178 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
Experience has demonstrated that as the lengths of after bodies of ships 
increase or decrease from the standard, the basic values of S decrease or increase 
and, in decreasing or increasing, follow a general law which may be expressed by 
the following equation— 
2) Wy, Ja, 18} 
H 
where A is the length of after body + H at which the basic apparent slip becomes 
zero, while the exponent a depends for its value upon A. The lower scale of abscissa 
is the scale of A upon which the curve of the exponent, a, is erected. The curves 
developed from the equation for coincident values of A and a are erected on values 
of 2L. A. B. + H as abscissas, the scale being that given on the upper scale of 
abscissas, Fig. 5, while the ordinates are values of basic apparent slip, S. 
The curves are direction curves according to which S varies as the ratio 
2L. A. B. + H varies from standard. Thus, suppose the S. B. C. of the vessel 
be .575. By examination of Fig. 5, it is seen that the curve marked A=22.5 
crosses the line of standard S at S. B. C.=.575 and 2L. A. B. + H=16.3. Now 
suppose the ratio 2L. A. B. + H for the vessel in question be 1714. The curve A= 
22.5 is seen to cross the ordinate of 174=2L. A. B. + A, at .113, which is the 
value of S to use in determining the basic condition of the vessel’s propellers. 
A(i—S)*= 
Computation for Basic Condition of Propeller. 
To illustrate the method of determining the basic condition of a propeller, 
let an example be taken, as follows— 
Suppose the corrected S. B. C. of the vessel be .575; the ratio 2L. A. B.+ 
H be 17.5; the propeller blades of standard form and three-bladed. 
Let Diameter = D= 16 feet 
Pitch = P= 18 feet 
Projected area ratio=P. A. + D. A.=.34. 
Then from Fig. 4 we obtain the following data:— 
1. Tip speed for P. A. + D. A. = .34 = 7,480 feet, from which, since Pitch X 
Revolutions = P X R = Posy Our 
aD 
, ee JAK Woo) 
Basic speed = V= Gora 
LS PXASORNE Solan 
Ti) io OTORSa aac MO 
2. Indicated thrust per square inch of disc area = I. T.p = 4.7 for P. A. + 
D. A. = .34, then basic indicated horse-power for a three-bladed propeller = 
PES ORISG LA OUD nL Oe nee ee ee, 
291.8 ii x X 291.8 
_ 7480 X 18 X 4.7 X 16 
a r X 291.8 
I. H.P. = 
= 11,045 
