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Originally Posted by Beery
I'm no scientist, but I know that if you blow ballast, the boat rises. If you fill the ballast tanks with water the boat will sink. If you have no compressed air and no propulsion the boat will sink. Air is lighter than water, which means that in water it has a 'negative weight' - a tendency to rise to the surface. Fill the volume of a ballast tank with it and the boat will rise because the boat has become lighter than the water around it. It's like a balloon. The balloon itself (the plastic envelope) has a certain weight, but its weight is modified by the gas that you put in it. Put air in it and it will just fall to the ground because you've filled it with ballast. Put helium in it and it rises. But it will only rise if there is enough helium to counteract the weight of the balloon.
If you put a propeller and dive planes on an air filled balloon you can probably get it to rise into the air. If you blow ballast by replacing the air with helium it will also rise. But if your propeller is broken and all your helium is gone, the balloon will sit on the floor, and there will be no way to make it fly.
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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?
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.
But tell you the truth, I now feel tempted to build my own type II and find out the hard way. :P