Anyway, this is a true statement, more or less. Once we submerged, we generally never felt any wave action.
However…there was this one time that we were out in the North Atlantic with this huge storm going on above us, and the sonar operators were telling us that based on the noise they were hearing on the surface, the predicted sea state was 9+. (The sea state scale only goes up to 9.) Anyway, we were due to go to periscope depth for communications, so the CO decided to go up slowly to see if would be possible or not.
At 400 feet deep, we didn’t really feel anything. At 250 feet, the boat started rolling, and by 150 feet, we were doing some pretty dramatic rolls that were getting progressively worse. At that point, the CO decided to bail on going to periscope depth and that we would try again after the storm had passed.
I doubt the diesel could detonate, because it would have to have some sort of oxidizer mixed with it. The air in the sub couldn’t mix with the diesel fast enough to create an explosion. The explosives in any torpedoes could detonate though.
It’s a lot easier to get gasoline to explode, because it can vaporize, mix with air, and then form an explosive gas. Diesel is a lot heavier and won’t vaporize and so it can’t mix thoroughly with air, and so won’t explode. Burn, sure, but not explode. The classic way to get diesel to explode is to pour it over ammonium nitrate. But even there the real explosive comes from the NH4NO3.
Most likely one point or other will fail first- a hatch, a seam, a port or flange where something goes through the pressure hull. Then the water rushes in and compresses everything, so the damage is limited as the pressure equalizes. You may have catastrophic chemical reactions once the oxygen partial pressure reaches flash point for whatever combustible materials are inside the hull, but I suspect that compared to external pressure, the force of any “explosion” will be muted unless there are interesting explosives on board and not submerged, so the compression process sets them off.
In a diesel engine, the fuel is deliberately sprayed in a fine mist into the cylinder. Unless the compression process creates a mist, a major explosion is unlikely in my inexpert opinion.
The British did a submerged crush test on a captured German submarine after World War II. I don’t know what depth they achieved, but failure occurred at the conning tower hatch. The hatch itself was found in the aft torpedo room. It had taken every hatch or bulkhead in between out on its passage aft, so, yes, failure would be catastrophic. The weakest points are always attachment points for something, or where a smooth continuous hull shape is broken by an irregularity such as a hatchway opening or a conning tower.
