Page 509 echo sounding 5512 



nected by means of a stuffing box, a %-inch diameter glass tube, installed vertically 

 with its midpoint at about the normal draft of the ship. The glass tube must be long 

 enough to cover the extreme range of draft. The upper end of the tube should be open 

 to prevent a pressure or vacuum developing above the water in the tube. A gate 

 valve should be incorporated in the lower part of the gage so that the connection may 

 be closed off, except while the gage is being read. 



A scale graduated in feet and tenths, with its zero at the level of the acoustic 

 units, should be secured behind the glass tube, so that the level of the water in the 

 glass tube will indicate on the scale^the draft of the acoustic units. A few drops of 

 oil, or a small cork ball, may be placed on the surface of the water to aid in reading^ 

 the draft. The length of the graduated part of the scale need be only long enough to 

 cover the extreme range in draft of the vessel. 



The scale of the gage can be set originally by test. With the vessel stationary in- 

 calm water, take a series of measurements from the reference marks described above 

 to the surface of the water, compute the draft of the acoustic units, and then set the 

 draft gage scale to this draft. Even though the scale may be adjusted, for use with 

 a particular acoustic unit, or pair of units, it may also be used for the draft of other 

 acoustic units in the vessel, by applying an allowance for the difference in draft of the 

 units. A sufficient number of comparisons must be made over the normal range of 

 the draft of the vessel caused by light or heavy loading. 



Draft marks on the stem and stern of a ship, or internal draft gages installed on some 

 ships, to indicate the ship's draft, also may be used to determine the draft of acoustic 

 units once the correct relationship has been determined. The marks or gages usually 

 indicate the depth of the keel below the water surface at their positions. If the hori- 

 zontal distances between the draft marks, or gages, and the acoustic units and their 

 vertical relationship are known, the draft of the acoustic units may be found by simple 

 proportion. A scale diagram of these relationships may be prepared and used to find 

 the draft of the acoustic units graphically from the gage or draft-mark readings. 



5512. Measurement of Draft 



To determine the draft of the acoustic units from reference points on the ship's 

 rails, the distance between these points and the surface of the water must be measured^ 

 accurately. Accurate leadlines marked in feet and half-feet or light wooden rods witht 

 a scale in feet and tenths may be used for the purpose. The measurements should be 

 made on both sides of the ship simultaneously while the ship is stopped and preferably 

 where the sea is calm. Several measurements should be made. Each pair of meas- 

 urements should be meaned and the average of the pairs subtracted from the vertical 

 distance between the reference points and the acoustic units. The result will be the- 

 draft of the units. 



To determine the draft of the acoustic units from an internal draft gage, it is only 

 necessary to open the gate valve and allow the water to seek its level in the glass tube 

 and then read the draft at the water level from the graduated scale. The ship must 

 be stopped while taking the measurement. The observer's eye should be at the level 

 of the water surface. After the draft has been measured the gate valve should be 

 closed. 



For sounding in depths of less than 10 fathoms the draft should be measured to the. 

 nearest quarter-foot. 



