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Old 12-14-18, 07:20 PM   #1
jscharpf
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Default Trying to understand buoyancy -- etc.

I'm trying to understand some principles about the sub.

So you have these ballast tanks that you let fill with water until your sub is I guess.. neutral or a little less such that it sinks. This tells me that you have only enough volume of air that would displace enough water such that the weight is about equal to the weight of your sub.

In addition, you're carrying these compressed air tanks.. And when you're down there you "blow" the air from the compressed air tank, into the ballasts.. thus pushing out the water..

So.. I don't understand if you are carrying a compressed air tank, that you had to fill with xx amount of air... why doesn't that compressed air tank keep the boat from sinking? (sorry if I sound silly lol).

You had to take the same amount of air and just squish it into a tank.. but it has a certain number of molecules.. now you release them into the ballast. You're not increasing the number of air molecules just reducing the pressure..

I'm confused lol..

And does an air compressor work if you're underwater with a limited amount of air?




Also..

My wife says that, at depth, the pressure is higher so you would need not as much air pressure in the tanks to start to rise than you would need higher up where the water pressure is lower..She says it's the difference that moves you up..

Jeff
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Old 12-14-18, 07:42 PM   #2
mapuc
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I found this

https://maritime.org/doc/fleetsub/air/chap1.htm

Guess there are a lot more about this on the web.

Markus
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Old 12-15-18, 05:28 AM   #3
ETR3(SS)
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Quote:
Originally Posted by jscharpf View Post
I'm trying to understand some principles about the sub.

So you have these ballast tanks that you let fill with water until your sub is I guess.. neutral or a little less such that it sinks.
Negatively buoyant to a point, but other wise yes.

Quote:
This tells me that you have only enough volume of air that would displace enough water such that the weight is about equal to the weight of your sub.
Not sure I understand your statement here.

Quote:
In addition, you're carrying these compressed air tanks.. And when you're down there you "blow" the air from the compressed air tank, into the ballasts.. thus pushing out the water..
Correct.

Quote:
So.. I don't understand if you are carrying a compressed air tank, that you had to fill with xx amount of air... why doesn't that compressed air tank keep the boat from sinking? (sorry if I sound silly lol).
I think I'm following your logic here, if air makes the boat float why doesn't the air bank keep the boat from submerging? The answer is the density of water versus the air and the mass of that air. This is all calculated before the boat even leaves the pier by the Ship's Diving Officer. What he, or she, will do is calculate the weight of all the stores loaded and compensate for that with the variable ballast trim tanks.

Quote:
You had to take the same amount of air and just squish it into a tank.. but it has a certain number of molecules.. now you release them into the ballast. You're not increasing the number of air molecules just reducing the pressure..

I'm confused lol..
If I took 1 cubic foot of air at atmospheric pressure and compressed it to 3000 psi, it no longer occupies the same amount of space. Same amount of molecules, yes, but decreased volume and not enough to fill the air banks. So when you fill the air banks you end up with more air, ergo more molecules. As pressure increases volume decreases and you're filling to the volume of the air bank.

Quote:
And does an air compressor work if you're underwater with a limited amount of air?
Yes, to a point. There's a few systems on board that regularly use compressed air, and depending on the system that air may be vented inboard. That is inside the hull. So the ambient air pressure varies inside the hull during normal operations. You can run the compressors to a point before you'll need to shut them down to maintain air pressure in the boat, that and it's harder on the compressor to run in a vacuum. When you need to fill the air banks at sea while submerged you can use the snorkel mast to bring air into the ship and bring the inside pressure over that of atmospheric.



Quote:
Also..

My wife says that, at depth, the pressure is higher so you would need not as much air pressure in the tanks to start to rise than you would need higher up where the water pressure is lower..She says it's the difference that moves you up..

Jeff
Water pressure increases with depth, that is correct. If you wanted to surface from a deeper depth you would need more air volume to overcome the greater sea pressure. If you're referring to the air banks and their volume and effect on buoyancy than that is irrelevant as discussed above.

Also, to avoid confusion, water goes in the tanks and air in the banks.
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Old 12-15-18, 08:45 AM   #4
jscharpf
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Thanks, both !!

I good information!
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