BY MEANS OF A CURRENT METER. 97 
Figs, 19, 20, and 21, Plate 34, give the results of tests upon Model 5, which was 
tested at only one draught. The results are in general accord with those of Model 3, which 
had much better lines. In testing this model the meter was placed below the line of the keel 
and showed a wake of 5 per cent when down a distance equal to about one-third the draught. 
The left-hand portions of Figs. 19, 20, and 21 give the increase in wake due to variation 
in fore and aft position of the meter wheel. Moving the meter wheel forward from the test 
position to about the position which the propeller would occupy on a single-screw vessel 
increased the wake by about one-fourth. In the twin-screw position the increase due to 
moving forward from the test position amounted to about one-fifth. 
Models 4 and 6 were not tested as extensively as the others. Fig. 23, Plate 35, shows 
the variations of wake for Model 4 as the meter wheel is varied in diameter. It will be 
noticed that the variation is quite marked in the single-screw condition, but that the varia- 
tion is much less in the twin-screw condition. In the single-screw condition the small wheel 
is in a region of maximum wake, but an increase in diameter brings the tip of the wheels out 
into regions of less disturbed water. This causes the average wake value to decrease quite 
rapidly with increase in diameter. In the twin-screw condition increase in diameter causes the 
tips of the blades to reach out into a region of high disturbance on one side and into a region 
of low disturbance on the other, and the result is a fairly uniform value of the wake for 
varying diameter. The results for Models 3 and 5 when plotted upon diameter show this 
same general variation. 
Fig. 25, Plate 35, gives the results of tests upon Model 6. In this case the bossing did 
not seem to decrease the wake on the center line, even though it did cause the model to drive 
easier. In this test the meter was carried down to a depth of 7 inches below the line of the 
keel, and a wake of 2.5 per cent was registered with a 3.75-inch wheel. 
Fig. 27, Plate 36, shows the results of tests with a 3.75-inch meter wheel upon Models 
1, 2, 3, 5, and 6, and also upon a 10-foot varnished plane. The conditions for all the models 
were not exactly the same but were very close, as shown by Table ITI. 
Tas_eE III. 
Length Be ais = Cp Cyr \1— aa as 
WL | Length 
Ft. Ins. Ins. 
Model 1......... 10.0 16.0 6.96 2. 535 735 272 50 
Qi vcastsee 10.0 17.83 7.2 2.48 71 .85 . 165 47 
Senso ae 10.4 16.9 6.81 2.48 80 88 .09 .376 
Departent neta: 10.0 16.3 7.07 2.3 .80 87 .08 30 
Ghios aobedn 10.1 15.72 7.0 2.25 .83 .86 , 034 20 
Varnished plane. . 10.0 0.75 7.0 0.1 
wee oe LE a 
Fig. 27 would seem to indicate that certain variations in wake values can be linked 
up with the vertical prismatic coefficient (see appendix). This quantity will give an indica- 
-tion of the fineness of the lower water lines which control the direction of flow of a consid- 
Ce 
WL 
will be a function of the virtual rise 
erable part of the wake water, and the quantity 1 — 
