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Vance,
Ok I stand corrected. MOST times the MBT are
totally flooded when diving.
On a K sub, I don't see the advantage of a MBT
bubble over a bit less water in the VBT. Actually I think my VBT vents
faster then my MBT. If I want to get heavy It's easier to let a little
water in the MBT.
With the slow venting of the K-subs MBT I think you
could get into trouble diving with a bubble in them. If you started to
ascend, I'm not sure if you could release air quick enough to over come the
expansion before you popped up on top.
I don't know if it's the accepted way or not, but I
usually totally vent the MBT's, flood the VBT to neutral, dive with the
thrusters, then fine tune the MBT near the bottom if I want to be heavier or
lighter. Neutral is easier to control but I try to keep it a slight bit
positive in case I "fall a sleep." I'd rather be found skimming the
surface rather then be searched for on the bottom.
I agree, the K sub VBT's are to slow to vent.
Even in calm water it's tricky to vent equally between the forward and aft tank
to keep an even keel and it takes a long time.
Still diving without a MBT bubble, ;-)
Dan H.
----- Original Message -----
Sent: Wednesday, March 19, 2008 7:52
PM
Subject: Re: [PSUBS-MAILIST] MBT air
lock
Hey guys,
Not to rain on anyone's parade, but it was, in
fact, perfectly normal in the oilfield subs to trim a little heavy and "ride
the bubble" meaning a shot of air in the MBTs to maneuver with. It's handy if
you are operating near the bottom, and very quick, assuming a fairly stable
depth. That way, you can goof around down there to your heart's content, and
vent quick to settle onto the bottom if and/or when you need to. Neutral
buoyancy via small trim systems is a fine thing, but slow and finicky to do
(except in the JSLs which have almost as much VBT as most boats have
MBT).
Don't sell yourself short by saying that there's only one way to
do things, as there are, in fact, a whole pile of variations out there, and
they're like spare ammunition for your personal defense firearm--mighty handy
to have when you need them. The Navy guys cringe at the idea of that bubble,
but it worked a treat, and was even included in the beginning pilot's training
classes, right alongside VBT-101 and all the rest.
As to the
fairwater vent valve size, bigger is better. George's were (are) too small.
When I vent, I want to GO!, and I mean right now. Three-quarter inch valves
are, in my opinion, about the minimum. One inch and up would be even
better!!! That's for ball valves, of course.
The Aquarius used
pneumatic mushroom valves designed pretty much after a dry suit vent valve,
only bigger. The vent area itself was an inch and a half in diameter (or
thereabouts) with a ring of half inch holes in the body. Pop the valve and out
came the air, and I don't mean maybe. You could vent a half ton of fairwater
ballast air in no time flat, and it was really nice to be able to get
underwater without all that waiting around.
Remember, anything you can
do to keep from spilling your coffee is worth a try. Wallowing around in three
or four meter waves is something you don't want to do any more of than you
have to. Taking that lesson forward, it stands to reason that the more
flexible you can make your sub, the more capable it will be, and that
translates to a better tool for you. It's a sort of belt and suspenders
approach, which I heartily approve of.
Vance
-----Original
Message-----
From: Dan. H. <jumachine@comcast.net>
To:
personal_submersibles@psubs.org
Sent: Mon, 17 Mar 2008 10:57 pm
Subject:
Re: [PSUBS-MAILIST] MBT air lock
Frank and Brian,
The MBT's are only used at the surface. When diving they should be
totally full of water and not used for depth control. When the tanks are
being flooded, all air gets expelled, even the air in the line. That's
how the water gets in the line. Once my tanks are full of water I leave
the vent valves open and never feel a difference in buoyancy while
diving..
The Variable Ballast Tank (VBT or hard ballast tank) is what is
use to adjust buoyancy. That's designed with all ports closable so once
it's set, the bubble can't shrink or expand while your diving.
Frank,
It looks to me like your system will work since your lines is sloped
back. Any water caught in them should eventually settle back
in the tank, but you can't have any dips.. A three eights line may be a
little small though. It will take a long time to vent and it's
harder for air to travel up as the water runs back. Half inch would be
better. Water will be caught in the pipe above the closed vent valve
though. If it's low to the water line, it may be blown out by the
pressure in the MBT but if it's not, a quick burst of fill air through your T
will surely clear the line.
Still, I feel the better way to deal with the vents is to place the valve
right at the top of the tank and run linkage rather then tubing. But
it's not the only way. As long as you can prevent water from getting
trapped in the line or have a method to blow it out it will work.
Dan H.
----- Original Message -----
Sent: Monday, March 17, 2008 2:04
PM
Subject: Re: [PSUBS-MAILIST] MBT air
lock
Hi guys. Is the air lock problem on the ballast tanks due
to the line from tank to valve being too long? Or is it that the valve
has a length of line above it that holds water ?
I know that the line can't have any low spots in it, or even just run
level, but must have a continuous up-slope. I remember Gary saying his
original installation had a low/level spot that trapped a little water and
effectively plugged the line, but he fixed it by moving the valve higher.
His valves are located outside the tower, with the
valve handles inside and a seal on the valve handle shaft. It's a
nice clean installation.
I am planning on running the lines into the sub, and using a
''T'' with air line to blow the tanks, and the line exiting the sub near the
top of the hull. This method has the same number of thru-hulls, but has
more valves. One on each thru-hull, and one on the ''T'' for the air line. (
three valves for each tank.)
I think Gary's has a separate air line going to the tank for
blowing dry so basically one thru-hull for the air line and one for the
valve handle shaft.
My ballast tanks are several inches higher at the connection, and I'm
hoping that with the short length of the plumbing, I should be able to avoid
any blockage.
The hole in the bottom of the tanks is low and 3 inch diameter. I was
planning on using a 1/2 inch thru-hull for releasing the air. Does anyone
think that's too small ?
I'll try to put a sketch of the system up so you guys can comment on
it.
Frank D.
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