3462 HYDROGRAPHIC MANUAL PaGE 264 



Most modern survey launches are capable of operating steadily at speeds sufficiently 

 slow for handlead sounding. When the slowest speed at which a launch will operate 

 satisfactorily is still too fast for handlead sounding, funnel-shaped canvas drags will be 

 found an efficient means of slowing the launch to the desired speed. They may be 

 towed astern by a simple arrangement of lines to adjust the amount of effective drag 

 or to spill them when desired to make them noneffective. They may also be used to 

 obtain a shorter turning radius than is normal for the launch and to assist in stopping 

 for wire soundings in order to reduce wear on the clutch. For temporary use, if canvas 

 drags are not available, one or two buckets can be substituted to obtain the same results. 



Echo soimdings may be taken at any practical rate of speed. A vessel using echo 

 sounding should be operated at a standard speed which is efficient as to accomplish- 

 ments and is economic as to fuel consumption. The standard speed must be reduced 

 in areas w^here dangers are suspected or where there is danger of grounding. Wlien 

 the interval between fixed positions cannot be shortened, the speed must be reduced 

 so that the proposed sounding lines can be followed reasonably well and there will be 

 no uncertainty in the positions of the intermediate soundings. 



Wlien soundings are obtained between fixed positions a uniform speed of the sound- 

 ing vessel must be maintained between consecutive positions so that the intermediate 

 soundings may be spaced with reasonable accuracy. Changes in speed should be made 

 only at the times of fixed positions, and an additional position should be taken for this 

 purpose, if necessary, as when approaching the shore or when a dangerous shoal area is 

 encountered unexpectedly. In such cases the launch may have to be turned, stopped, 

 or backed instantly and a fix should be obtained at the time this occurs. If it cannot 

 be obtained, the time must be noted. 



The time and amount of any change in speed must be entered in tlie Soimding 

 Record whether or not a position is obtained, the amount being noted in the "Remarks" 

 column. 



The construction and use of gra])hic speed scales in hydrography are described 

 in 4826. 



3462. FoUouing Proposed Lines 



In hydrography, an eft'ort should always be made to follow the proposed lines as 

 indicated on the boat sheet (see 3241). 



The greatest care should be taken to make the actual sounding Ime coincide with 

 the proposed line when the lines are parallel to the coast and, when a position plots oft" 

 such a line, to return to it as soon as practicable. For other lines coincidence is not as 

 essential as is uniformity m spacing over the entire area. In many cases, if a position 

 plots some little distance oft' the proposed line, it may be desirableto change course only 

 enough to parallel the latter and then to shift the entire system of future linens to agree 

 with those actually run. (See also 3141.) 



Where the use of ranges or distance angles is impracticable, the sounding vessel is 

 kept on, or as near as practicable to, the proposed line by small changes in course made 

 after each fixed position has been plotted and the relation of the vessel's position to the 

 proposed sounding line has been determined. It is only after considerable experience 

 has been gained that this can be done quickly and by the correct amount. By the 

 time the data are available on which to base the amount of the change, the vessel has 

 moved beyond the plotted position and this fact must be taken into consideration in 

 judging the amount of change necessary to bring the sounding vessel back on the line. 

 When the positions are fixed by strong three-point fixes, the plotting of which con- 



