3421 HYDROGRAPHIC MANUAL PaGE 254 



soundings. The echo method has replaced the use of the wire and lead almost com- 

 pletely and is rapidly replacmg the handlead. Vertical direct measurements by both 

 handlead and wire are still required for purposes of comparison. The handlead is still 

 most effective ior feeling over shoals and is used on launches or small boats when no 

 echo-sounding apparatus is available. The sounding machine and wire, except for. 

 comparisons, are used mostly in obtaining temperatures and water and bottom samples. 



3421. Handlead Sounding 



Handlead soundings are usually taken from a slowly moving vessel. They are 

 practicable only in depths where the leadsman can heave the lead far enough ahead to 

 obtain a vertical cast from a vessel proceeding at an economic speed. This depth limit 

 will vary considerably, depending on the skill of the leadsman and the height of the 

 sounding chair above the water. Under average conditions in small launches the limit 

 will be approximately 10 fathoms and on ships and auxiliary vessels approximately 15 

 fathoms, although an exceptional leadsman can obtain soundings in depths of 20 fath- 

 oms. On ships and auxiliary vessels where the height of the sounding chair above the 

 water is sufficient, the elevation and the stability permit the leadsman to throw the lead 

 farther ahead than is practicable from lesser heights and from small launches and boats. 

 The end of the leadline should always be made fast to the sounding chair. 



To obtain a sounding the leadsman stationed in a sounding chair heaves the lead 

 far enough ahead to give it time to sink to the bottom just before the leadsman is over 

 that point. A right-handed leadsman coils in his left hand as much line as must be 

 heaved out to obtain a vertical cast. He grasps the line at the toggle with his right 

 hand and swings the lead over his head in a vertical arc for one or two complete circles to 

 give the lead sufficient momentum, releasing it at the bottom of one of the swings so 

 that it is cast ahead in the direction of progress. As the lead is tlirown ahead he pays 

 out the coiled line in his left hand. When the lead strikes the water it begins to sink 

 vertically, but the speed of the vessel is often greater than the rate the lead sinks, so 

 that the leadsman has to gather in the slack line as the vessel approaches the spot 

 where the lead hit the water. If the proper amount of line has been thrown out the 

 lead will have sunk to the bottom just before the vessel arrives over it. The leadsman 

 then hauls the line taut and raises the lead slightly off the bottom to straighten the line 

 and lead, lowering it again to touch the bottom just as the sounding chair comes verti- 

 cally over the spot. Immediately after the depth has been measured the line is hauled 

 in by an assistant, except in shallow depths, and coiled in readiness for the next sounding. 



From a rolling launch or small boat where the sounding chair is only a few feet 

 above the water, the leadsman generally cannot swing the lead over his head; otherwise 

 the procedure is the same. 



After a sounding has been obtained and the line is being hauled in, it trails aft 

 along the side of the vessel. During this time the hydrographer must be careful in 

 changing course not to risk fouling the leadline in the propeller. When a right-handed 

 leadsman is sounding from the starboard sounding chair the course may be changed to 

 the right with impunity, but changes of more than a few degrees to the left should be 

 made with caution, until after the leadline has been hauled clear of the propeller. 



The leadsman should be trained to gage the probable depth from the soundings 

 immediately preceding in order to heave out just the right length of leadline. Too 

 much may be just as objectionable as too little. Wlien sudden changes of depth are 

 expected by the hydrographer, the leadsman should be warned and given an estimate 

 of the amount of shoaling or deepening (see 3234), 



