3664 HYDROGRAPHIC MANUAL PaGE 284 



3664. Isolated Rocks and Shoals . 



An isolated rock or shoal may be found by a system of closely spaced sounding 

 lines and, when found, its slope to the top may be followed by feeling with the leadline. 

 The use of one or two marker buoys is almost essential for this purpose. On arrival at 

 the area one marker buoy should be anchored at the supposed location of the shoal 

 and another held in readiness in the boat for immediate use when the exact location 

 has been discovered. (See 3665.) 



The lines and the soundings along them must be closely spaced for this purpose 

 and if the leadline is used the progress of the vessel must be slow. It is good practice 

 to use two leadsmen, one on each side of the boat. 



If there is a current and the leadline is used, the most effective method is to let 

 the boat drift along a series of closely spaced parallel lines to cover the suspected area. 

 The boat should be run into the current until it is up-current from the supposed location 

 of the shoal and on one of the lines to be followed. It is then allowed to drift while 

 the leadsmen sound or feel along the bottom with leadlines until it is down-current 

 from the supposed location of the shoal. It is then propelled up-current again and 

 allowed to drift along the next line, and so on until the shoal is found or the area is 

 covered. When the shoal is found the second marker buoy should be thrown over at 

 once. If practicable the lead which has struck the shoal should be left on the bottom, 

 the leadline being payed out while the boat is propelled upstream to a position vertically 

 above the lead, whence by feeling in the vicinity with the leadline the least depth may 

 be determined. 



After the position of the shoalest area has been fairly well determined, a good 

 substitute for the wire drag is to cover the shoal area with drift soundings. Tlu-ee or 

 more leadsmen may be distributed along the side of the boat, each with two leadlines, 

 one in each hand. The boat is maneuvered upstream from the shoal and turned broad- 

 side and allowed to drift over the shoal spot while the area is felt out by all of the 

 leadsmen. If the area is close inshore, ranges should be used to keep track of the 

 courses drifted over. The exact ranges should be noted where the least depth on each 

 line is found. The boat should then be maneuvered upstream and allowed to drift 

 over the shoal again, overlapping the previous area by approximately a half boat 

 length. This should be repeated until the shoal has been entirely covered. 



The position of the least depth has now been closely determined and by means of 

 the observed ranges the boat should be anchored at the spot with the least scope of 

 anchor line which will hold. With the leadsmen placed as before the boat is now 

 sculled in an arc around the anchor. The anchor line is then let out a half boat length 

 and the same procedure followed. The operation is repeated until the least depth 

 is found. 



The leadlines may be prepared by attaching a float to each a few feet farther from 

 the lead than the least depth expected. Then when the rock is found the leadline may 

 be let go, leaving the lead on the bottom with the float marking the spot. With this 

 as a guide, the least depth can be determined in a few minutes by feeling over it. 



3665. Use of Marker Buoys 



In the examination of shoals the use of marker buoys is almost essential not only 

 to mark the position of a shoal after it has been found, but also for reference in maneuver- 



