THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 207 
Determination of the Design Point. 
The data supplied the designer in order to determine upon the proper propeller 
to meet any given set of conditions usually come in one of three forms:— 
1. Required speed, effective horse-power for this speed, and revolutions of 
propeller desired at this speed. 
2. Effective horse-power curve for the ship, designed power of the engines 
and revolutions of the propeller at this power. 
3. Designed power of the engines and revolutions of the propeller, desired 
speed of vessel. 
With each of these are supplied the hull characteristics of the vessel, together 
with distance between center of propeller hub and skin of ship or between center of 
hub and limiting dimensions of the propeller well. 
When data of form 1 are given, the problem consists in determining not only 
the characteristics of the propeller but also the engine power necessary to drive 
the vessel at the required speed. 
When data of form 2 are given, in addition to the characteristics of the pro- 
peller, the maximum speed of vessel which can be obtained with the designed power 
of the engines is also determined. 
Finally, with form 3 data, the propeller designer is in the hands of the naval 
architect. So far as the designer of the propeller is concerned, the ship may or may 
not make the desired speed with the designed power at the designed condition of 
load. He can only feel confident that she will make it at some load condition. To 
call on him for a guaranteed performance is, to say the least, an absurdity, and to 
blame him for failure is to carry injustice to the extreme. 
In selecting the point upon which to base the design, Fig. 7 is used, and the 
preliminary point or points taken will depend upon the type of vessel and the derived 
basic slip of the vessel. The point or points taken should be at sufficient height 
above the cavitation curve for the basic slip and the value of K as to insure that 
for any probable increase in resistance of the hull, due to deep loading, foul bottom, 
or ordinary heavy weather conditions, the value of v+V will not be reduced to 
such an extent as to lower the performance point below the curve of cavitation. 
Ratio of Pitch to Diameter. 
The limitation for this ratio is not definitely known, but for safety it is recom- 
mended that a lower value of about .75 be taken as a limit, although conditions 
may arise where it will be found necessary to go even lower than this value. The 
curve E, Fig. 7, need not be adhered to rigidly as the upper limit of design, but 
this limit and the maximum limit of ae = used for the adopted point of design 
should be regulated by the values of P+D and of P. A.+D. A. which are obtained 
in the calculation. It is undesirable to use a basic value of P. A.+D. A. of less 
than approximately .23 on account of the bluntness of blade sections which result 
with the smaller values. 
