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HYDROGRAPHIC MANUAL 



5-130 Dead reckoning. — Dead reckoning 

 is navigation by account, or reckoning, from 

 the last known position. It is a procedure 

 by which the position of a vessel at any in- 

 stant is determined by applying the ship's 

 run to the last known or well determined 

 position, using for this purpose the course 

 steered and distance traveled. Dead reckon- 

 ing is used to plot track or cruise lines be- 

 tween fixed positions, and is frequently used 

 to aid in evaluating EPI position data. 



Dead reckoning is seldom exact, and the 

 uncertainty of position increases in propor- 

 tion to the distance from the last fixed posi- 

 tion. Errors may be introduced in a number 

 of ways. Those affecting the course include 

 imperfect steering, incorrect allowance for 

 compass error, leeway, and current; and 

 those affecting distance are imperfect logs, 

 unknown log factors, and incorrect allow- 

 ance for current. 



In order to attain greater accuracy a few 

 refinements are applied to dead reckoning 

 practices normally used in standard naviga- 

 tion. On a track line the engine speed should 

 be maintained at a constant rate. On oceano- 

 graphic cruises it is necessary to stop and 

 lay to while making observations and the 

 value of a constant rate of speed is dimin- 

 ished, but constant speed will be useful in 

 making adjustments between fixed positions. 

 The ship should be equipped with an ac- 

 curately rated submerged electric log. The 

 revolution counter should be read at all fixed 

 positions in order that these readings may 

 be substituted if the log fails to function 

 properly. Courses must be steered very 

 closely, within 1° if possible. When a gyro- 

 pilot is not being used, only the most compe- 

 tent helmsmen should be assigned to steer- 

 ing duty. The officer on watch must check 

 the course being steered at frequent inter- 

 vals. The errors of any compass used must 

 be accurately determined. 



A complete record of events shall be kept 

 on Form 722, Dead-Reckoning Abstract, as 

 the line is run. On this abstract all control 

 data should be entered which are to be used 

 in plotting and adjusting the dead-reckoning 

 line. It is particularly important that all 



entries be correctly related to time and that 

 each event be recorded promptly. The data 

 on the abstract, the Loran line of position 

 computations, and the astronomic sights are 

 used to make final adjustments of the plot. 



Dead reckoning can be run most accurately 

 in a dead calm, but this condition seldom 

 exists. Experiments should be made to de- 

 termine the amount of leeway which will be 

 caused by winds of different velocities at 

 various angles to the ship's heading. Graphs 

 can be prepared from these data for use in 

 estimating course changes to compensate for 

 leeway. 



An allowance must also be made for the 

 set and drift of the current. In addition to 

 oceanic currents encountered in many areas, 

 a persistent wind will generate surface cur- 

 rents. An estimate of the wind-driven cur- 

 rent may be based on the following : 



(a) Observations have demonstrated that 

 a persistent wind will set up a wind-driven 

 current with a velocity of approximately 2 

 percent of that of the wind. This ratio can 

 be expected to hold both in coastal areas 

 and in the open ocean. 



(b) The direction of the wind-driven cur- 

 rent in the Northern Hemisphere has been 

 found to be about 20° to the right of the 

 wind in coastal areas, but theoretically this 

 deflection is probably nearer 40° to the right 

 of the wind in the open ocean. The rule for 

 coastal wind-driven currents is not always 

 applicable, especially near the shore where 

 the direction of the current depends on the 

 angle between the wind direction and the 

 coastline. 



The dead reckoning should be corrected 

 for all known factors affecting course and 

 distance before it is plotted. The difference 

 between the dead reckoning position and the 

 position determined by astronomic or other 

 fix is the dead-reckoning closure. This dif- 

 ference is adjusted by distribution in propor- 

 tion to time between fixed positions in the 

 same manner as a traverse adjustment is 

 made with respect to distance. This may not 

 be a straight line adjustment, since sun 

 sights, Loran lines of position, or other re- 

 corded data must be considered and eval- 



