Tank position to trim?

[simbrooks]

As a precursor, i will say i have a good understanding of what a horizontal trim looks like and have been trying to attain that for a while now, this question pertains to tank position and weight placement.

Whilst out today, during the safety stop and general shallow water air burn off practice time after a cavern dive, i was playing around on my trim and trying to hover motionless. I feel i am well weighted at 1# (on my tank strap) plus my SS BP with my 3mm and AL80 tank, however i know my trim is out as i typically even when i try to hang still my feet seem to twitch slightly, i had thought this was just them not being used to being in such a position - but essentially they very slowly scull me along and i know it is to do with my trim being out. So whilst we were playing about my buddy took out her 2#'s of weight and put them down on a conveniently located platform which we were using as a reference point. I then decided to transfer them to another platform as a whole bunch of OW students descended on us and the platform at that time - look out below

I swim along and notice i am a little more head heavy as i hold these 2 1# weights in my hands, i then move them around and find that they are ok at about my shoulder and i can hang there with absolutely no movement of feet and no marginal tilting (out of trim) and subsequent correction - this was quite an epiphany for me. Ok, so now i have to try to recreate this in a real world weight placement (which means i will be overweight, but probably only by a pound or two). During work on this before i was advised to move my tank up my back a little to get more weight up top - it also allows me to reach the valve quite well too. The thing i wonder is if this is really a good thing for weight placement/balance? I can understand that with a steel it is going to be negative to neutral near the end and that weight will act to make your body trimmed, however with an aluminium tank it gets positive as it empties, so in fact you are getting a tilting buoyant moment pulling you heads up as your tank empties. I believe with my engineering mind that this should be the way it works around my centre of buoyancy and gravity and as such should i really put the tank back down a bit (as long as i can still reach the valve) and that should help whilst placing weight up on the top cam strap to recreate the moment i had this morning with the hand weights? Its something i have been pondering, but i just want to check my mechanics as i continue to work through to an ideal trim.

 

[jbd]

As a precursor, i will say i have a good understanding of what a horizontal trim looks like and have been trying to attain that for a while now, this question pertains to tank position and weight placement.

Whilst out today, during the safety stop and general shallow water air burn off practice time after a cavern dive, i was playing around on my trim and trying to hover motionless. I feel i am well weighted at 1# (on my tank strap) plus my SS BP with my 3mm and AL80 tank, however i know my trim is out as i typically even when i try to hang still my feet seem to twitch slightly, i had thought this was just them not being used to being in such a position - but essentially they very slowly scull me along and i know it is to do with my trim being out. So whilst we were playing about my buddy took out her 2#'s of weight and put them down on a conveniently located platform which we were using as a reference point. I then decided to transfer them to another platform as a whole bunch of OW students descended on us and the platform at that time - look out below

I swim along and notice i am a little more head heavy as i hold these 2 1# weights in my hands, i then move them around and find that they are ok at about my shoulder and i can hang there with absolutely no movement of feet and no marginal tilting (out of trim) and subsequent correction - this was quite an epiphany for me. Ok, so now i have to try to recreate this in a real world weight placement (which means i will be overweight, but probably only by a pound or two). During work on this before i was advised to move my tank up my back a little to get more weight up top - it also allows me to reach the valve quite well too. The thing i wonder is if this is really a good thing for weight placement/balance? I can understand that with a steel it is going to be negative to neutral near the end and that weight will act to make your body trimmed, however with an aluminium tank it gets positive as it empties, so in fact you are getting a tilting buoyant moment pulling you heads up as your tank empties. I believe with my engineering mind that this should be the way it works around my centre of buoyancy and gravity and as such should i really put the tank back down a bit (as long as i can still reach the valve) and that should help whilst placing weight up on the top cam strap to recreate the moment i had this morning with the hand weights? Its something i have been pondering, but i just want to check my mechanics as i continue to work through to an ideal trim.

With the aluminum tank by itself you might want to move it lower on your back, however, keep in mind that you have a first stage attached to that tank. It has been my experience that by moving the tank up on my back I get a little more head down results and the opposite when I move the tank down. I know when I put my gear in the water to demonstrate donning gear in the water the top of the tank(with the regulator) would sink.

