Why does my tank rock

I dive a Halcyon eclipse with a STA. I love my Halcyon but find that the tank tends to rock at times. I am having a hard time understanding the physics behind the rocking as my cam bands are super tight, my STA is tightly bolted to the BP and the bolt is relatively low profile and so I dont believe that it is pushing up against the tank. Also moving things around on land cannot duplicate the rocking. Any ideas?

Thanks

Craig

Craig,

Do your cam bands have some rubber pieces to compensate for the change in length of the cam bands as they get wet and stretch out? Have you tried soaking your cam bands in advance of the dive?

Is it a steel tank?

Mr Carcharodon:

Craig,

Do your cam bands have some rubber pieces to compensate for the change in length of the cam bands as they get wet and stretch out? Have you tried soaking your cam bands in advance of the dive?

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Yes, I always soak prior to use. The bands do have pads but those are mainly to prevent scratching the tank by the buckles

supergaijin:

Is it a steel tank?

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I believe it happens with both steel and aluminum.

Can you be a bit more descriptive. Is your tank rocking side to side or is it rocking top-bottom wise?

What are you doing underwater when you notice the tank rocking?

Are you using the Halcyon STA? Have you tried placing the bolts so the wingnuts are between your back and your plate?
Are you sure it's not the plate thats rocking?

Sorry, I'm just handing out more questions than answers here. =\

g1138:

Can you be a bit more descriptive. Is your tank rocking side to side or is it rocking top-bottom wise?

What are you doing underwater when you notice the tank rocking?

Are you using the Halcyon STA? Have you tried placing the bolts so the wingnuts are between your back and your plate?
Are you sure it's not the plate thats rocking?

Sorry, I'm just handing out more questions than answers here. =\

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The rocking is side to side and only when I rotate side to side. When I roll it rocks one way and when I right myself it rolls the opposite way.

I am using the halcyon STA.

I do have the wing nuts between my back and plate

I am not sure it is not the plate rocking as all I feel is the knock but I do make sure everything is super tight.

Hope that helps

Craig

I have had this happen with two different causes. The first was what I was trying to get at in post #2, the cam straps stretching out during the dive (pre-wetting the straps, and having something elastic to take up the slack was the solution to that). The second was the STA nuts coming loose. That is pretty easy to diagnose since the STA will be loose after the dive. Nylock nuts solved that problem. They have a nylon insert at the top of the nut that prevents them from turning off on their own. Of course the tank needs to contact the STA in a stable way to begin with. Some STAs will not work with all diameter tanks. You can tell by just holding the STA up the tank and trying to rock it back and forth in your hand.

Mr Carcharodon:

I have had this happen with two different causes. The first was what I was trying to get at in post #2, the cam straps stretching out during the dive (pre-wetting the straps, and having something elastic to take up the slack was the solution to that). The second was the STA nuts coming loose. That is pretty easy to diagnose since the STA will be loose after the dive. Nylock nuts solved that problem. They have a nylon insert at the top of the nut that prevents them from turning off on their own. Of course the tank needs to contact the STA in a stable way to begin with. Some STAs will not work with all diameter tanks. You can tell by just holding the STA up the tank and trying to rock it back and forth in your hand.

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I am pretty certain I am still tight on the nut at the end of the dive but I like your idea of nylock nuts and will get a couple. Will make sure that my bands are totally soaked before tightening. Thanks

You may want to try two things.

Go down in a pool and see if you have the same rocking problem. If you do, ditch your BC and rock it in your hands underwater to see if the tank is moving. If you see no movement, then it's not the tank.
You can also brief your buddy to rock your tank and see if it moves when you're on an ocean dive. Just make up a signal for it when you want him to give the tank a rock.

Secondly, your Straps shouldn't be super tight persay, whether you're using the quick cinch or a standard Hog harness.
Your shoulder straps should be loose enough that you can fit 2 fingers between your shoulder joint and the straps (w/ usual exposure protection on). You should also be able to pull the straps off your shoulder easily.
The top of the Bp should be where you can reach your hand over the same side shoulder and touch the top of the plate.

From there: The waist strap comes on as tight as is comfortable; I stress comfortable. The crotch strap comes up to meet exactly where the top of the waist strap is; some will lengthen it by 1in, but I feel this allows my Bp/W to ride up too much for my liking.
These things together make the Bp/W stable underwater, even with wetsuit compression.