90 EXPERIMENTS ON THE FROUDE. 
measured by an indicator spring against which the end of the long arm 
bears. The deflections of this spring are transferred to the recording device 
and recorded there on the strip of paper in the following manner: A wire 
is led directly up from the head of the indicator spring, and over multi- 
plying and guiding pulleys to a pen which traverses across the strip of paper 
of the recording device. ‘This pen is carried by a carriage which runs on a 
horizontal transverse rod with antifriction rollers; it draws a continuous 
curve of deflections of the spring with a scale of about 25 pounds to the inch. 
The entire device is kept taut by a small weight on the free end of the wire 
beyond the pen and its carriage. If the weighing arm is forced down out 
of contact with the spring, the deflection pen is at its zero and the zero-line 
pen can then be correctly set so that the weight for keeping the wire taut 
introduces noerror. Considering that the major part of the thrust is weighed 
by the sliding weight the delicacy of the measurement of the remnant 
thrust by the spring and recording pen is even greater than necessary. 
In order to obtain a satisfactory record of the propeller these various 
devices are used: the propeller shaft is free in its bearings; the thrust-block 
has ball-bearings; the inboard stuffing-box and the outboard steady-bearing 
were reamed to give a very small clearance, and the inboard gland is set 
up very lightly. To exclude water, the shaft-log is filled with oil with a 
slight head so that it oozes out slowly when the boat is running. When the 
boat is at rest, the motor, chain-gear and propeller shaft can easily be spun 
by hand. When underway a change of two pounds effective thrust will 
make the thrust-block crawl forward or astern; this is about three-fourths 
of one per cent. of the thrust at full power. The record of the thrust shows 
a like delicacy, and slight changes of the helm are immediately recognizable. 
The boat is steady on a course and the helm is practically steady during a run. 
As a matter of convenience a Hutchinson electric tachometer made by 
the Industrial Instrument Company of Foxboro, Massachusetts, was con- 
nected with the propeller shaft in the usual way and was rated by the 
observing staff by comparison with the recording device. By its aid the 
boat could readily be run at approximately any desired speed. During 
brake tests the speed of the shaft was controlled by the aid of the tachometer 
which was found satisfactory for this purpose. 
TOWING OF MODEL. 
A model of the Manning one-eighth size (23.5 feet long) was made in 
the Institute model shop and was forwarded to the Navy Yard at Washington. 
Through the interest of Admiral W. L. Capps, U. S. N. (then Chief Con- 
structor of the Navy), permission was granted to test this model and also 
SS eo 
