Page 265 hydrography 3463 



siunes only a small part of the total time interval between positions, the problem is 

 comparatively easy. As the three-point fixes become weaker and there is less reliability 

 in the plotted positions, and as their plotting requires a larger proportion of the total 

 time interval, it is increasingly difficult to judge whether to make a change in course and, 

 if so, how much. 



In R.A.R. the problem is more complex. The position data are usually not avail- 

 able until 4 or 5 minutes after each position and, if the position plots off the line, by 

 the time this is known, the vessel may be much farther off the line if on an erroneous 

 course. Added to this is the fact that the R.A.R. method of control is often not 

 sufficiently precise for the relation between two consecutive positions to be used with 

 assurance for this purpose. The hydrographer must be an expert judge of dead reckon- 

 ing to decide whether or not to trust the R.A.R. data for this purpose. (See 6824.) 



3463. Changes in Course and Turns 



If practicable, a fixed position should be obtained at each change in course greater 

 than 10°, and the effective time of all turns must be entered in the Sounding Record. 



For most survey scales a turn of 90° or less by a small launch may be assumed 

 to occur instantaneously and need not be plotted as a curve. For a ship, a change in 

 course greater than 10° should be. plotted as a curve to represent as nearly as practicable 

 the actual track followed, taking into account the turning radius of the vessel and the 

 time spent in the turn. In such cases the times of starting and completing the turn 

 must be entered in the Sounding Record. 



A large vessel has considerable momentum and docs not start to turn until some 

 time after the rudder has been moved. This tendency of a large vessel to maintain 

 its original course must be taken into account in plotting the vessel's track around 

 turns. When the turning radius of the vessel is known, the curved track around a 

 turn shovdd be plotted backwards from the fix, or position of the vessel, after the com- 

 pletion of the turn. There will then be a gap between the beginning of the turn and 

 the position before the turn which should be connected by a straight line. The mo- 

 mentum of the vessel makes the component of distance traveled in the direction of the 

 original course roughly twice as great as that traveled in the direction of the new course. 

 (See also 7682.) 



The turning radius of each survey vessel at sounding speed should be determined 

 and the amount should be noted in the Descriptive Report of each survey sheet (see 

 842C). The turning radius may be determined by plotting at a large scale successive 

 strong three-point fixes observed as rapidly as practicable, the various movements of 

 the steering wheel being correlated therewith. 



For dead-reckoning or R.A.R. controlled hydrography on large scales the following 

 method may be used to determine the relative position of the ship before and after a 

 turn: A large paper carton or wooden box, or similar object, is thrown overboard at the 

 beginning of the turn and the time noted. It is kept in view until the ship has com- 

 pleted the turn and steadied on the new course when the time is again recorded and the 

 bearing of the carton is observed by pelorus and its distance is determined by a 

 depression angle. 



3464. Sounding in Currents 



It has been definitely established that neither handlead nor wire soundings can be 

 accurately obtained or located when the sounding lines are run across or against strong 

 currents. The measured depths will be invariably greater than the true depths, the 

 error being in direct proportion to the strength of the current and increasing greatly 



