----- Original Message -----
From:
Dale A. Raby
Sent: Monday, December 30, 2002 5:37
AM
Subject: Re: [PSUBS-MAILIST] Ballast
Question
I am far from expert... but if I screw up, I'm sure somebody will correct me.
I believe you to be laboring under some misconceptions. As you descend, the air in a ballast tank that is open to the outside does indeed compress... though I am not sure it compresses by half at twice the distance. That does seem logical, however.
The thing is that with the air compressed to half size at depth, the water it displaces is also "heavier", though it is not compressible. The wight of the water is what causes the air to compress. It has all the weight of the water above it to add to its weight. Thus, though the compressed air is denser (heavier) and theoretically not as buoyant, in water that is also heavier by the same factor, it retains its buoyancy... in fact it becomes even more buoyant, I believe.
The reason you adjust ballast as you go down is because you can't descend unless you do. In the example you are using, trying to double your depth by using electric propulsion. This might work if you had unlimited power because you would have to maintain downward thrust in order to keep it down there. It is kind of like a helicopter in reverse. A helicopter opposes gravity by downward thrust, you would be opposing buoyancy (or really just gravity again) by upward thrust. As soon as you shut off your power, you would rise to the level you had adjusted your buoyancy to.
Now, if your ballast tanks are pressurized so that the air in them cannot be compressed, you could have more buoyancy from the same volume of air at sea level and this might allow you to have smaller ballast tanks... but you'd pay for it in the weight of the tanks themselves.
Try this trick some day. Dive to the bottom of a pool and pick up a rock... the biggest one you can iift and try to bring it to the surface. You will find that while you can lift it on the bottom, as you rise, you will need to use more and more effort to keep rising. This is the same principle in reverse.
Now, for a better example. Go into the pool again and dive down. You will find that unless you are one of those unfortunate people who have negative buoyancy, you must maintain muscular effort to remain below the surface. The air in your lungs, your "ballast tanks", though compressed, is still buoyant... and the deeper you dive, the more effort you must expend to stay down. If you expell some air, you will find that you can stay down with not nearly as much effort... but don't go too deep, you also reduce the air reserve in your lungs that you need to breath.
Keep on asking questions. It is easier to answer your questions than the questions your wife, children, and other relatives will ask after your death.
On Mon, 2002-12-30 at 09:30, Pierre Poulin wrote:
Hi there every body! It's the newbi again. I'm working on an a dry ambient sub. The hull is 60% finish. I tought that the ballast system was pretty simple but then I figured that if I have a main ballast system witch is opened at the bottom and I ajust the buyancy at 33 feet. Then, I go at 66 feet using electric motors. The buyancy will need to be ajusted again since the air in the ballast will be 2 times smaller. Right? So the deeper I go, the faster I sink? Would it be better if the ballast was able to support the pressure like an 1 atm design and you close the bottom of the ballast witch is normaly open? So the volume of air will remain the same? I do realize that when I will try to put more air in the ballast, I will have to play with 2 valves at the same time to conpensate. Is that a variable ballast system? Is there a difference between those two systems (main and variable)? I'm a bit confused with that! I'm looking for something as simple as possible. I tought that some of you guys who are more experienced surely have the answer to my problem! Thanks for your help! Pierre Poulin >From cold Québec, Canada. _________________________________________________________________ MSN Messenger : discutez en direct avec vos amis ! http://www.msn.fr/msger/default.asp
Dale A. Raby Editor/Publisher The Green Bay Web http://www.thegreenbayweb.com |
