THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 171 
is to increase the nominal block coefficient, but the wake will not increase but will 
remain practically the same as with the initial draught; while should the draught 
be increased the nominal block coefficient will be decreased but, as in the opposite 
case, the wake will remain practically unchanged. 
Having corrected for varying ratios of length to beam and of draught to 
beam, it might be considered that the coast had been cleared and that a position 
had been reached where the propeller could be taken up, but this is not the case. 
There is still a variable, and that a very important one, to take care of, namely, 
varying ratios of lengths of after body, or if preferred to so call it, length of run to 
draught. This variable has a very strong effect on the wake, varying both the 
character and the quality of the flow of water to the propeller. 
All of the varying dimensions now having been covered, it will be found that 
still another correction must be made, and that is the correction for type of hull. 
The types of hulls are sharply defined by the character of the after-body lines as 
the hull fines towards the stern, whether this fining is produced by a rapid dead 
rise of the bottom, with a very slow decrease in beam at the load-water line, or 
whether it is produced by a rapid decrease in beam. The latter type can be sub- 
divided into two types, the first having a full and the second a fine midship section, 
the first tending to concave body section lines and the second to convex. In the 
first subtype the propellers are to a great extent, or entirely, covered by the sub- 
merged hull, while in the second subtype the propeller may be entirely outside the 
limits of the load-water plane. The first subtype will be spoken of here as ‘Hulls of 
Type 1” and those of the second subtype as “Hulls of Type 3.” 
Where the hull fines rapidly by means of a rapid rise of the bottom towards 
the stern the vessel is denoted as having a ‘‘Hull of Type 2.” 
As examples of the different types the following may be taken: 
Type z.—U. S. dreadnaughts and destroyers. 
Type 2.—Vessels of the ordinary fantail form of stern such as U. S. colliers 
and the ordinary type of merchant ship, also submarines. 
Type 3.—Vessels having the so-called cruiser stern; gunboats Annapolis and 
Sacramento, battleships of the Virginia class, battle cruisers, scouts. 
METHOD OF MAKING HULL CORRECTIONS (FIGS. I AND 2, PLATES 71 AND 72). 
The determination of after-body form, or, as it may be expressed, the deter- 
mination of the slip block coefficient of a ship, together with the effect of the varia- 
tions of ratios of length, beam and draught, can be expressed graphically. With 
the exception of the correction for ratio of length of after body to draught, which 
correction is used rather for the correction of apparent slip rather than of slip block 
coefficient, the preliminary estimate of this coefficient, together with its correction 
for ratio of draught to beam and of length on load water line to beam, for all the 
various hull conditions which may be considered as of normal form, are shown on 
Figs. 1 and 14, Plates 71 and 72. 
On Fig. 1, the line X passes through the points whose abscissas are nominal 
