RE: [PSUBS-MAILIST] variable buoyancy

...this type of input is really quite invaluable, otherwise I have no choice but to imagine....thanks Vance!

Joe

From: vbra676539@aol.com
Reply-To: personal_submersibles@psubs.org
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] variable buoyancy
Date: Sat, 30 Dec 2006 16:44:29 -0500

Dean,

You are quite right. A small bubble will work in one of the MBTs. However, your double valve idea is over-complex. We did very much the same thing on the professional circuit using just the 1/4" ball valve on the panel at 2250 psi, and it worked fine--just give her a little squirt. It takes a practiced wrist, but you'll get the hang of it pretty quick. We had trim systems (on the Perry boats) which we usually set slightly negative and left alone, then when we got to wherever it was we were going, I could squirt some air into a tank and go neutral. The nice thing is that you can go negative again very quickly, just by opening the big valve and venting that bubble, or part of it. Keep in mind, actual operations is about staying near the work, whether in the water column, or on the bottom.

Having said all that, I also operated Aquarius for two seasons with Hyco, and they had, in fact, stripped the original VBTs out of the boat so all we had to go on were lead weights for coarse adjustments and air in the MBTs. It meant that we had to run the boat negative (with MBTs flooded) ALL the time, and use the mains to go neutral. That worked fine, once I got used to it (and probably STILL works fine, as I don't see any trim system on the current iteration at Nuytco). And guess what? It cut a whole system of valves and operators and tanks out of the boat. That's a bonus, when you can get away with it. It was a pain in the ass from time to time, but worked mighty well most of the time.

As a sidenote, I did surveys and light work stuff in that little thing from the surface to the bottom, in all kinds of weather, and it did everything I ever asked it to. AND, if anyone asks, a 48" diameter hull is about perfect for two grown-up children to work in. We could lay down, sit up, stretch out, pee in peace and didn't even kick each other very often. I consider 42" about minimum for comfort, which is why we all sit up in the Ks. There just isn't any room to do anything else.

Phil Nuytton has carried all this a bit further, using a small internal flood tank of captured water (in an external bladder) to trim neutral on each dive. That way you don't have to tote lead, and you can STILL blow air into the mains for positive buoyancy if you need to. And once your trim is in the onboard tank, you can leave it for the next dive, and the next. When the next pilot hops in, you shift the water back to the bladder and he goes through the same process, tailoring the trim to his weight and preference.

The only real issue for our psubs that I see is shallow water ops. A MBT bubble expands really fast in just a few feet, and you'll have to be quick to control it. Otherwise, who cares? Folks on the psubs site worry about this a lot, but you can trust me on this. A ton or two of submarine (never mind eight or ten tons!) just doesn't react all that fast to a few pounds of buoyancy either way. Any pilot worth his salt (and you get salty in that pilot's seat, it's really the only way) will be able to control his (or her) boat just as well with the mains. VBTs are fiddly, very handy for minor adjustments, but not terribly useful otherwise.

It's nice to be able to do EVERYTHING in a sub--and flooding a little water in or pumping it out is a good thing. A very good thing. But there are trade-offs. I'm overhauling a K-350 as we speak, and the VBT is about rusted out. I've got to make some decisons along these lines myself. I'm going to put some kind of VBT in, just haven't decided whether hard tank or soft tank inside with a pump. I just can't help but remember how sweet it was when one of the boats was perfectly trimmed--you could steer the thing by leaning from side to side, and it didn't take any effort at all to steam along doing video surveys of pipelines or rig bases or whatever. Mostly that kind of thing takes a good variable ballast system.

Now, you've got lots of answers, but no solutions. Sorry, that's kind of how it works. Given the room, the money and opportunity, I'd opt for a VBT. But if it worries you, then launch without the silly thing. You can always cuss me later and put one in. T'ain't like the ABS is going to come knocking on your door, is it?

Happy New Year,

Vance

-----Original Message-----
From: Recon1st@aol.com
To: personal_submersibles@psubs.org
Sent: Sat, 30 Dec 2006 12:03 PM
Subject: Re: [PSUBS-MAILIST] K350 Parts

I have read many posts discussing secondary bt

for fine neutral control.

My question is can't the mbts be used for this?

It seems to me it could be done by using a simple

method.

my thoughts are to put 2 inline control valves on the

hp air line. the valve closest to the mbts would be left

open and the 2nd one would operate normally. Then

when fine control is needed, you would close the front

valve, open the back valve then close it. This would allow

a small amout of hp air to be delivered giving fine tuning.

the same could be done on the air release side.

The amount of control would be the volume between the two

valves.

So am I ignorant again? am I missing somthing here?

just a planning away

Dean Ackman

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