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Discussion Starter #1
I got a cheap twinset.
The problem is they are dumpy 12s and top heavy.

I'm using them with an OMS bungeed wing and backplate.

First couple of dives I had to stay very head-up, if I tried to get flat air would go to my feet and then I'd be going feet up.

I ended up actually moving cylinders higher on backplate, I could just reach valves then, and moving wing up to highest of 3 holes, it is reasonably comfortable but still a little bit top heavy. I've done about 15 dives like this.
It seems to be at it's worst with empty tins.

With one more hole to go higher on the backplate I want to nudge the tanks slightly higher too as it really is quite painful to get my (previously highside-affected) left shoulder round that far.
So what I've done is move the tins higher and make a P weight to sit right at the bottom of the backplate, it's about 1.7kg. But not dived this yet.


So if this doesn't dive right what can I do? Please don't suggest euro cylinders or buy a new wing, I'm poor due to ongoing chronic projectitus!

Figure my options are

try a bigger P weight or make the backplate lower (but it's nicely comfortable as it is).
remove the top bungees from the wing to try and persuade some gas to sit at the top rather than bottom. Anyone done this?



Oh and my mates twin 10s with dive right wing feel like they trim out lovely and it's easy to stay comfortable with them so I don't think it's just me!
 

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Conscientious Objector
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sorry, but the only answer here is to get rid of the dumpy 12s. seriously, it's a bad idea!

Unfortunately, the bands and manifolds on your set are only any good for twin 15s or dumpy 12s.

You'll probably make enough money stripping them and selling them in bits (or with cheap pillar valves) to be able to afford a set of "proper" 12s.

Alternatively, have a rummage through the garage, shed, loft, Mrs's wardrobe etc and flog a load of tat on Fleabay to fund proper 12s.

Seriously, dumpy 12s are the devil's work even as singles, so as twins...?
 

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MIFLEX DIR = "Did It Rupture"??
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sorry, but the only answer here is to get rid of the dumpy 12s.

Seriously, dumpy 12s are the devil's work even as singles, so as twins...?
Same might be said for a bungee'd wing.... :angel::D:D:tongue:
 

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Dive tart, just can't say no :-)
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Seriously, dumpy 12s are the devil's work even as singles, so as twins...?
Seems like a massive generality to me. Surely that depends on the user's height. I've never dived dumpy twins but single dumpys work really well for vertically challenged folk and I'd expect the same applied to twins.
 

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remove the top bungees from the wing to try and persuade some gas to sit at the top rather than bottom. Anyone done this?
I've done the reverse. Tightening the bands to keep the air lower. It made some difference but not a lot.
 

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moving the tanks up on the plate will make you even more head heavy/feet light. are you doing it for trim or shutdowns? obviously moving lead further down in some way will trim you out.
 

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Can one flounce in a drysuit??
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What kind of backplate do you have? If it's SS, and especially if the P weight you're adding will be over and above your current weighting needs, how about swapping it for an ali one? It didn't completely solve the same problem for my twin 10s, but it was a noticeable difference.
 

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have you tried a keel weight a bit like a v weight but with a bar attatched that bolts onto the bands and sits right at the bottom of the cylinders or get some heavy fins but not sure how much of a difference these will make.
imho split the twinset up sell as single 12's and buy some nice faber(or whatever you want) twins you may be able to use the manifold if you can fond some bands that will fit if you buy 2 single faber 12 you can just swap the valves over and get some new bands if you can fond some
 

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For now, drop them down, close the manifold and dive as indies.

Dumpies have a double whammy of being short and the center further away
from the BP, which makes the pivot point more critical. By diving as indies
you can forget the need to reach for shutdowns and place it where it feels
right instead.
 

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Nigel Hewitt
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First couple of dives I had to stay very head-up, if I tried to get flat air would go to my feet and then I'd be going feet up.
Way too much air in the suit.
That's why you have a wing.

I have a pair of dumpies here. On my back they are a bit heavier than normal 12s but they are so close to the centre it makes no odds.
 
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I dive with 300bar double 10s and have no problem untill I used to stuff to much air in the suit.
Just enough to take the squeeze off and then use the BDC/wing.

As I was told, the clues are in the names.
 

