Page 503 echo sounding 54 



acoustic signals. In appearance and operation the indicating device of this instrument is quite 

 similar to that of the 312 Fathometer which is described in 521. A neon tube rotating behind the 

 depth scale indicates the depth by its flash. A governor-controlled motor rotates the neon tube and 

 operates the keying cam. The energy to operate the transmitter is supplied by the discharge of a 

 condenser into the coil windings of this unit, thus producing shock excitation to the magnetostrictive 

 elements which in turn set up the acoustic waves in the water. The transmitting and receiving units 

 are installed in strong water-filled tanks, mounted inside the ship's hull as described in 543, or an 

 opening may be cut in the hull over which the tanks are installed, the opening being replaced by a 

 thin metal diaphragm. The instrument has two depth scales, one from to 100 meters, and the other 

 from to 1,000 meters, selection between which is by means of a switch. A graphic attachment 

 may be used in conjunction with the visual-indicating device and in this way the advantages of both 

 types of instruments may be realized simultaneously. 



54. ECHC-SOUNDING INSTRUMENT INSTALLATION 



Much of the effectiveness of an echo-sounding instrument depends on the installation — not only 

 as to convenience but as to the results obtained. Each type of instrument requires a different instal- 

 lation, which must often be modified for different ships. The general factors to be considered are as 

 follows: (1) The available space for the various parts of the instrument- — the indicator must be 

 conveniently located for use in surveying, and all parts should be where they are protected from 

 weather, heat, dirt, and vibration, and they must be accessible for adjustment and repair. (2) 

 The parts should be arranged so that the connecting electric cables can be installed conveniently and 

 where they are not exposed to damage. (3) The transmitting and receiving units should be located 

 where they will be most effective. 



Since there is a wide variation in these factors even on ships of the same type, installation pro- 

 cedure can be described only in generalities. 



541. DoRSEY No. 1 Installation 



The indicating device of an echo-sounding instrument is located in the pilothouse of most survey 

 ships of the Coast and Geodetic Survey. This is the most convenient location for use both in hydro- 

 graphic surveying and in navigation. The after bulkhead of the wheelhouse is usually a desirable 

 location and affords a rigid support for the indicator cabinet. 



The indicator cabinet of the Dorsey Fathometer No. 1 must be located where there is a space 

 on each side which will permit opening the doors, and where there is a sufficient clearance above it 

 so that the photoelectric tube, exciter lamp, and index adjustment are accessible. The echo amplifier 

 should be close enough to the indicator cabinet so that the operator can reach the amplifier gain 

 control and still have a full view of the depth dial. This arrangement is also desirable for electric 

 reasons. 



In most cases the equipment can be fastened directly to the deck or bulkhead, but on ships where 

 vibration is excessive, some of the equipment, such as the echo amplifier, must be mounted on vibra- 

 tion-absorbing mounts. 



The poweT supply can be installed in any suitable place in the ship, but the numerous cables 

 between the power supply, indicator cabinet, and amplifier, make it desirable to keep these three 

 units reasonably close to each other. The power supply and echo amplifier should be separated by 

 at least 5 feet to avoid possible electric interference. 



The most careful consideration must be given to the location of the transceiver. It should be 

 located where the maximum echo signal to noise ratio is to be expected. This ratio is a function of 

 the impedance to transmission of acoustic waves and the spurious noises generated either in the ship 

 or by the passage of the ship through the water. Aeration and turbulence (5143) are causes of both 

 sound attenuation and noise and should be considered when selecting the site for the transceiver. Ex- 

 perience has proved that a point a little forward of amidships is generally relatively free from aeration 

 and turbulence. Farther aft turbulence increases, and farther forward there are likely to be installa- 

 tion difficulties because of the shape of the hull. The transceiver should be located as near the keel as 

 possible, since aeration and turbulence diminish as this point is approached. The engine room and 

 the propeller are usually principal sources of noise, so it is important to place the transceiver a reason- 

 able distance forward of the engine room. It should be at least 10 feet from, and never directly aft of, 

 any discharge or intake pipes, submerged log mechanism, or any other protrusions from the ship's hull. 



