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THE EFFECT OF WAVES UPON A TAFFRAIL LOG. 
By Proressor Haroyp A. EVERETT, 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.] 
Taffrail logs have had, in general, a bad name for accuracy, though the 
reasons advanced have seemed hardly sufficient to cover the gross errors 
which occasionally appear. In speed trials they have been practically 
abandoned as a means of accurately measuring speed through the water, 
though the need for this information is great. At the Institute of Tech- 
nology we have used them occasionally in conjunction with other means for 
measuring speed and invariably with erratic results. The causes of this 
erratic behavior have been variously ascribed to poor mechanical construc- 
tion, wake, and torsional elasticity of the log line. 
Sketch of F lec tre Log 
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Trials of ‘Greshom' a? Sankey” 
Body. Containing gear redvctan 
and contact making device 2 
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ody from 
Folaling 
Fic. 1. 
Some years ago the Institute purchased a log intended to eliminate the 
first and last sources of error and in order to do away with the second it was 
proposed to tow this log from a long spar projecting from the side of the ship. 
This log, a cut of which is shown above (Fig. 1), had the ordinary form of 
rotator, but it was attached directly to a body which did not rotate and which 
was towed from the taffrail by a long cable with two wires contained therein. 
The rotator drove through a mechanical reduction gear a shaft carrying a 
cam that made and broke the circuit in the cable. This make and break 
by means of an electro-magnet threw a counter in the dial located on board 
the ship which read directly in 2oths of a knot. Runs were made with this 
log towed from the end of a 25-foot boom and still erratic behavior was 
