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Originally Posted by Kissaki
But the uboat rises so long as it weighs less than the water it displaces. As long as the ballast tanks are empty enough for the boat to rise, it shouldn't matter how much compressed air you had left. If it did, you have a hard time explaining the following case:
You have no propulsion, and you have just enough compressed air to blow the ballast tank one last time. You do, and the sub rises. After it's done rising, it starts sinking because of lack of compressed air.
Question is, if the lack of compressed air makes it sink, why did it rise in the first place?
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That's easy. You have a leak in your ballast tank and compressed air is being replaced with sea water. At first the sea water would be pumped out - just like if you had a leaky balloon filled with air and started filling it with helium, at first it would rise, but after a minute the helium would escape and air would start to enter the leaks. Then the balloon would sink.
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Now, a cannister of compressed air is a heavy thing. Let's say it's a tiny one of about 10 lbs. both full and empty. If you put one on the boat, that's going to make the boat 10 lbs. heavier, whether it's full or not.
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Yeah, but that 10lbs of compressed air can remove many times its weight in seawater from the boat.