344 F. H. Todd 
In order to have some actual figures on which to base a discussion, calculations have 
been made of the horsepower required for both surface and submarine ships of displacements 
from 25,000 tons up to 150,000 tons. From these figures estimates have been made of the 
powers required for surface and submarine ships of the same deadweight, as this is the only 
fair way of assessing their respective merits. This particular range of displacement was 
chosen because the lowest figure represents the smallest ship for which marine nuclear 
installations at present envisaged would be suitable and the highest figure is comparable 
with the largest tankers at present being built and having deadweights in the neighbourhood 
of 100,000 tons. 
For the surface ships the results have been taken from the David Taylor Model Basin 
Series 60 [2]. For a number of displacements within the above range the EHP has been cal- 
culated for Series 60 models having block coefficients of 0.60, 0.70, and 0.80. The ships 
all have a length to beam ratio of 6.5 and a beam to draft ratio of 3.0. The ship powers have 
been estimated from the model results using the A.T.T.C. 1947 line with a correlation allow- 
ance AC of +0.0004. In designing ships of different speeds, it would be natural to use a 
finer block coefficient for the higher speeds, and the comparisons have therefore been made 
at a speed appropriate to each fulness. These speeds have been determined from the 
modified Alexander formula 
V 
MV. 2 (1.06 = Cp). 
In order to convert these EHP values into DHP at the propeller, it is necessary to make 
certain assumptions regarding the propulsive efficiency and appendage allowances. In the 
first place, it has been assumed that where the DHP is less than 40,000, the ship will be 
propelled by a single screw, where it is between 40,000 and 80,000, it will have twin 
screws, and above this higher figure, will have four propellers. For the single screw ships 
a quasi-propulsive coefficient of 0.72 has been used, in accordance with the experiments 
with Series 60, and no allowance has been made for appendages. It is assumed, in effect, 
that with modern large ships of all-welded construction, the allowance of AC; = +0.0004 
will be sufficient to take account of any ordinary appendage resistance as well as any hull 
roughness effects. In such cases, therefore, the DHP has been taken as equal to the EHP 
divided by the QPC. For the twin and quadruple screw ships, the QPC has been taken as 
0.68 and an appendage allowance of 10 percent and 20 percent respectively has been added 
to take account of the bossings or shafts and A-brackets in these arrangements. 
For the submarines, estimates have been made for two different prismatic coefficients, 
namely 0.60 and 0.65, using various published works by Weinblum, Amtsberg, Crago, etc. 
These apply to streamlined bodies of revolution having their maximum diameter at a point 
40 percent from the nose. Again, the skin-friction resistance of the ship has been estimated 
by using the A.T.T.C. 1947 line and including an allowance of AC = +0.0004. The 
length/diameter ratio has been taken as 7.0, which is approximately the optimum for vessels 
of this form having the appropriate tail surfaces to give adequate directional stability. 
The dimensions of the surface ships and submarines for the displacement ranges 
covered are shown in Tables 1 and 2 and Fig. 1, and charts of EHP/V® in Figs. 2 through 6. 
The EHP values for submarines of different displacements are also listed in Table 3, and 
those for surface tankers in Table 4. This latter table also includes estimates from NPL 
data, which show close agreement with those made from the Series 60. As will be seen in 
making the comparison for vessels of equal deadweight, the use of a circular submarine form 
leads to very unrealistic dimensions as regards draft. In order to be able to assess the 
