Page 24 - Electronic Navigation Cyber Book

This type of paper should be kept in its airtight tins before use. If an echo
trace on damp paper is to be kept for reporting or other purposes, a line
should be drawn down each side of the paper while it is still damp to
indicate the limits of the scale. The bottom trace and transmission line
should be drawn in pencil, the paper dried, preferably in dim light, and
then the paper should be rolled up to prevent fading. An indelible pencil
should preferably be used or a ballpoint pen for all writing.

The Dry paper is a carbon impregnated paper base, metallized on one
side and covered on the other with a very thin film of fight colored semi-
conducting chemical. The metallized side makes contact with the metal
plate at the back, and the stylus moves over the chemically treated side.
When the echo returns, a pulse of current is applied to the stylus which
destroys the chemical film and exposes carbon beneath to show a black
record in contrast to the gray paper, carbon dust and possibly fumes will
be released and these may be a health hazard. The recorder must be
sited so that adequate ventilation its possible. The dust, which is
deposited on parts of the recorder must be removed at require intervals
using a soft rag or brush.

TRANSDUCER SITING

Satisfactory operation of an echo sounder depends on the transmission
and reception of the largest possible signal for a given amount of power.
The siting of the transducer is important in this respect to reduce
attenuation on transmission and reception as far as possible. The ideal
position is one in which there is "solid" water free from aeration beneath
the transducer, and where the effects of surface, engine and propeller
noise are at a minimum. There are few positions which are suitable in
every respect and a position found to be satisfactory in one design of ship
will not necessarily give equally good results in another.

The principal source of aeration is the bow waves created by the ship.
This wave rises some way up the stem, curls over, and then is forced
down beneath the ship, taking a quantity of air with it.
The resultant bubble stream normally starts about a quarter length of the
ship from the stern and divides about three-quarters of the length from
the bow. The bubble stream varies in form and intensity according to the
speed, draught, shape of bow and hull, the trim of the ship as well as the
sea state. In ships with a bulbous bow, the wave appears to dip water
just abaft the stem so that the flow of bubbles is over almost the whole

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