Thanks for the reply, Bill.
Yes I was referring to air from the ballast tanks to pressurize the cabin. For this proof of concept vehicle I was thinking of an unpowered underwater elevator. I was envisioning a tube divided into 3 compartments, the central compartment being dry; the fore and aft sections being the ballast tanks. For descent, I was thinking that some form of piping would direct the ballast tank air to the cabin, from which it could be vented to the outside.
It was when I thought about ascent that I realized that a relatively small amt of water expelled from the ballast tanks (by a compressed air tank) would result in positive bouyancy and the craft would ascend. But the ballast tanks would not "drain" to any appreciable degree and I could see pressure building in the cabin.
I've thought about that hole in the bottom, but my concern is how tall the "centerboard trunk" would have to be. And in
writing that last sentence I just figured out that it doesn't have to be in the center of the cockpit where I want to sit: it could be split in two and placed at the sides. The craft could still sit low in the water and still have a dry cabin.
Ouch. All this thinking makes my head hurt.
Well, thanks for the feedback. It looks like I have a way to go yet.
Akins <lakins1@tampabay.rr.com> wrote:
Hi Philip.
You said on descent that air from the ballast tanks has to be vented to the cabin in order to kieep the pressure equal with the outside.
Is this actually what you meant to say,
and do you mean that as you decended you would utilize the ballast tank air to help pressurize
the cabin and counteract the outside water pressure or did you mean to say air from pressurized tanks instead of ballast tanks would be used?
If you meant exactly what you said and didn't make a mistake and actually mean "compressed air tanks" then I have this below to advise you.
If you just use the air in your ballast tanks to pressurize the cabin on decent, what are you going to do when the outside water pressure exceeds
the volume of air that your ballast tanks held? Then there will be no more air volume from the ballast tanks to compress into the cabin to counteract the outside
water pressure. This means you will run out of enough ballast tank air to compress into the cabin to counteract the outside water pressure.
You asked if on accent if the pressure inside the cabin needs to be vented to prevent the sub from exploding. The answer is yes. If you did not make a mistake in what
you said earlier about your ballast tanks venting to the cabin, perhaps if you were in a POWERED accent you could actually vent some of the now overpressurized air from the cabin
back to the ballast tanks and function PARTIALLY like the hunley submarine's recycleable ballast air system did. Unless you made a mistake in what you said, your system sounds
like you plan to use use your ballast tank air to pressurize
the cabin against the outside water pressure, but this means you need a pressure hull once your outside water
pressure exceeds the air volume you have available to compress into your cabin from your ballast tanks or else you will implode the sub. This would mean you would not have an ambient sub, or
you would be ambient until your ballast tank air ran out, and then you would need to have a pressure hull or else use compressed air tanks to make up for no more ballast tank air
being available once you had used it all and it was now compressed inside your cabin, and your outside water pressure still needed to be counteracted. Personally I would not trust a cabin
pressure release valve to always work. What if it malfunctioned? If it
stuck you could blow the sub apart. Of course you could always have several
pressure relief valves for safety backup, but it would be a whole lot easier if you just had an always open hole in the bottom of your ambient sub. This way no pressure
relief valve would be necessary. You can make the hole in such a way so that no water sloshes into the sub. Think of a sailboat keel/skeg box for the way to make
your hole. You would have a box like projection coming up from the hull bottom and at the top of that box would be your hole. This way you never have to worry about
a pressure relief valve malfunctioning and blowing your sub apart, and the air pressure in your ambient sub will keep the water from coming into the cabin. If any of this is confusing
to you let me know
and I will try to explain it better than I have. I was handicapped a bit in giving the above answers by not knowing if you meant to say "ballast tanks" or "compressed air tanks" when
you described how you plan to pressureize your cabin. Let me know if this helped or if you need further explaination.
Bill Akins.
----- Original Message -----
Sent: Saturday, March 18, 2006 7:35 PM
Subject: [PSUBS-MAILIST] internal pressure question
Hi All,
I'm working on the design of a proof-of-concept vehicle to test the ballast system of an ambient dry sub. I've figuered out that on descent air from the ballast tanks have to be vented thru the cabin in order to keep the pressure equal with the outside. On ascent, once sufficient water has been expelled from the ballast tanks, will the pressure inside the cabin need to be vented in some way to prevent the sub from exploding as the surroundind water pressure lessens?
Any thoughts or suggestions are appreciated
Philip Ridenauer