230 SELECTION OF THE BEST KIND OF PROPELLING MACHINERY. 
found, taking into consideration the smaller all-round year horse-power required for the 
motor ship, due to the better immersion of screws under all conditions of draught and more 
uniform running conditions, that the motor ship would be appreciably smaller than the equiv- 
alent steamer, resulting in a smaller investment of capital and a higher earning rate on 
same. (See Plate 64.) 
Mr. W. W. Smitru, Member:—On page 226 the author states that weight is the least 
important of the economic factors. From my study of the subject, I conclude that weight 
is, next to fuel consumption, the most important factor. Since the cost per pound for this 
class of machinery does not vary greatly, the first cost is largely a function of the weight. 
In addition, the deadweight and the cargo-earning capacity are reduced in proportion to the 
weight of machinery for deadweight vessels. 
With reference to loading, the class of cargo determines whether it is a cubic or dead- 
weight vessel. Certainly there are many vessels which are loaded to the draught marks, 
including tankers, colliers, ore carriers, general cargo vessels handling weighty cargo, etc. It 
would seem, therefore, that the author’s statement in this respect is not entirely correct. 
Referring to the propulsive efficiency on page 226, the twin-screw vessel will probably be 
slightly more efficient, but the difference will probably be not more than 3 or 4 per cent. 
Referring to the problems in Mr. Stevens’ paper where the speed is about 11.5 knots, we 
have for single-screw vessels propulsive efficiencies based on shaft horse-power as follows: 
(1) .62; (3) .645; (4) .595. The only twin-screw vessel of this speed is problem 13, where 
the efficiency is .54, which, however, is probably low. These values do not show that single- 
screw vessels are particularly inefficient. 
In comparing the propulsive efficiencies of single- and twin-screw vessels, it should be 
remembered that, while the single-screw loses considerably due to a high effective slip ratio, 
it, on the other hand, gains greatly due to the higher hull efficiency. 
With reference to revolutions, increasing the revolutions for given conditions of speed 
and power will reduce the propulsive efficiency. This effect is well illustrated by the p—é 
diagrams in Admiral Taylor’s book. Thus the tendency to high revolutions for Diesel en- 
gines results in reducing the propulsive efficiency. 
Referring to the advantages pointed out on page 227 for double-acting engines, we have 
made a number of comparisons which agree with this. In addition to the larger powers pos- 
sible, these engines are much lighter and should therefore cost less. 
Referring to Plate 63 the data given are too general in character, and there are not suffi- 
cient particulars to make a really accurate comparison; a description of the machinery in- 
stallations is desirable. The types and makes of engines, turbines, gears, etc., will have an 
important bearing. 
In addition to the general type of machinery, the design and conditions are important. 
There may be wide variations in weight, cost and efficiency due to these causes. For ex- 
ample, the fuel oil consumption of a geared turbine vessel may vary from .85 to 1.3, de- 
pending on these factors of which we are not informed. The weight and cost may vary like- 
wise. The revolutions will influence the weight and efficiency considerably. 
In making a comparison of this kind, the best designs and conditions should be selected. 
Apparently this has not been done for the 2,500 shaft horse-power geared turbines. At any 
rate, the results given are inferior to those which we have obtained with certain designs of 
Parsons turbines. 
