THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 179 
and assuming that the shaft horse-power equals 92 per cent of the indicated in 
well-built and adjusted engines, a figure which experience demonstrates to be 
nearly correct, basic shaft horse-power = S. H. P. = .o2 X I. H. P. = 11,045 X 
-.92 = I10,161.4. 
3. The propulsive coefficient for a projected area ratio of .34 is seen, from 
Fig. 4, to be P. C. = .667, therefore the basic effective horse-power = E. H. P.= 
EEE PA res Cit OAS xe607 = 17307 
Variation of the above for four blades and for two blades. 
Suppose the propeller should have four blades with a total projected area 
ratio = .453 or a two-bladed one with a projected area ratio = .227, then— 
Blades 4 2 
Rotalye Aw DN Auw— Nise. 227 
Peay = DANI F aa - 34 
that is, the basic P. A. + D. A. equals 34 total for the four blades and equals 3/2 
total for the two blades. 
The values of T. S. and I. T.p are those pertaining to the basic P. A. + D.A., 
while the propulsive coefficients which are dependent upon the total area ratio 
will be those corresponding to the total projected area ratios. 
The formulas for basic I. H. P. become, for four blades:— 
RS ee 74 SO PAS AE 7G LO 
Teel .865 X m X 291.8 wT X 252.41 mee 
for two blades :— 
LHP. aes: OEP Se ey Dax S| TACO SALT LO = fom 
7 X 291.8 7 X 389.1 
The basic effective horse-powers now become :— 
ll 
Four blades; E. H. P. = 12,768 X .585 
Two blades; E. H. P. 
7:470. 
8,283 X .706 = 5,850. 
Should the blades be of other than standard distribution of projected area, 
other corrections occur which the limits of this paper will not allow of discussion. 
Conditions Other than Basic (Fig. 6, Plate 77, Curve 4). 
Usually the performance of a vessel and its propeller will be very remote 
from the basic conditions of the propeller. In such cases it is necessary to have a 
ready means of converting from basic to actual conditions, and such means, so 
far as power is concerned, are provided as follows:— 
