Pony Tank and Buoyancy/Trim

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eweingarden

Contributor
Messages
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Location
Canton, CT
# of dives
100 - 199
Just got a 19cuft aluminum pony tank which I plan to sling from the left side of my BC. Any way to get an idea on how I'll need to change the amount of weight I'll need, and it's distribution, before I get in the water? Thanks.
 
I wouldn't change the amount of weight you use. I dive with an ali 7 litre stage (ali 63) most of the time, and I keep my weighting the same. I barely notice it, it's maybe a kg negative when full, and maybe a kilo positive when at 50 bar.
 
You may notice it more if you dive a wetsuit. In a drysuit you can easily shift the bubble around to remain stable if you have differing weight on either side of your body (due to all the crap you have clipped off). In a wetsuit, you have to rely on the wing/bladder to provide additional loft (initially on the side the pony is on when full, then on the opposite side if it's empty).

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.
 
I added a 2 lb weight on the opposite side since my pony is mounted on one side of my main tank. That reduces the roll.
 
I cold water dive with a wetsuit and an AL19, I don't change my weighting at all.
 
I did not notice any trim change when slinging an AL30. I probably compensate for it unconsciously. I don't change my weighting.

Best wishes.
 
I dive wet with my AL19 and usually drop 1-2 lbs when carrying it. Also shifting a little weight to my right side to compensate for clipping it on my left. With an AL19 it is going to be slightly negative when full and essentially neutral when empty. That said, as others mentioned, we are talking about ~ 1lb of difference here so shouldn't have a material impact on your trim or overall buoyancy.
 
I've been diving with an Al40 bailout with my rebreather and haven't noticed any issues with weight distribution.
 
https://www.shearwater.com/products/swift/

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