Re: [PSUBS-MAILIST] MBT air lock

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 -----

From: vbra676539@aol.com

To: personal_submersibles@psubs.org

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 -----

From: ShellyDalg@aol.com

To: personal_submersibles@psubs.org

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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