5511 HYDROGRAPHIC MANUAL PaGE 508 



draft gage may be installed by which the draft of the acoustic units may be read directly, 

 or the draft marks on the bow and stern or on the sides of a vessel may be used to find 

 the draft of the acoustic units. A third method involves the use of permanently 

 marked reference points at convenient locations on the main rail or deck of the vessel. 

 In the latter method, knowing the vertical distance of the reference points above the 

 acoustic units, their draft at any time may be determined by measuring the vertical 

 distance of the reference points above the water surface and taking the difference 

 between the two vertical distances. 



5511. Facilities for Draft Measurement 



The draft of an acoustic unit is the vertical distance from the water surface to 

 some reference point on or in the unit. This point is usually the sound transmitting 

 or receiving surface, which in most cases is a diaphragm. For systems employing 

 separate transmitting and receiving units the draft is the vertical distance to the 

 midpoint of an imaginary Ime joining these two units. The draft of the acoustic units 

 of echo-sounding instruments employed by the Coast and Geodetic Survey shall be 

 measured from the surface of the water to the following: 



(a) 312 Fathometer— to the midpoint of an imaginary line joining the center of the diaphragm 

 of the 324 oscillator and center of the hydrophone. 



(b) Dorsey Fathometer No. 1 — to the lower surface of the diaphragm of the transceiver. 



(c) Dorsey Fathometer No. 3 — the same as the Dorsey Fathometer No. 1 for the transceiver, 

 and the same as the 312 Fathometer for the 324 oscillator and hydrophone. 



(d) Veslekari — ^to the midpoint of an imaginary line joining the tops of the reflecting cones. 

 The proper point on the top of the cone is in the plane indicated by the line HH in. figure 109. For 

 the MS 12 D, measurements should be made to this same plane, although, of course, the cones are 

 <5ontained in a fish, 



(e) 808 Fathometer — to a point 6 inches below the top of the fish, and not from the bottom of 

 the cover plates. 



The draft of an acoustic unit permanently installed in the ship's hull may be 

 determined (1) by measuring the vertical distance from the surface of the water to 

 reference points on the ship's rail or on deck, which are at a known vertical distance 

 above the acoustic unit, (2) by an internal draft gage especially installed for the purpose, 

 or (3) by existing draft marks or gages. A draft gage is usually installed while the ship 

 is in drydock and the required measurements to establish any permanent reference 

 marks may also be conveniently made at this time. 



If reference points on deck are to be used, points should be selected on both sides 

 of the ship, preferably on top of the main deck rail, at positions abeam of the acoustic 

 units. These points should be at a known vertical distance above the acoustic unit, 

 or above their mean position if there are separate transmitting and receiving units, 

 and permanently marked for identification, as by metal plates, and their positions and 

 vertical distance above the acoustic units marked on the ship's blueprints. If there 

 is more than one echo-sounding instrument, the plates should contain the data for each. 



The measurement of the vertical distance of the reference points above the mean 

 level of the acoustic units, or the measm^ements to set the zero of the scale of an internal 

 draft gage to the horizontal plane of the acoustic units, may be best made when the 

 vessel is in drydock, with the aid of an engineer's level and a steel tape. 



An internal draft gage for use in determining the draft of acoustic units should be 

 installed near the acoustic units and connected to an independent opening through the 

 hull. The gage may consist of a brass pipe connected to the intake, to which is con- 



