Page 351 equipment and instruments 4453 



4453. Use of Logs in Hydrographic Surveying 



In ship hydrography controlled by dead reckoning or astronomic observations it 

 is important to use two logs, which are read simultaneously to the nearest 0.01 nautical 

 mile at each position. The mean of the distances given by the two logs, after their 

 respective log factors have been applied, is used except when the results indicate that 

 one has been functioning incorrectly. The use of two logs provides a check and is a 

 safety factor in case one of them functions improperly. 



When a taffrail log is used in ship hydrography, the ship's crew must be prohibited 

 from throwing overboard rags or other materials which might foul the rotator. 



4454. Calibration of Logs 



Logs shall be rated at the following times: at the beginning of each season's work; 

 every 3 months during the field season if used to any appreciable extent in hydro- 

 graphy, except R.A.R.; and at any time there are indications that the rate of any log 

 has changed appreciably. 



A log factor is determined by comparing a distance as measured by the log with the 

 true distance, assuming that no current has been encountered during the calibration 

 run. This operation is similar to the calibration of the taut- wire apparatus described 

 in 4467, except for the disturbing influence of the current on log measurements. Any 

 one of the four methods described in 4467 may be used to calibrate logs. 



Logs should be rated at the speed of vessel at which they will be used. This will 

 generally be the standard speed used in navigation and hydrography. (The extensive 

 use of echo sounding has practically eliminated handlead sounding at slow speeds in 

 areas where the use of a log would be necessary.) If it is expected that the logs will 

 be needed for speeds other than standard speed, however, they should be also rated 

 at the slower speeds. The taftrail and submerged logs and the revolution counter shall 

 be rated on all runs made for determining log factors. 



In determining log factors, the same precautions are required and the maneuvers 

 should be performed with the same accuracy as in calibrating the taut-wire apparatus. 

 In addition, certain other precautions are necessary, principally owing to the effect of 

 the current on the log measurements. A calibration run must be started far enough 

 from the beginning of the course, to permit the taffrail log-s to assume their normal 

 rotation for that speed. Around turns tafl'rail logs follow an arc described by a shorter 

 radius than that followed by the ship, and for that reason are rotating at a slower speed 

 than normal at such times. The course to be followed during the calibration must be 

 steered for several minutes before the initial end of the calibration distance is reached. 

 A proficient helmsman must be at the wheel to steer a straight course throughout the 

 run. The logs and the revolution counter must be read accurately and at the instant 

 of the mark, and the times recorded. 



Logs should preferably be rated where there is no current whatsoever, but such a 

 condition is rarely obtainable. In an area where current is to be expected the calibration 

 runs should be made at a time near slack water and the course selected should be 

 parallel to the direction of the current. Cross-ciu"rents should be avoided. 



A current which does not exceed 0.25 knot may be disregarded in log calibration 

 runs. Stated in another way, when the dift'erence between the log distances for con- 

 secutive runs in opposite directions is not more than 5 percent of the distance, the 

 current may be ignored. 



