Wow. You're either clairvoyant or I've been trying to re-invent the wheel. (Too bad there's no money in reinventing the wheel.) I was not aware of the Hunley's ballast system. Everything I've read indicated that the Hunley was really an awash boat not a true submersible. Looks like I need to do more research.
As for the underwater glider concept, that hasn't been too far from my mind. The proof of concept route was simply to verify my thinking but I was planning on a motorized final version. The idea of the glider was merely a way to extend underwater range and endurance. It's neat to know that someone else has thought of it and made it work.
One question about the ascent phase of the glider would be: how fast is too fast. I'm no diver but I've read never ascend faster than your bubbles. If the boat were restricted in vertical movement between, say, 10 and 100 feet, then you would
glide down 90 feet (say 1 ft/sec or 90 sec.) then glide up the same amt at roughly the same speed. Would that be too fast?
I've been around the psubs site but I'm sure I haven't found everything in it. Is there futher info on the Hunley there or if not could you direct me where to look?
Thanks again for your time. If I can I'll try to draw up something on paintshop and e-mail it, a picture being worth a thousand words, and all.
Hi Philip.
From your description it sounds like a non powered self surfacing/decending ambient diving sub. Hook a cable on it and you could pull
it with a surface boat.
After you are done with it as a proof of concept project, you could have fun using it with a surface boat while you build your final concept project. Sounds very
cool. I'll try to show you some things on your buoyancy and air compression.
You said...."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."
My reply....Picture this Philip, You are on the surface wanting to decend, so what do you do? Ordinarily you would vent air out of the ballast tanks into the atmosphere so they take on water
and you submerge, right?
But you want to vent those ballast tank's air into your cabin instead of just out into the atmosphere right? If your sub is already very heavy even with the
ballast tanks full of air, and you ride low in the water with very little freeboard, and you opened a valve to allow the ballast tanks to vent to the cabin IF the boat is heavy enough in
the water to compress the air in the ballast tanks you might get some compression of the air in the ballast tanks and therefore some of the air in the ballast tanks would enter
the cabin and start compressing the atmosphere in the cabin UNTIL that atmosphere in the cabin became just as compressed and with as much outward pressure as the water
pressure on the outside. Then your ballast tank's air would stop flowing air into the
cabin because the interior cabin pressure would be equal to the outside water pressure pushing on
the air in the ballast tanks trying to force that air into the cabin that is the same pressure as the air trying to get into the cabin. Like I said, if your boat was very heavy with low freeboard
and it took little air removed from the ballast tanks to submerge, you MIGHT submerge but you wouldn't go very deep because the interior cabin pressure would soon build and that would
stop the air from the ballast tanks trying to enter the cabin. There is however a way you COULD make this work.
If you force the boat under with her motor (you said yours would be unpowered) and dive planes rather than just trying to submerge with ballast tanks alone, this would allow the
outside
water pressure to exceed your interior cabin pressure and this would compress your ballast tank air you have piped going into the cabin and compressing the cabin. Because you
are forcing the boat under and exerting pressure against the ballast tank air forcing that air into your cabin. But as I said, you need a motor to do that. All the above was directed to most of
your above statement, but for the last part of your statement you said "from which it could be vented to the outside". Why would you want to go to the trouble to make a sub that would allow
its ballast tank's air to be compressed by outside water pressure which is then routed to compress your cabin interior to keep you ambient, and then vent that air to the outside? Why not
do
what they did in the hunley and have a air pump inside the cabin and just pump the air from the cabin back into the ballast tanks for surfacing instead of wasting it and venting it to the atmosphere
when you took such trouble to save it by venting it into the cabin?
You said...."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."
My reply....As I
mentioned above, just pump the air from the cabin interior back into the ballast tanks and then there is no need for the compressed air tank you mentioned. Of course you would have
a compressed air tank for safety and emergencies. This would also take care of the last part of your statement. There would be no overpressure in the cabin if you pumped the compressed air out of the
cabin and back into the ballast tanks.
You said...."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."
My reply....Exactly. Good thinking. The trunk would not have to be that high. Just high enough to stop water from lapping over the hull edge into the boat.
The centerboard truck hole would also be a good safety in case you accended so rapidly that your interior cabin overpressure did not have a chance to all exit
out of the cabin back into the ballast tanks before it would build so much pressure it might burst the hull before it could all exit to the ballast tanks, then your hole in
the bottom would allow any excess
to go out without any fear of overpressurizing the interior and exploding. Of course since this is an ambient sub and if you were under for any appreciable length of time and depth, and you accended that rapidly
overpressurizing your hull would not be your only worry. Rapid decompression and the bends could result in death. This is the main bugaboo about ambient subs. You have to remember your body is just as
pressurized and saturated with nitrogen as a diver on the outside at the same depth would be. So an ambient sub ABSOLUTELY MUST HAVE a reliable ballast system that will not rocket you to the surface
and kill you just as a too rapid accent would kill a diver diving at those same depths and for the same length of time. I think the best thing to
have on an ambient sub is a SLOW ballast system that gently rises
the boat and is incapable of rocket accents.
Consider the things I wrote, but you are on the right track and whether you realize it or not you are talking about making a proof of concept vehicle that has a ballast system that is very like the civil war Hunley's recycleable air ballast system
as long as you vent the cabin air back into the ballast tanks instead of into the water needlessly wasting it, then your concept is a Hunley type ballast system on an ambient sub. Just be sure and use
ballast tanks isolated from the cabin interior and only venting their air into that interior via a valve. No open to the cabin's interior, open top ballast tanks like the hunley had. Leave that bad
idea out of yours.
I know you were thinking of a unpowered underwater elevator going just up and down, but a system like this could also actually travel forward without any motor. As you decended, your dive planes could decend you downward at an angle
so your would travel forward. Then when you accended your dive planes could angle you upward as well resulting in forward motion from both decending and accending. Just like Carl Stanley's sea bug does without any motor. You could
have an underwater glider like Carl's. Since you would be constraned to shallow depths (unlike Carl's 1 atm that goes very deep) you probably wouldn't travel that far forward, but you would travel some nonetheless. No accounting for underwater
current here of course. In theory you could have
an ambient, shallow diving to scuba depths, non powered submarine glider, using a recycleable air ballast system never needing compressed air. I would like to see that built.
You said...."All this thinking makes my head hurt."
My reply....Don't worry Philip. Mine hurts all the time from trying to figure out sub stuff too. Everyone's here does. Take a break and come back at it fresh. The important thing is to never give up.
Bill Akins.
----- Original Message -----
Sent: Saturday, March 18, 2006 9:55 PM
Subject: Re: [PSUBS-MAILIST] internal pressure question
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 t! o 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 pres! surize 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
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