The depth of enemy waters in
which our submarines patrol has
been fairly well charted. Nevertheless, it will often be necessary to
determine accurately how deep the
water is beneath your keel. This is
done on the NM gear by echo-sounding, which is merely echo-ranging
with the ocean bottom as the reflecting surface. Instead of sending signals straight out, the NM projector
sends them straight down.
The NM Projector looks some
thing like a small steel coffin. It is
bolted flush with the bottom of the
submarine in the forward trim tank.
The NM projector gets the current
needed to send out pings from the
same driver that is used with the. QC
The NM Fathometer. From the
projector, a cable runs up to the
depth-sounding indicator, which is
located in the control room. This
is called a fathometer, because it is
marked off in fathoms instead of
yards. It is practically the same as
the range indicator used in echo
ranging. A receiver-amplifier is in
side the lower part of the unit.
How to take a depth sounding
Set the remaining switches according to the table below:
Approx. depth of water
Signal interval switch
Less than 300 fathoms
Between 300 and 700 fathoms
Between 700 and 1800 fathoms
More than 1800 fathoms
Important: Echo-sounding pings are likely to be picked up by the enemy.
Do not take unnecessary risks.Return the signal interval switch to its
center position as soon as the depth has been obtained.