We used to do North Atlantic runs off of Murmansk in winter.We were there to listen to the screws of the Russian boats so we could ID each one.One time we sat at the bottom at 500 feet.The old pig boat I was on was groaning like an old whore from the pressure.I thought for sure we were going to bite the bullet.
What boat? Only one I can recall in the later '60s in CT. was the USS Bang.... If it was a 'thick-skinned' boat, crush was 600 w/o pressurization. If you were on a thin-skinner, then you hadda pressurize to stay intact - and I'm not even sure 500 was possible, but maybe so..
I was on Redfish when we had to do a dive to test depth.. Cap'n did it slow, about 50 feet at a time once we passed 300 or so.. I was in the after torp room and was watching the tubes and hatch pretty closely... But no issues - went to 412, stayed there for 30 mns and brought it back up..
