337 HYDROGRAPHIC MANUAL PaGE 232 



ship where the tracks of both ships are plotted on the boat sheet. Unexpected shoal 

 soundings should also be radioed to the R.A.R. ship. 



To assist the auxiliary vessel in maintaining station, all changes in course made 

 by the R.A.R. ship should be communicated to the auxiliary vessel by flag signals or 

 by radio. 



At the end of a pair of sounding lines a standard maneuver should be followed to 

 place both ships on the next pair of sounding lines to be run, without risk of collision. 

 In such operations the auxiliary vessel is always kept on the same side of the 

 survey ship regardless of the du-ection of the lines. In turning at the ends of lines to 

 start a new pair of lines, assuming that the turn is to the left, the right-hand ship turns 

 about 90° to the left, passing astern of the other ship, which maintains its course for a 

 distance equal to the spacing between lines. The latter then turns 90° to the left 

 and runs to the next adjacent sounding line, turning again 90° to the left and steadying 

 on the new course. Meanwhile the ship which was to the right origmally has continued 

 on its course for a distance equal to three times the spacing between lines and then 

 turns to the left onto the new course. When this maneuver is correctly executed the 

 ships will be exactly abeam at all turns and at a distance from one another equal to 

 the spacing between lines. By maintaining relative distance by rangefinder and 

 executing the turns accurately both are in position to start the next pair of lines very 

 close to the desired positions. To make a similar turn to the right the maneuvers of 

 the two ships are, of course, exactly reversed. 



When it is necessary to turn onto new lines where the above maneuvers are im- 

 practicable, the auxiliary vessel should be instructed by radio to fall back to the quarter 

 or astern of the survey ship and maintain position there until the survey ship has got 

 into the position desired to start the new lines. The auxiliary vessel can then take 

 her position. 



Some disadvantages of the method are: 



(a) That it cannot be used at night or when the two ships are not clearly inter- 

 visible. 



(b) The officer-in-charge of the auxiliary vessel cannot plot his data at the time, 

 nor know how much development is required, nor whether it is being achieved. His 

 job is merely to follow instructions. 



(c) More than ordinary care and judgment are required in making turns where a 

 close spacing of lines is required. 



337. Dead Reckoning 



Dead reckoning in hydrographic surveying is neither more nor less than dead 

 reckoning as practiced in navigation, with refinements. Dead reckoning enters, in 

 more or less degree, into practically all of the methods for determining the position of 

 the survey vessel. Rarely is each individual sounding fixed in position, and when this 

 is not done, the positions of the intermediate soundings are determined by dead reckoning 

 with reference to the fixed positions. Dead reckoning is used to verify positions of 

 doubtful accuracy, and in Radio Acoustic Ranging it is frequently used to aid in 

 evaluating the data (see 7634). 



Dead reckoning is navigation by account, or reckoning, from the last known position. 

 It is the procedure by which the position of a vessel at any instant is determined by 

 applying the ship's run to the last well-determined position, using for the purpose the 

 course steered and the distance traveled, generally as indicated by log. . 



