Page 505 echo sounding 544 



The contactor box which furnishes the energy for the magnetostrictive transmitting unit must* be 

 located not more than 15 feet from it. This box should be mounted in a vertical position, in a readily 

 accessible place, and so it is free from vibration. The front cover is hinged, and space should be al- 

 lowed for it to swing either to the left or to the right sufficiently to make the interior accessible. 



The site for the tanks containing the acoustic units should be selected with care. The same 

 considerations apply which were mentioned in 541 in connection with the location of the transceiver 

 and in addition there are other factors to be considered. The transmitting and receiving units should 

 not be so far apart as to introduce an appreciable error in shoal soundings due to their physical 

 separation (see 556), yet they should be far enough apart to reduce the effect of the transmitted 

 signal on the receiving unit, so that the registration of the transmitted signal will not be unduly 

 prolonged, making shoal soundings impossible. 



Tanks to house the magnetostrictive units and associated reflectors are designed to fit the particular 

 ship on which they are to be installed. The selected location, frame spacing, and other constructional 

 details influence their design. The following is an abridged excerpt from specifications for the con- 

 struction of such tanks installed on two Coast and Geodetic Survey ships: 



Tanks to be made of quarter-inch iron, rolled into a cylinder 25 inches long with an inside di- 

 ameter of 19 inches, and welded along the seam. Tanks to be provided with watertight covers of half- 

 inch iron held down to the tank top by four half-inch brass bolts. No bolts or fittings to be on the inside 

 of the tank. The following holes to be drilled in each cover: (1) center hole, to pass spindle for hold- 

 ing the reflector, must be accurately located on the axis of the tank to make the reflector concentric 

 with the inside of the tank; (2) two holes for watertight glands for the passage of electric cables; and 

 (3) two half-inch tapped holes for filling the tank with water. 



Sometimes the transmitting and receiving units are placed on opposite sides of the keel, but not 

 necessarily between the same frames. A separation of 4 to 10 feet is usually satisfactory. The tanks 

 should be located where the slope of the hull plates does not exceed 19°, and the lower edges of the 

 tanks must be shaped to the plates to ensure watertightness and that the sides of the tank are vertical. 

 All cement and paint must be removed from that part of the ship's plates that will form the bottom 

 of the tank. The tanks may be secured to the inside of the hull plates by welding, or by means of a 

 pressure fit. The welding may be on the inside or outside of the tank, but if inside it should not ex- 

 tend into the area of vertical projection of the reflector. To secure the tank by pressure fit a strong- 

 back may be used, fitted between the frames and arranged so as to apply vertical pressure on the tank 

 and so press it securely against the ship's plating. A rubber gasket should be used between the lower 

 edge of the tank and the plating. 



The receiving-unit tank should be located with the same care required for a transceiver (see 541), 

 and in addition, this tank should have no direct contact with bulkheads or ship's water tanks. The 

 magnetostrictive units should not be installed in the ship's water tanks, nor should the tanks they 

 are in be located so near the keel that they will be in the bilge water. 



Three shielded cables, insulated for 2,000 volts, connect the recorder, contactor box, and the trans- 

 mitting and receiving units. A three-conductor shielded cable of No. 14 wire runs from the recorder 

 cabinet to the contactor box; a two-conductor No. 12 shielded cable connects the contactor box to the 

 magnetostrictive transmitting unit; and a third cable of the same specifications connects the magneto- 

 strictive receiving unit to the echo amplifier located in the recorder cabinet. These cables should be 

 separated from one another by at least 2 feet, and should all be bonded to the metal frame of the ship 

 at intervals. The cables should pass through stuffing boxes where they enter the transmitting and 

 receiving unit tanks. These tanks must be kept filled with fresh water. If there is danger of the 

 water freezing, Prestone or an equivalent should be added in amount equal to 20 or 30 percent of 

 the water. 



544. Semiportable Installations 



The general procedure for the installation of a semiportable instrument, such as 

 the 808 Fathometer, has been described in 5236. The same kind of an installation is 

 applicable in the case of the Hughes Mis' 12 D recording instrument, making allowance 

 for the constructional differences of the two instruments. 



465382—44 34 



