A STUDY OF THE WAKE OF CERTAIN MODELS BY MEANS OF A CURRENT 
METER. 
By Proressor E. M. Bracc, MEMBER. 
[Read at the thirtieth general meeting of the Society of Naval Architects and Marine Engineers, held in 
New York, November 8 and 9, 1922.] 
Our knowledge of the wake of models has been obtained by means of Pitot tubes and 
from experiments upon self-propelled models. 
The objection to the Pitot tube is that it gives the wake value for a filament of water 
while the propeller works in a column of water whose average wake value is wanted, and 
the integrated value of the filament wakes is uncertain. 
The self-propelled model results give values of wakes for conditions similar to those in 
which the full-sized propeller is to work, but any extensive search of the wake involves 
a great amount of labor and expense. Further, the wake value depends upon the assump- 
tion that, given the relation between thrust, slip, and speed of advance for a propeller tested 
in the “open” condition, the same thrust and speed of advance when working behind a model 
is accompanied by the same slip. This may be very nearly true in the case of twin screws 
behind fine models, but I doubt very much if it is true for single screws behind models with 
full sterns. 
In the “open” test the water is flowing parallel to the center line of the screw shaft and 
the velocity is uniform over the area swept by the screw. In the “behind’’ test the water is 
closing in and rising up around the stern of the model, the flow is anything but uniform 
in the area swept by the propeller, and the conditions are such that the propeller efficiency may 
be greater or less than that obtained in the “open” test, as results given later will show. 
The results given in this paper were obtained by means of a current meter fitted with 
wheels of varying diameter. A photograph of the meter and wheels is shown on Plate 29. 
The experiments extended over a considerable length of time, and it could not be foreseen 
in the beginning just what conditions might be met with later on. A position for the meter 
wheel was chosen which could be obtained in any model which might be available for 
testing purposes. This position will be referred to as the “test” position and was such that 
the hub of the meter wheel was 0.375-inch back of the rudder post. 
When the meter wheel was off to one side of the center line of the model there was 
probably a slight slewing of the wake in that direction, due to the obstruction offered by the 
meter wheel, but I do not believe it was sufficient to affect the results materially. 
Since the meter wheels were not exactly in the position that the propellers would occupy, 
it should be kept in mind that the wake values here given are for comparative purposes 
only, and certain corrections would have to be made for fore and aft position and probably 
for scale before they could be used for design purposes. 
The models were tested at speeds ranging from 140 feet per minute to 200 feet per 
minute in the case of Models 1130, 1131, 3,4, and 5; from 150 to 250 feet per minute in 
the case of the 10-foot plane and Models 1 and 2; and from 130 to 175 feet per minute in 
