Page 235 hydrography 3374 



and log reading should be noted both when the ship starts to swing and when she has 

 straightened out on the new course. An officer should observe the compass at such 

 times and mark the time for the recorder; otherwise the time is apt to be recorded too 

 early at the beginning of the turn and too late at the end. For dead reckoning on large 

 scales the ship's course should be plotted as a curve for changes in course larger than 

 15°, using the ship's known turning radius for this purpose (see 7682). 



The courses steered should take into account the effect of the wind, sea, and cur- 

 rent insofar as these effects can be estimated, so that the courses plotted after correc- 

 tion for these effects will coincide with the proposed lines. 



3374. Dead-Reckoning Distances 



It is axiomatic that positions on a short dead-reckoning line will be proportionately 

 more accurate than positions on a longer line. It is important, therefore, to plan dead- 

 reckoning loops so as to be beyond the range of fixed control for as short a time as 

 practicable. 



The faster and more constant the speed of the vessel the better can the dead reckon- 

 ing be maintained. The engine room should be notified when a dead-reckoning line is 

 to be started and warned to maintain the revolutions at as constant a rate as possible. 

 The effect of leeway is minimized in proportion to the speed of the vessel and its applica- 

 tion is easier when the speed is constant. 



Echo sounding now permits soundings to be taken continuously at standard speed. 

 The ship does not have to be stopped nor slowed down for soundings as was formerly 

 necessary. For best results, the speed should be maintained at a constant rate and 

 the ship should never be stopped during a dead-reckoning run. However, other con- 

 siderations may necessitate this, such as the safety of the vessel or the need for obtain- 

 ing serial temperatures at the outer limits of the survey. But so far as the dead 

 reckoning alone is concerned, much more accurate results will be obtained by eliminat- 

 ing all stops and changes of speed. 



At least two accurately rated logs should be used in all dead reckoning (see 4454) , 

 If the ship is equipped with an electric submerged log (see 4452) , several of the difficulties 

 encountered in connection with conventional patent rotating logs will be eliminated. 

 The submerged log records correctly the distance traveled by the ship around turns. 

 When a rotating log is used, log loss can be ignored for changes in course less than 15°, 

 but for larger changes it is necessary to substitute time for log readings around the 

 turns and for a period of several minutes thereafter. Around turns a patent log 

 follows a course through the water shorter than the ship's course and friction on 

 the line is often greater than normal, making the log unreliable at such times. After 

 any change in course larger than 15° the operation of the log should be observed until 

 it is normal, when its reading and the time should be recorded. (See also 3381.) 



If a buoy is passed close-to while patent logs are streamed, the ship should always 

 be kept to leeward of the buoy and not too close to it, because of the danger of fouling 

 the log line on the buoy or its anchor line. Also when patent logs are used on dead- 

 reckoning lines started at buoys the survey ship must be placed on the proposed line 

 4 or 5 minutes running time before the buoy is reached in order for the logs to attain 

 normal operation during this period. 



The revolution counter should be read at positions, so that these readings may be 

 substituted for log readings any time the logs fail to function properly for any reason 

 whatsoever, and to furnish a check on the constancy of the speed. 



