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.

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.

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

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.

Yeah, but that 10lbs of compressed air can remove many times its weight in seawater from the boat.

I have a leak in my ballast tank? Stempel, grab a monkey wrench and have at it!

Seriously, though, are there not little hatches or such in the ballast tank that open to take in or expell water, and close to maintain buoyancy? Otherwise you'd be constantly using compressed air - even while travelling at the surface.

Yes, but now all the seawater is gone. So is the compressed air. We're at the surface, and for the sake of argument the dive planes are located on the conning tower. In fact we're docked, and nobody's minding the sub. Why should it sink?

Of course, but if you have a leak, the tank will tend to equalize pressure with whatever the hole leads to. That means seawater if the boat is submerged. You can't plug a leak by opening or closing a hatch. The defining characteristic of a leak is its tendency to leak. :lol:

Who said it was surfaced? You asked me why a boat would rise a bit and then sink. We never talked about surfacing. On the surface it's a different matter entirely because the boat can be kept buoyant by filling the ballast tanks with air from outside using the pumps. You don't need compressed air.

Check it out:

http://www.onr.navy.mil/focus/blowballast/sub/work3.htm

You can make a submarine rise or sink while at neutral bouyance. Normal and no leaks without using very little air. It is what they do when using flood recovery. It is very slow compared to using compressed air to force water out of the ballast.

Using a high pressure pump, you can pump water inside the submarine interior or ballast area out into the open sea under the pumps high pressure. It will leave a less thin air inside the sub which can be replaced with a little air from the 'low' air supply line if you wish. This small amount of air is just replacing the water that was pumped out of the sub into the ocean outside under pressure.

This does not work well if you are realy deep since the outside pressure is so high, you cannot pump againts it -thus in the old days that is why they would say subs had a tendency to sink if you did not keep it moving via the dive planes. It was impossible back then to pump against extreme high pressure at very deep levels.

That demo is very basic and a submarine can rise and fall with very little or no air usage very slowly. Only if it is an emergency need it expell it high pressure air. But they did use a small amount every time they moved up/ down back then using the pumps due to engineering reality.

Example, take a rubber compressed can with a compressed spring inside it. Make it so it just barely floats neutral. If you release the spring under water, the expanded spring will make the rubber can bigger displacing water weight around it and it will rise to the surface using no air. Of course the air pressure inside the can is droped some due to the air filling in where the water was, and that is were the small amount of low pressure air usage comes in.

Now a submarine has no rubber hull to expand. It is solid, so they use a high pressure water pump and valve to force the water from the ballast tank or water flooded inside the sub to the outside ocean. It is slow and will not work if too deep due to engineering limitations.

I had a great link on how the military used this in real life to minimize compressed air usage. Modern diesel submarines under normal operation and not damaged, could rise and sink without expelling air to a great degree slowly using this trick. It is like a expandable badder in a fish but using a pump instead.

http://www.geocities.com/gwmccue/Sys...st_System.html

Anothe point I forgot but is modelled in the game, under high dep water pressure the boats hull is compresed some, making it displace less water, and thus tend to sink. Shoot a torpedo out and you lose weight and you tend to rise. Ther are auxillary tanks to help control over this stuff. It gets to be quite a bit of stuff to handle. They do remakr it is imposible to achieve perfect neutral bouyance. It is a saddle point, you will either rise or sink very slowly. So many subs moved alittle to keep at neutral depth.