Size, Type, and Speed of Ships in the Future 31 
Noteworthy here is that Electric Boat Division currently considers 60,000 shp the 
maximum which can be installed per shaft. 
TYPES OF PROPELLERS 
After the numerical data mentioned in the preceeding section, a discussion of propul- 
sion seems desirable. 
A frequently used method of expressing the characteristics of a propeller type is the 
relationship between Bp- 5 and 7p for optimum propeller diameter. These are defined as: 
the design coefficient: 
i i) 2 Sane 
Bp yanisi 2 e J 
the diameter coefficient: 
NDE ie 
Bayt yct. 
This optimum relationship is given in Fig. 4 for various types of propellers, namely, 
(1) supercavitating propellers of the type TMB 3-50, (2) wide-blade propellers of the type 
Gawn 3-110, (3) propellers of the type Wageningen B 4-40, (4) propeller with nozzle rings of the 
type K 4-55, with a length-diameter ratio of 1/2. The range of profitable application ofeach — 
type of propeller is given on the basis of B,. The considerable difference between the 
optimum diameters of supercavitating and conventional screws and screws in nozzle rings 
is noteworthy. 
Since an evaluation of types of propellers on the basis of Bp- 6 can be troublesome for 
those who do not work with this type of data every day, a more convenient and instructive 
method of presentation is given in Figs. 5(a), 5(b), and 5(c) illustrating the general tenden- 
cies of the propellers discussed. 
In Fig. 5(a), the power-rpm relationship is given for various speeds and diameters, as 
derived from the optimum relationship presented in Fig. 4, for supercavitating propellers. 
The cavitation thresholds, according to Tachmindji and Morgan, and the limit of efficiency 
Bp = 3, beyond which this type of propeller would not be utilized, are given for 60, 80, and 
100 knot speeds. Only one line of constant propeller diameter, i.e., D = 1 meter, is given. 
Other lines of constant diameter greater than 1 meter would appear to the right in the dia- 
gram. For reasonable application of a supercavitating screw with a diameter of 1 meter, a 
minimum speed of 54 knots is desirable. 
In Fig. 5(b), the optimum Bp- 6 relationship is presented in the same manner for conven- 
tional screws of the Wageningen B 4-40 type. If the limit of 60,000 shp given by Electric 
Boat is adhered to for installed shaft power, then this appears to lead to inadmissably large 
propeller diameters for speeds of 20 knots. For 40-knot speeds with 250 rpm there still 
appear to be possibilities. 
The same data are presented in Fig. 5(c) for the K 4-55 series operating in ducted 
nozzles with a length-to-diameter ratio of 1/2. The superior properties of this type of 
propeller with respect to maximum installed power for relatively low speedsis clearly evident. 
