100 A STUDY OF THE WAKE OF CERTAIN MODELS 
When self-propelled models show different wake values for inward and outward turning 
screws it is not because there is an actual difference in the wake, but rather because there is 
a virtual change in the pitch of the propellers due to the direction of flow in the wake. 
A study of the wake curve for Model 6 in Fig. 27 will show that wake values derived 
from the tests of center-line screws on self-propelled models of full form must be subject to 
considerable error. When tested in the “open’’ the water is fed to the screw with uniform 
velocity. The central portion of the screw does not contribute much to the thrust, due to the 
alteration in virtual pitch caused by the thick blade sections at the root. The slip angle 
is slight anyway, and there may even be a negative thrust over the inner part of the thick- 
bladed screws. 
If the water were fed to the central portion at a slower rate than to the outer portion, 
the thrust would be increased and the efficiency of the screw improved. This is exactly what 
happens to a screw whose center is located in the region of maximum wake. Fig. 27 gives 
the variation in wake in a vertical plane passing through the center line of the model. In 
a horizontal plane passing through the center of the screw the wake is a maximum at the 
center line and decreases rather rapidly as one leaves the center line. Thus we may have in 
a full model such conditions of flow in the wake that the propeller may develop a greater 
efficiency than when tested in the “open.” 
In the case of Model 6 the benefit to be derived from lowering the propeller from an 
elevation of 0.4 the draught, an ordinary position, to an elevation of 0.2 the draught can be 
easily seen. We know from experience that great improvement follows such lowering of the 
propeller, and these results show that it is not due alone to placing the propeller where it will 
not react so strongly on the hull, but is due in part to improved efficiency of propeller. 
As regards the extension of wake values obtained from models to full-sized ships, there 
is a division of opinion as to whether they ought to be increased or decreased. Eminent au- 
thorities claim that the wake is larger in models than in corresponding ships. This is hardly 
in accord with our treatment of surface friction in short and long planes. 
In the case of surface friction it is assumed that the coefficient of friction for a short 
plane will be greater than for a long plane, due to the fact that the long plane is in contact 
with the water for a longer time and has a chance to give a greater forward velocity to the 
adjacent water. This causes the average rubbing velocity of the long plane to be less than 
for a short plane, and a smaller coefficient is used for the long plane. 
The greater forward velocity of the adjacent water at the aft end of a long plane would 
mean a greater wake value for a long plane than for a short plane. 
This study of the wake of models, while not extensive enough to be absolutely conclu- 
sive, would seem to warrant the statement that any complete system for determining wake 
values must take account of the following conditions: 
1. The diameter of the screw relative to the draught of the ship. 
2. The draught of the ship relative to the breadth of the ship. 
. The fore and aft position of the screw. 
. The transverse position of the screw. : 
. The vertical position of the screw relative to the keel. 
. The vertical prismatic coefficient of the ship. 
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