THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 211 
Attention should be called to one danger that is incurred where I. H. P., or 
S. H. P., are used in obtaining the characteristics of a propeller. This danger is 
produced by an incorrect estimate of K. Should the K used be too small, the 
propeller obtained will be too large while the efficiency promised will be greater 
than will be realized. The propeller will, however, be lifted higher in the safety 
zone than expected. Should K be too large the opposite results occur. 
Propellers of Reduced Diameter. 
In many problems that arise, the permissible diameter of propeller will be too 
small to reduce the revolutions under standard conditions of design to so low a 
number as the problem requires. In such cases it becomes necessary to resort 
to what may be called a method of “reduced diameter.” 
There are two of these methods, as follows:— 
1. Method by changing form of blades to “fantail.” 
2. Method by changing form of blades to “broad tip.” 
By the first method, the propeller, by using the actual effective horse-power 
to be delivered or a modification of the propelling I. H. P., or S. H. P.., is designed 
for a larger diameter and lower number of revolutions than specified. The tips 
are then cut off the blades and the original projected areas retained by widening 
the blade tips beyond the centers of pressure by an amount of area equal to that cut 
off from the tips. This method produces higher pitches, considerably lower revo- 
lutions and slightly less efficiency than the standard method of design and must be 
used when the reduction in revolutions is considerable. 
By the second method, the propeller is designed for a greater diameter than can 
be carried, using a higher effective or engine power than required for the ship. 
The tips are then cut off the blades, reducing them to the diameter allowed. 
This method maintains practically constant efficiency with the basic full 
diameter propeller, with a higher pitch and lower revolutions than would be obtained 
by the regular method of design, but the revolutions are not decreased to the same 
extent as by the first method of reduction. 
Formulas for Reduced Diameter. 
Form 7: “Fantail.” 
(i) Ds iDienan, ORSINI. oo oedcoagdes D D D (Constant) 
(2) 22— Assumed diameters Da sn a: I 2 3 (Variable) 
(Q)) SB IBD SS ae SEUNG AA AN IE EN ET I 2 2 (Variable) 
CANCE SDD) PR HES Ae ANN erent ela Na Ni: Ge I 2 3 (Variable) 
(5) e. h. p. (effective horse-power on one 
FAROE ety) Were y Mra fulve le Uariu ee weed e. hi p- e. h. p. e. h. p. (Constant) 
Now proceed as in preceding methods, using D,, until I. H. P.,’or S. H. P.,' 
have been obtained. The actual corrected power will then be— 
iy JEL, 12 y (Compseusch) oleh, Je aera 
