174 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
line crosses X, which is at an abscissa value of .585, will be the corresponding 
corrected value of the B. C. for wing screws, while where it crosses W, at .64, will 
be the S. B. C. for a single-screw vessel or for the center propeller of a multiple- 
shaft vessel with an odd number of shafts. 
Should the vessel be fitted with wing propellers, a correction should be made 
for variation of midship-section coefficient from standard, by multiplying the 
value of the S. B. C., line X, by the ratio actual midship-section coefficient + 
standard coefficient. 
Vessel B, Type 2. 
Turning now to vessel B, passing the line through its plotted point, nominal 
B. C.=.7, B+L. L. W. L.=.13, and abscissa value I.0, and extending the line 
to C—D, it will be found to cross this line at the B. C. value of .582. On the 
curve marked H~B, condition 2, standard, the value of H+B is found to be .423, 
therefore the actual value of H+B being greater than this standard, turn to 
Fig. ta and plot the point H—B=.5, B. C.=.582, above the lower standard 
H—B curve, and from this plotted point follow the curves of direction down to 
this standard line. It will intersect this line at B. C.=.675. Through B. C.= 
.675 on C—D and abscissa unity, again draw a line. Where it intersects X, at 
B. C.=.697, will be the S. B. C. for twin propellers, while where it intersects Z, 
at .797, will be the S. B. C. for a single or center propeller vessel. 
Vessel C, Type 7. 
In vessels of type 1, the determination of S. B. C. is made as for type 2. 
With vessels of this type, the character of the wake appears to be nearly 
similar to that for type 2, while quality is more nearly like that of type 3. 
By “character of wake” is meant the degree of obliquity of flow of the water 
to the propeller disc, while by “quality of wake”’ is designated the degree of dis- 
turbance of the wake and general decrease in its density due to contained air in the 
water in which the propeller operates. 
Conditions of wake are the most favorable with ships of type I, less so with 
ships of type 3, and decidedly unfavorable with ships of type 2, although it may 
be claimed that vessels of this latter type carry much heavier wakes than do the 
other types and more gain should therefore be realized from the wake than with 
the other two types. This claim would be a just one were the quality of the wake 
with the vessels of this type, with which designers ordinarily have to deal, equal 
to that experienced with vessels of the other types. 
It has been attempted in the foregoing to give a slight idea of the difficulties 
in arriving at the very foundation of the screw-propeller problem before the pro- 
peller can even be approached. It is hoped that it has also shown the absurdity 
of basing any propeller design upon nominal block coefficient without correction. 
In connection with this the author had the rather amusing experience of being 
approached by a gentleman with the question as to how he “would determine 
the slip block coefficient of a vessel to use by looking at her in the drydock.” The 
