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Originally Posted by Kissaki
The only way I can make sense of this is if the ballast tanks are only "buoyantly" full, and not empty - and with no compressed air to push more water out. The presence of compressed air itself shouldn't do anything for buoyancy, because unless it has negative weight (which it doesn't) it doesn't make the boat lighter. And the uboat displaces the same ammount of water regardless.
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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.