Charla:

Generally, small hull penetrations in the pressure hull located near the bottom of the keel for the the diving control station, and midway on the pressure hull for the forward and aft torpedo rooms and engine room, provide the source for the seawater pressure (depth) guages. They all have a hull valve at the penetration and an isolation valve at the guage. However, this sea-pressure guage doesn't help if you're transiting to the bottom, it cannot "look ahead".........It only tells you what the sea pressure is at the present depth. The reason for the hull penetration for the control
room diving staion's location (bottom of keel) is that the depth indicated on that guage is a "depth-to-keel" indication.

The torpdomen need to know the sea pressure in order to equalize pressure in the
tubes for opening their outer doors. Injection temperature and sea pressure is generally provided for the enginemen; if the trim pump is located there, then sea pressure indication is available for operation of the pump at depth.

It is understood that sending out a "ping" from active sonar is a deadly practice if
escorts are in the vicinity, however. If VIICs were equipped with the gear, I'm sure
the CO would use it rather that than "bottoming" and causing a ruptured pressure hull. I don't believe the active sonar itself, if so equipped on a U-boat, would be used for indicating depth. A smaller unit, similar to an echo-sounder (FATHOMETER) would be used. The passive sonar hydrophone on a VIIC was
mounted topside forward near the bow, and was trained by the operator, either
mechanically or electrically, and could not indicate depth.

Modern U.S. Navy fast-attack and missile submarines use a "bottom-bounce"
depth recorder, which scrambles the signal (ping), so no other craft can detect it.
This equipment can be used to navigate the boat, 'cause sea-bottom charts were
developed just for that purpose in the late '50s. The regular FATHOMETER (trade
name for the equipment) is de-energized on patrol and activated only on return to home port.

Does this answer your question?

Cheers,

ROBERT M.