In Clancy’s novel would 50 feet be relevant to sub depth when firing torpedos? Meaning at that time? Still carrying on with the (Hunt for) Red October stuff?
When a Ship (Submarines, likewise) mention “Depth”, they talk about KEEL depth (distance from bottom of Vessel to the surface). Most Submarines (in today’s Navy) are 30 feet plus in diameter and draw (while on the surface) 27 feet plus (Tridents draw 36 feet or so). 50 feet (keel depth) is too shallow for MODERN DAY SUBS, OK for WWII subs (much smaller)
Technically Torpedoes can be fired at nearly any depth, even at the time of the Hunt for Red October. The bigger issue is firing blind (at a sonar blip) versus using a periscope. So the reference is still fuzzy. My guess is it’s a missile reference.