Page 21 - Electronic Navigation Cyber Book

sound pulse is directed downwards through the steel bottom shell by a
reflector. The advantages of nickel ring types are that it is cheaper to
construct and damping is greater.
In both designs, the sound pulses are directed downwards in a cone-
shaped beam to avoid loss of sounding when the vessel is rolling. The
process is reversible, as, when the echo returns, it applies varying
pressure to the working faces of the transducer, which causes the
magnetic condition of the nickel to fluctuate at the same frequency. This
varying magnetic field strength induces a voltage in the winding round
each piston leg and this voltage is amplified before being applied to the
indicator.
As oscillators must be in water dry forepeak tanks. Tanks may be flooded
sufficiently to keep them submerged. Forepeak tanks are usually
arranged so that when they are pumped out, enough water is retained to
keep the oscillators from becoming dry.

Echo sounding equipment may be divided into two main classes:
.1 Those that transmit and receive sound vibration through the shell
plating of a ship referred to as 1nternal installation" class.

.2 Those that are in direct contact with the sea generally referred to
as "pierced-hull installation" class.

In the internal installation class, because of the shell plating, energy is
wasted during transmission and reception. For a shell plating thickness
of 9.5 mm, about 15 percent of the energy gets through the plate and
only 2 percent gets through when the shell plating is 31.8 cm thick.

The advantages of having an internal installation are:

.1 Equipment may be fitted without dry-docking the ship.
.2 Projectors or oscillators may be serviced or changed while the
ship is afloat.

Sound wave energy is wasted if it is required to pass through a plate. The
plate will prevent sound waves to pass through if the thickness of the
plate is close to a quarter wavelength of the sound wave; but if the
thickness of the plate is about a half wavelength then the steel plate
becomes transparent to the sound wave.
For a pierced-hull installation, the shell plating of the ship is first pierced
and the gap filled in by a thinner plate. If a steel plate is to be fitted, then

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