224 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
Answer: No correction is made. 
3. Would it not be as accurate and less complicated to use the prismatic in place 
of the block coefficient? 
Answer: I do not believe complication would be any the less should the prismatic 
in place of the block coefficient be used, as in either case we would have to apply the 
correction for variation of ratios of draught to beam for ratio of length of after body 
to draught and for length of middle body. 
4. To what type would high-speed yachts, torpedo boats, and the earlier destroyers 
belong? The fining of the after body of these vessels is produced neither by a rapid 
dead rise of the after sections nor by a rapid decrease of beam, but by the raising of the 
keel. As the propellers of these vessels are, to a great extent, covered by the submerged 
hull, the inference might be that they belong to type r. 
Answer: Practically all of these boats are of type 1. 
5. What is meant by a full midship section? Is it one that is as full or fuller 
than the standard as shown on Plate 71; and is a fine midship section one that is consid- 
erably finer than this standard? 
Answer: A full midship section is one that is as full or fuller than the standard 
as shown on Plate 71. A fine midship section is one that is considerably finer than this 
standard. ‘The curve of midship sections on Plate 71 fits vessels of types 1 and 2. 
6. How would Plate 76 be applied to ships of type 2? Although one of this type 
is shown in Problem 1, the method of getting the slip is not explained. 
Answer: The curve marked ‘‘Slips for S.B.C. on X—W, Types 1 and 3 Hulls” 
has marked on it values of S.B.C. To find the basic apparent slip for any S.B.C. with 
any given ratio of length of after body to H, pass a direction curve through the point for 
this S.B.C. as marked on the first-mentioned curve. Where this direction curve inter- 
sects the ordinate through the given ratio will be the basic apparent slip to use. 
Mr. Stevens says: ‘“‘It would appear that when a propeller is to be designed for a 
ship, where the effective horse-power is not given but has to be estimated, or if the 
value of 2 L.A.B.+H is not known, the original method (of design) is about as good 
(as the new), at least for ships of this class. For small single-screw steam yachts, with 
a large ratio of beam to length and with small midship-section coefficient, the method 
as shown in this paper would, I believe, be decidedly more accurate.”’ 
My experience up to date has indicated that the second method gives approxi- 
mately the same results as to power as were obtained by my original method but it is 
considered more accurate in determination of revolutions and in designing the screw 
so as to plot more surely in the safety zone. 
Where the value of twice the length of after body divided by H is not known and 
we have reason to believe that the after body is not abnormally long as compared to the 
draught nor abnormally short as compared to the draught, the two cross curves of 
Plate 76 marked for vessels of type 2 and vessels of types 1 and 3 can be used. 
Following is given a table of ship characteristics with the type to which each vessel 
belongs, and as performances of different vessels become available their screws should 
be analyzed in order to determine accurately the type the hulls belong to and add to 
the table. By doing so it will be found that it will aid very much in the future, as there 
are many cases where it is quite difficult to determine whether a vessel is of type 1 or 
type 3, or whether she is type 2 or type 1, although the latter distinction is easier 