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Aquanauts tea boy & GUE instructor
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Trim problems stem from, and are generally sorted by correcting three inter connected issues. 1) Centre of gravity. In your case it's too far forward, ie all forward of your waist. This is made worse by the weighting characteristics of your set. The natural reaction is to sit back at about 40-45 degrees to move that weight back and over your pivot point. Correcting this in a twinset with a p-weight or tail weight will help (remember to adjust your total weighting if adding trim weight) I suggest a weight belt rather than v weights (between the tanks) to keep it all as close to the centre of gravity as possible.
2) Kit set up. This is generally down to a poorly fitted and adjusted harness. Too tight to help getting at the valves and the weight is forward, to loose to help getting in and out and the whole rig is unstable. The top tank band should be just on the break of the neck, and with the harness set up properly this will move a little of that weight back down your back.
3) Body technique/position. Eyes forward, head back and against the isolator, elbows up, gentle arch in the back, a bit of tension on the butt.

Bobs your uncle.
 

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Owner of the Frankinspo
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You could try inverting the cylinders.

Cost would be a slob-knob to reach the manifold, and a halo to protect the valves & regs.

Plus-side is the valves are easier to reach and the weight of the valves goes to your middle so may make trimming easier.

Down side is you may need to re-route your reg hoses and might need some longer hoses.

You could test them inverted to see how they trim out before you go to the expence of the slob-knob, etc, but when testing be aware that your valves are vulnerable when kitting up and de-kitting, and you can't easily isolate without the slob-knob.
 

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Conscientious Objector
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Cost of inverting would be custom length hoses (certainly wing inflator and short reg hose?) plus a halo or other valve protectors. Nigel Hewitt has a good page on his website about inverting twins:

Nigel's twinset rig
 

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Discussion Starter #18
Wanted to stay away from ankle weights as it's one more bit of kit to buy.
Same with inverting them, by time I've bought slob knob, valve cage, loads more hoses I could have sold tanks and bought euro 12s or 10s.


moving the tanks up on the plate will make you even more head heavy/feet light. are you doing it for trim or shutdowns? obviously moving lead further down in some way will trim you out.
Trying to get it so it will be useable in both respects.

Way too much air in the suit.
That's why you have a wing.

I have a pair of dumpies here. On my back they are a bit heavier than normal 12s but they are so close to the centre it makes no odds.
I thought that was the case, but I do generally run the suit fairly tight and use the wing. And when trying to sit the same way in the water (flat wit knees bent) using the twin 10's I don't tend to pitch forward.

Trim problems stem from, and are generally sorted by correcting three inter connected issues. 1) Centre of gravity. In your case it's too far forward, ie all forward of your waist. This is made worse by the weighting characteristics of your set. The natural reaction is to sit back at about 40-45 degrees to move that weight back and over your pivot point. Correcting this in a twinset with a p-weight or tail weight will help (remember to adjust your total weighting if adding trim weight) I suggest a weight belt rather than v weights (between the tanks) to keep it all as close to the centre of gravity as possible.
2) Kit set up. This is generally down to a poorly fitted and adjusted harness. Too tight to help getting at the valves and the weight is forward, to loose to help getting in and out and the whole rig is unstable. The top tank band should be just on the break of the neck, and with the harness set up properly this will move a little of that weight back down your back.
3) Body technique/position. Eyes forward, head back and against the isolator, elbows up, gentle arch in the back, a bit of tension on the butt.

Bobs your uncle.
The harness is set up so I can just touch the top edge of the backplate, it's comfy and doesn't flop around (with crotch strap). I can get in and out of it OK topside and underwater.
OMS stainless backplate.

I have been using 4kg (2x2kg) on a weightbelt. I'll drop this down to 2x1kg if I can scrounge some 1kg blocks as I think 4kg was about right, could hold 3m with 20 bar in.



As it is I can reach the RH post and isolator no problem, but I can't get the left arm up that far. To get to the left valve I have to shove the set up on my back with right arm and "walk" my fingers onto the valve and it bloody hurts and takes longer than it should. Am I being picky?
 

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The issue is get-roundable, but in all honesty I would split the tins and flog them, and try to get a 'normal' set of twin 12s. I suspect it would not cost a great deal more if all done 2nd hand.
 

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Discussion Starter #20
Aye, probably doable fairly cash neutral. Will see how they go next weekend, hopefully the P weight will make everything nice and comfy and flat (hahahaha), or at least liveable for another 100 dives or so until I get the rebreather built...
 
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