The S-Boats are "tested" for 200 feet, but dependent upon damage, might not make it to PD, so the damage sustained will change these figures, but:
- S-18 - 115.0m = roughly 377.3 feet * Test Depth= 200 feet
- S-42 - a clone, same as S-18
- Narwhal - 165.0m = 541.3 feet * Test Depth= 300 feet
- Porpoise - 167.0m = 547.9 feet * Test Depth= 250 feet
- Salmon - 176.7m = rougly 579.7 feet * Test Depth= 250 feet
- Sargo - 178.7m = roughly 586.2 * Test Depth= 250 feet
- Tambor - 180.0m = 590.5 feet * Test Depth= 250 feet
- Gar - clone of Tambor, same
- Gato - 185.0m = 606.9 feet * Test Depth= 300 feet (some sources say 350)
- Balao - 250.0m = 820.2 feet * Test Depth= 400 feet
- Tench - 280.0m = 918.6 feet * Test Depth= 400 feet (some sources say 600)
The crush depth info is found in the zon files for the subs. The cfg file is ~supposed~ to match, but not all of them do. The "test depth" is from online, and subject to the usual internet warnings about "accuracy". Also, final caveate: no two boats are exactly alike.YMMV - Note that while "Test Depths" might increase in real life, the crush depths go down, the older the boat is, and allows less leeway.
Now, all that said - just remember that you do not have to exceed the crush (crash) depths above to sustain damage. There is a "speed" with which you incur damage, and a set amount of hit points for going beyond, but you might start taking damage above that crush depth, boat & speed dependent. You can usually figure twice the test depth is how deep a new US sub could generally safely go. Note that the Tench in the game is not like that. As previously mentioned, just a small amount of hull damage can adversely effect the survivability of your submarine beneath the surface, just as in real life. It is difficult to know when you have exceeded that "line", until you dive and then die... as you apparently already know.
Edit: Again, I are too slow!!! lol
2nd Edit: Forgot to mention, that successfully diving to the actual listed crush depth does "damage" your boat, and subsequent dives are not able to go as deep, until you get back to home port, and "repairs" are made. Just like in the SeaTrial missions, which came from TMO, if you take your boat down until you see signs of failure in the conn, and do not immediately come back and FAST... you might just sink right there. Also, any further diving is a risk to just get down to test depth now. Once you go back in from patrol though, that "damage" is repaired and your boat should be good to go once again.