THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 209 
(5) S=(GDK IE a EES ORS) SEIS a2) 
= (SiS Je le) es <i) Ss Il 
SANG OS Sb) eNO sla any Bal PRUE I 2 3 (Variable) 
(26) Rz=(vX 101.33) +(PX(1—s) )= 
J BS Gy AEN ane WOU eo) Sonal ON RMS I 2 3 (Variable) 
Should the revolutions obtained be less than those desired and a good ratio 
of P+ D exist, the revolutions may be increased in two ways, both of which produce 
a reduction in efficiency. 
(a) Retain original diameter and increase values of v+ V. 
(6) Reduce the diameter and retain the original values of v+ V. 
By the (a) process, the projected area ratio will be increased and the values 
of P+D and of s will be reduced, but the safety distance above cavitation will 
increase. 
By the (6) process, the values of the projected area ratios will be increased, 
P-+D will remain constant and s will be increased, while the safety remains as 
originally. 
(c) A third method may also be used; that is, the rise above the original point 
may be made by keeping the effective thrust constant. This will maintain constant 
projected area ratios, reduced efficiency, decrease in s and increase in revolutions, 
but the reduction in efficiency will not be so great as in (a). 
Should the revolutions obtained be higher than those desired, follow the oppo- 
site course to (a), (8) or (c), and the effects will be opposite. Care must be taken 
to preserve sufficient safety clearance above the cavitation curve, and to do this 
it may be necessary to hold the 7 constant and use a lower value of = = 5 : 
Form 2: Data as per (2). 
(1) v (covering range of expected speed) :- 2 
(2) ~Xe. h. p. (total ef. H. P. for v’s).. 2 
(3) 7=number of propellers........:.. n n (Constant) 
(4) e. h. p.=ef. H. P. on one propeller. I 2 3 (Variable) 
Proceed as in form 1. Should the revolutions for the designed power be 
greater or less than those desired, proceed as in (a), (0) or (c). 
The expected speed will be that corresponding to the designed power and 
revolutions as determined by the calculations. 
3 (Variable) 
3 (Variable) 
S 4 
Form 3: Data as per (3). 
(1) 2 S. H. P., (designed shaft horse- 
DONT ME na UAE Sie. val merce § Ssiel lea? Se lal, Paap Sy leblegs (Coase) 
@) i MeSrinsPy (desioned (I) He) P.)\— , 
iO), (Shy TaL Be Se aye Ud Re Re ees lal UIs Ie) Welle lel leas Yelle lees ((Cermsie)) 
(3) (number of propellers).......... nN n n (Const.) 
(ees (na Pon onelpropeller)) Ic Hy Pe) (lye Py a Pe (Const) 
(5); wi(expectedispeed) vo uk v v v (Const.) 