 

[teknitroxdiver]

Also keep in mind that if you get the tank too high you can hit your head on it. I didn't think it would be that big of a deal, but one time I had my tank too high, looked up real fast and nearly blacked out. It hurts REAL BAD! Especially when you aren't expecting it.

 

[simbrooks]

With the aluminum tank by itself you might want to move it lower on your back, however, keep in mind that you have a first stage attached to that tank. It has been my experience that by moving the tank up on my back I get a little more head down results and the opposite when I move the tank down. I know when I put my gear in the water to demonstrate donning gear in the water the top of the tank(with the regulator) would sink.

Yes that is what i was wondering, if the result of moving an AL tank is the opposite of a steel tank, ie you want it a little lower, even with a little extra from the 1st stage, you still have all that buoyancy "pulling" you up. I shall have to play when my AL80's finally come in, these rental ones do cause trouble as they arent the same every time. Still thinking of adding the little pockets though to trim out better.

Also keep in mind that if you get the tank too high you can hit your head on it. I didn't think it would be that big of a deal, but one time I had my tank too high, looked up real fast and nearly blacked out. It hurts REAL BAD! Especially when you aren't expecting it.

Already finding that one out, with a yoke adapter on these rental tanks, it really sticks out quite a bit, havent had any bangs, but i know it is there. With the new tanks i have a DIN valve, so that should take it all back an inch or so, giving me more room. My tank isnt ridiculously high, but its high enough.

Thank you both for confirming my thoughts somewhat.

 

[JimC]

If your only talking 1 or 2#'s off horizontal, move your feet back and forth. You should beable to, with some pratice, control a lot of "trim weight" just by bring your heals in and out.

Hint: This will help you counteract the change in boyancy/trim caused by breathing down a tank.

 

[simbrooks]

If your only talking 1 or 2#'s off horizontal, move your feet back and forth. You should beable to, with some pratice, control a lot of "trim weight" just by bring your heals in and out.

Hint: This will help you counteract the change in boyancy/trim caused by breathing down a tank.

Good point Jim, my legs are already pretty well cocked up at 90-deg to swim, much more would probably strain them slightly, but if i balance myself with a full tank and legs slightly more stretched, then bring them in as the tank pressure drops, i can see some of that working.

Today i dropped in with a 3# on the top band and lowered the tank about 2", however this was an upwelling spring, so i needed that extra little bit of weight anyway, but out of the flow i felt more trimmed (just needed a little air in the wing). This seems to be working, so tank strap trim weight pockets here we come (where did i put that LP web address?).

However when i get my jets then of course my feet will be heavy again, and this merry-go-round will start again i am sure.

 

[JimC]

Oh, not in jet fins, eh?

Get them before you throw some bucks away on weight pockets. You'll have more trim controll attached to your feet then.

 

[The Chairman]

I work on my trim with my eyes closed... really.

In a pool with ALL of my gear, I get close to the bottom and I close my eyes. I wait a minute and see where I am. Move tank up to get the head down and back to get the head up. Each time I close my eyes longer and longer, until I feel I have it down pat. The 6 lbs or so you lose from emptying the tank still has the same center of gravity as the tank, so the difference is not all that noticeable.

 

[JimC]

The 6 lbs or so you lose from emptying the tank still has the same center of gravity as the tank, so the difference is not all that noticeable.

Not im my experince. I find there is a definite trim change as you empty an AL tank.

AL tanks get bottom light as the empty. They start off so that they rest on the bottom, then as you empty it the tail rises untill its sticking straight up.. then it goes shooting off to the surface if you keep emptying it. Its only a small change, I am suge most ppl who work on trim use body english to corect it. Concously or not.

 

[The Chairman]

Well, a tank just by itself will let the valve/regulator outweigh the rest... but it starts that way to begin with.

But if you start with great trim, it's quite easy to make those micro adjustments throughout the dive. If you start with poor trim you will be fighting it the whole dive. BTW, this is one reason why I like steel tanks!