172 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
block coefficients, while the ordinates are the ratios B+L. L. W. L. The value 
L. L. W. L. may be replaced by L. B. P. (length between perpendiculars), without 
any sensible error, as the entire method must be considered as one of approximation. 
For the vessels of types I and 2, there is given a curve of coefficient of midship 
section at the top of Fig. 1, for the block coefficients as given by the intersection 
of the diagonal through the plotted point of nominal block coefficient and the 
abscissa value I with line X. Where the coefficient of midship section for any 
block coefficient referred to line X is less than the standard for this referred block 
coefficient as given in Fig. 1, the after-body form approaches the form for type 3. 
Correction for B + L. L. W. L. 
Of the standard hulls chosen on which to base the chart, those of type 3 had 
short middle bodies, which, when eliminated, would have raised the ratio line 
of B + L. L. W. L. to A—B, while those of types 1 and 2 would have raised this 
ratio line to C—D, the nominal block coefficients decreasing in both cases. Now 
let it be supposed that the forward and after bodies remain constant but that the 
length of the middle bodies be varied. As this length increases, the nominal block 
coefficient will increase until at length the effect of the fineness of the forward and 
after bodies on the value of the nominal block coefficients will become inappreciable 
so that the nominal block coefficient as calculated will be unity. On the other 
hand, the middle body may be entirely eliminated when the nominal block co- 
efficient will be that of the combined fore and after bodies only. These changes of — 
nominal block coefficient will take place approximately along straight lines passing 
through the plotted point of nominal block coefficient and the point of 1.0 on the 
scale of abscissas where the ordinate B + L. L. W. L. is zero. The equation to 
these lines is— 
Nominal block coefficient on 
Ba Ea Wa Et 
A CNG 1 OLS olla ie 
Now the line X is for S. B. C.’s of standard twin-screw ships of all the types, 
and the standard B. C. for single-screw ships of type 3 would be that where the line 
through the plotted position of the actual ship crosses the line W, while for type 2 
and type 1 it would be at the point of crossing of the line Z. 
Should the vessel have four shafts, for vessels of standard ratios of H + B 
and 2 L. A. B. + H, the slip B. C. for the outboard propellers will be the inter- 
section of the correction line with the line X, and, for the inner propellers, the inter- 
section with the line Y. Should the ratio 2 L. A. B. + H be much greater than 
the standard, the above corrections will be reversed. As it takes quite a large vari- 
ation in S. B. C. to make a heavy variation in revolutions, this correction for posi- 
tion may be neglected without serious error and the point on X only be considered. 
