in estimating the performance 
of propellers, and it is hoped that the Society will encourage members to present papers of this 
nature. 
Mr. Stevens deserves the thanks of the Society for preparing this very interesting and 
valuable paper. 
REAR ADMIRAL CHARLES W. Dyson, Member of Council (Communicated) :—Refer- 
ring to the first paragraph where it is stated that the method has been criticised because that, 
while the anticipated power and revolutions obtained by the calculation compare favorably 
with those developed on the trial trip, the same cannot be said regarding the power and 
revolutions obtained at sea, the author’s explanation of this is perfectly satisfactory. It is 
very queer that people cannot understand that, where a propeller is designed for a certain con- 
dition, the revolutions and power estimated for this condition cannot be expected to be realized 
when the ship is loaded down several feet deeper in the water than on the trial and designed 
conditions. : 
Referring to the effect of spectacle frames, my experience has been that, where the fin 
of the spectacle frame is horizontal, or nearly so, the basic slip of the hull is very much de- 
creased from that which would exist were struts used instead of the spectacle. Also the K 
factor is reduced so that the new K factor equals the normal K factor of the hull to 1/7 
