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.  
Illustration showing ping down and echo back.
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 system.  
Photo of NM projector.
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.  
Photo of Fathometer indicator in control room.

How to take a depth sounding

Photo of fathometer indicator showing switch settings for a depth sounding.

Set the remaining switches according to the table below:

Approx. depth of waterVisual-audible switchSignal interval switchShoal-deep switch
Less than 300 fathomsVisualStandardShoal
Between 300 and 700 fathomsVisualAlternateShoal
Between 700 and 1800 fathomsAudibleStandardDeep
More than 1800 fathomsAudibleAlternateDeep

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.


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