|
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.
|