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AUTOMATIC RECORD OF PROPELLER ACTION IN AN 
ELECTRICALLY PROPELLED VESSEL. 
By W. L. R. Emmet, Eso., MEMBER. 
{Read at the eighteenth general meeting of the Society of Naval Architects and Marine Engineers, held in 
New York, November 16 and 17, 1911.] 
In studying the problems connected with electric ship propulsion, the 
author has encountered some contradictions and differences of opinion con- 
cerning the conditions necessary for satisfactory reversal. ‘The discussion 
of these matters and the lack of definite information which has been dis- 
covered has led to the making of the experiments which are here recorded, 
and although the case is not a representative one, the data being definite and 
certain have some engineering interest as a basis of comparison. 
The fireboat Graeme Stewart is one of two boats owned by the City of 
Chicago. They are equipped with General Electric turbines which drive 
centrifugal fire pumps. These turbines are also connected to direct current 
generators and each of the twin screw propellers is driven by a motor. The 
fields of these motors are separately energized from a constant potential 
exciting generator and speed changes are accomplished by changing or 
reversing the field excitation of the generators. 
The automatic record of conditions shown by the accompanying curve 
sheets was taken by recording ammeters, one of which is shown in the illus- 
tration. With this instrument the record of current variations is drawn 
upon a strip of paper and arrangements are made by which records of equal 
time intervals and of propeller revolutions are marked upon the same record. 
In addition to these records, which are easily provided for, a propeller log was 
rigged on an outrigger near the bow of the boat and connected through an 
electric circuit in such a manner that a mark was made upon the record 
strip every time the propeller made a revolution. The log was calibrated by 
repeated runs over established distances on the Chicago water front and the 
results of this calibration are shown by the accompanying curve. As this 
curve indicates, the speed of the vessel through the water can be accurately 
determined from the marks on the paper down to a speed of about 150 feet 
per minute. 
Theaccompanying cutsof these automatic records show a rather irregular 
line for the current record. The current was regulated by the use of the field 
