THE APPLICATION OF DYSON’S METHOD TO PROPELLERS OF OCEAN- 
GOING MERCHANT VESSELS. 
By E. A. STEVENS, Jr., Esg., MEMBER. 
[Read at the thirtieth general meeting of the Society of Naval Architects and Marine Engineers, held in 
New York, November 8 and 9, 1922.] 
During the transactions of this society in 1920, Admiral C. W. Dyson presented a paper 
entitled “The Problem of the Hull and its Screw Propeller,” in which a number of examples 
were shown. These were all navy vessels with the exception of the first problem. It has 
been claimed by many that, while this method is very accurate for these vessels, especially 
those fitted with twin screws, the block coefficients of which do not exceed 0.65, it might 
not be applicable to merchant ships with full lines and fitted with one propeller. The writer 
will present in this article several ships of this type, together with other types of merchant 
ships, and show the results of Dyson’s analysis, together with the results obtained on the 
trial trips. Another argument used against this method is 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 per minute obtained at 
sea. This, however true it may be, is not due to the method but to the way it is used. In 
the latter case, an addition of 20 to 30 per cent should be made to the effective horse-power 
of the ship as found from the model test. 
Before attacking any of the problems it might be well to mention one thing. The basic 
slip (S) is dependent not only on the corrected block coefficient but also on the ratio of 
twice the length of after body divided by the draught. In a few cases coming to the writer’s 
attention, it was impossible to obtain the length of after body. In order to make a reason- 
able estimate of this length, about forty-five ships were plotted with block coefficients as 
abscissas and length of after body divided by length between perpendiculars as ordinates. 
These varied considerably, but a fair average would be as shown in the following table: 
L.A. B. L. A.B. 
B.C. L.B.P. B.C. L.B.P. 
0.60 0.50 0.725 0.407 
.625 494 750 376 
-65 -48 775 .339 
675 46 80 302 
.700 -436 825 265 
If the ship is fitted with a cruiser stern, the length of water line should be used in place 
of length between perpendiculars. 
If the above table is used for obtaining the length of after body, quite accurate results 
will be obtained as, in the majority of cases, it requires a material change in the basic slip 
to appreciably affect the revolutions. 
