170 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
All of the above relate to the hull, and the responsibility for them rests with 
the owner, the builder and the naval architect, all of whom, apparently, are so 
short-sighted as to be able to see only the few dollars saved to-day by compara- 
tively petty economies, losing sight entirely of the thousands of dollars which 
would be realized in the future were the small trial savings of to-day ignored. 
Turning now to the propellers we find :— 
(kh) Blade forms varying according to the taste of the designing engineer. 
(t) Blade sections varying to the same degree as blade forms. 
(j) Variations of pitch in a blade and in the blades of the same propeller. 
(k) Blades abnormally thick and blunt. 
(1) Blades exceedingly rough. 
(m) Blades made with pitch expanding radially or axially, or both. © 
(n) Hub of such form as to cause abnormal eddy losses. 
(0) Propellers so located with respect to the hull of the ship as to produce 
abnormal losses. 
The guilt for all of these rest with the designer of the propeller. The naval 
architect is, however, particeps crimints in the last-named offense. 
3. Derwwation of Fineness of After Body of the Ship.—In making the selection 
of data upon which to base formulas and design data, it is necessary to confine 
consideration to hulls of fine lines, with propellers so placed that the chances of 
decrease in propulsive efficiency due to hull effect are reduced to a minimum. 
The propellers which are fitted to these hulls must all have practically the same 
radial distribution of the projected areas, must have the same general forms of 
blade sections, must be true to pitch, fine-edged and smooth, in order to eliminate 
as many variables as possible from the equations. 
VARIATIONS IN HULLS OF VESSELS. 
Suppose a vessel to have a certain set of forward and of after-body lines with 
a parallel middle body of a certain percentage of length of the vessel. She will 
have a certain block coefficient. Now, maintain the same forward and after-body 
lines but modify the length of the middle body. As the length of the middle body 
increases over the initial length, the block coefficient increases, and vice versa; 
but the after-body lines, which control the character and quality of the flow of 
water to the propeller, have remained as at first. It is impossible, therefore, to 
base propeller conditions on block coefficient, but there must be provided a means 
of bringing measurements of block coefficient to a standard condition. 
In the foregoing example the constants, in addition to the forward and after 
lines, are the beam, B, and the draught, H, while the ratio B+L. L. W. L., or 
B~+L. B. P., is variable. 
Having obtained a means of correcting for the variable, there will imme- 
diately be discovered another condition which contains the same variable as 
before but, in addition, H has become variable. Experience has taught that with 
constant form and area of load-water plane, to decrease the draught of the hull 
