446 HYDROGRAPHIC MANUAL PaGE 352 



If the velocity of the current is too great to be ignored, the log factor must be 

 obtained by solving simultaneous equations based on the two consecutive runs in 

 opposite directions. The fundamental formula is 



Ry±Tx=D 



in which R is the log distance, y is the factor, T is the time interval, x is the velocity 

 of the current, and D is the true distance of the calibration run. 



In using the formula it is assumed that the ship encounters an identical current 

 throughout the double run. For this reason it is important that a calibration run 

 in a current be made at a time when the current is constant, or nearly so, during the 

 entire period of the double run. 



Where the current is weak enough to be ignored, better results will be obtained if 

 the calibration distances are comparatively long — at least 3 or 4 miles. In this case 

 it should be sufficient to steam along the course three times, twice in one direction 

 and once in the other direction. The mean of the first two runs maj^ then be com- 

 pared with the mean of the second and third runs to detect any blunders which may 

 have been made. 



Where there is enough current to be taken into account, the runs should be made 

 along a shorter calibration dista,nce — one of approximately 1 mile. In this case the 

 complete calibration should consist of no less than three double runs. The reason 

 for using a shorter distance where there is current is to shorten the time required for 

 each double run so that there will be less likelihood of a change of current during it. 



Log factors should be determined to three decimal places. Graphic scales for use 

 in applying log factors during hydrographic surveying are described in 4827. 



446. Taut-Wire Apparatus 



The taut-wire apparatus furnishes the most accurate means of measuring hori- 

 zontal distances at sea. The apparatus is constructed so that a wire of small diam- 

 eter, one end of which is anchored on the bottom, passes around a calibrated sheave 

 as the ship steams along the route to be measured. The apparatus requires no mo- 

 tive power. If the anchor does not drag, the wire w^ill remain stationary with refer- 

 ence to the ground, being draw^n off the apparatus as the ship moves ahead. A unique 

 feature of the design, adapted from submarine-cable laying equipment, is a revolving 

 arm that unreels the wire from a large stationary reel so that the wire alwa^^s remains 

 taut. 



The taut-wire apparatus was developed in England and has been used in hj^dro- 

 graphic surveying by the Hydrographic Department of the British Admiralty since 

 about 1921. The first apparatus acquired by the Coast and Geodetic Survey was 

 installed on the ship Oceanographer in March 1933; others have since been installed 

 on five other ships. The apparatus is an important factor in the position accuracy of 

 recent offshore surveys and has made it practicable to extend accurate surveys to 

 increased distances beyond the visibility of shore signals. 



The manufacturer of the taut-wire apparatus, the Telegraph Construction and 

 Maintenance Company, Limited, of East Greenwich, England, makes a large and a 

 small-sized apparatus. They differ in operating principle as well as in the length of 

 wnre contained. The larger apparatus requires considerable space for installation and 

 its use is limited to the larger ships; the smaller is more compact and is adapted for use 

 on board small vessels. 



