Pony Tank and Buoyancy/Trim

[eweingarden]

I got in the lake with the 19cuft pony slung on my left side. I had a very difficult time keeping from rolling to the left. I also found that my lower body felt more buoyant than I've experienced in the past. I was diving with a steel 100, 7mm suit, and was at a depth of 12'. I did another dive a few days later. I shifted 2lbs from my left to the right. My trim was better, but my lower body continued to feel more buoyant. Perhaps the latter was due to tank position on the BC? Any thoughts?

 

[Michael Guerrero]

If by more buoyant you mean that you were going head down, then you need to shift some weight backwards or pull your hands in, or extend your legs. Something to even out the fulcrum.

 

[eweingarden]

Thanks

 

[g1138]

Looking at cylinder specs, both the Luxfer and Catalina AL19s are neutral when empty and -1.3lbs when full (with the valve included). That means you'll be a little over a pound heavy on the side you sling the bottle. I would try just keeping more air in that side of the wing/bladder. If you are trying to stay still in perfect trim, and pushing the bubble to the side of the wing the pony is on isn't working, you could shift a pound to the opposite side. That will even you out when it's full, but you'll run back into the same problem when it's empty.

Really though, 1lb shouldn't be that significant.

-1.3lbs does not include the reg, which may be an additional 2-3lbs. 3-4 lbs offset; that's a lot of offset in trim. If you're sporting a stock Jacket BC, you're going to have to offset your weight placement.
Keep in mind your SPG or console clipped to the left side also has negative bouyancy, normally it's so minimal that you don't notice, but it does add up into the equation as well.

If you're using a Back-inflate, wing, or drysuit, you can compensate with an offset air bubble so long as you're not in the (-)4-5 lbs offset range. If you're in that range it becomes more of a burden, especially in a drysuit.

We use 13cuft ponys on my work's Jacket BC's and I have to offset 4 lbs to keep from rolling.
We use Sherwood Brut regs with a Button SPG & Second Stage; which doesn't necessarily put it on the heavy spectrum of regs. The 13's are -2.1 full though, unlike the 19.

With a Jacket BC, your pony bottle will also be blocking your weight pockets. This could make ditching or weight recovery complicated. So adding a weight belt may be preferable to split up your weight. But then you also need to learn a weightbelt roll recovery if your belt ever comes undone. Because standing on the bottom to lift a weight belt behind your back doesn't work on the anchor line.

Then there's the issue of handing off your pony when you're terribly offset. Not a huge issue if you're sticking to no-deco diving, but it does mean to be safe, you have to be 3-4lbs overweighted and offset.

 

[GrumpyOldGuy]

I drop 2lbs from the side the tank is mounted, it evens out my trim. The tank is full and with a reg, it runs 2lbs neg. I wear enough gear I don't mind loosing 2lbs on the climb out. Its not critical, just what I do. I don't hand off my pony, but if I do, loosing the 2lbs of ballast is not a big deal.

 

[eweingarden]

g1138:
I use a back inflated BC. What do you mean by an offset air bubble?

 

[g1138]

g1138:
I use a back inflated BC. What do you mean by an offset air bubble?

You can roll slightly to place the majority of your air in your BC onto one side.
Depending on the shape of your BC bladder when you level back out, you can trap more air on one side so it's not evenly distributed. This in effect puts more buoyancy to one side of your body.

So if you're heavy on your left side you can compensate by rolling very much right (air bubble rises to your left side), and then leveling out. This could trap more air onto your left side of the BC and even you out. Ie. offsetting your air bubble.

This is one of the downsides to having a Jacket BC, as you're not able to do this. But with your back-inflate you should be able to do this.

 

[scubadada]

As per several posters, I take 1-2 pounds off my left side when I'm diving with my 19 cu ft Al pony. Works very well for me.

Best, Craig