Would like to get my mind around a tank/buoyancy concept

[dumpsterDiver]

At any rate, I am able to dive trimmed out; I just have to do different things with weight placement between aluminum tanks and steel ones (for the ones I have dived to date).

Thanks all,
Blue Sparkle

PS: ZKY, your post came in while I was typing. Thanks!

If you mostly dive steels and occasionaly dive aluminum. a simple solution might be to buy an extra tank cam strap and then throw 4-5 lbs on it and clamp it to the lower portion of the tank. This might allow you to leave the weight belt, BP/W etc. untouched and still sink your butt.

I normally dive a single steel, but when I grab an aluminum, i just throw some weight on the belt amd/or add lead to a BC pocket and it is not optimal, but good enough for me. I'm not too worried about perfect trim when diving open water anyway.

If I wear worried about it, I'm pretty sure i would make a rig that converts an aluminum to a steel bouyancy and trim.

 

[Blue Sparkle]

Hi NetDoc,

Yes, the best part is finding something that works well! I'm a bit of a nut about taking notes and experimenting (and I enjoy that part of the process as well ), so I've been able to come up with my various "best" combos for what I have dived so far; but like I said, I still like to "get" it on a theoretical level.

I hope this does not come out like I'm negating your suggestion, but I don't think I can really change the geometry of the waist-strap/crotch strap. My waist strap is as low as it can go (will not go past hips), and the crotch strap tight. I keep the shoulder straps a bit loose. I mean, not "bad" loose, but they don't seem to need to be tight like "fitted" tight. Because of the waist and crotch strap dynamic there can be no shifting up or down anyway.

I think part of it has to do with (at least my particular) female body shape. I know when I first used a backplate, it was riding too high on me (top of plate past shoulder level), and the DM commented that I should loosen the shoulder straps so it would go "down." The thing is, that would work on a male, or on a female with slim hips, but on me the shoulder straps could be a mile long and nothing is going to go lower at all. The waist strap is set, as low as it can go, at the top of my hips ("low" in the sense of towards my feet, if I were standing up). On the other hand, with "boy hips" and perhaps a longer back to work with, I can see where changing the shoulder straps and crotch strap would give one up/down adjustment.

When I bought my own gear I got a small backplate, so now with the waist strap at my waist the top of the plate is how they show it should be, wherein if you are standing up with just the plate on, you can reach back over your shoulder and just touch it with your hand (the plate I had rented was a "regular" size).

I hope that was even slightly clear; it's a little weird describing up and down and such as if one were standing up, but then in the water it becomes back and forward, etc.

Blue Sparkle

 

[aquaregia]

One of my fundies instructors had some pretty interesting input on characteristic differences between weight positioning on men and women.

I'd be sort of interested to see photos of you wearing your plate.

 

[Blue Sparkle]

If you mostly dive steels and occasionaly dive aluminum. a simple solution might be to buy an extra tank cam strap and then throw 4-5 lbs on it and clamp it to the lower portion of the tank. This might allow you to leave the weight belt, BP/W etc. untouched and still sink your butt.

I mostly dive aluminum tanks (Keys/renting), and that is almost exactly what I do. It does seem to trim me out just right. Instead of a cam strap though (buckle clearance a pain on boat tank racks), I have a length of rubber strapping and a metal buckle like is on a harness or weight belt. That way I can pull the rubber and stretch it, then buckle it (I also had the rubber strapping and the buckle on hand). I use 3# on an AL63 and 4# on an AL80, in salt water.

I'm thinking about getting my own tanks(s). Contenders are two steel 72s (one for each dive), which I think I could support even with no exposure protection (in salt water); or one LP95. I think I could get two Keys reef boat dives out of one LP95 fill, and then use it for deeper dives too (only one dive per fill tho'). That might be cutting it a bit close, but I could give it a whirl. I don't need any extra "tail" weight with the LP95, and I'm guessing the 72 would be similar; I would like to try one of those.

That said, the AL63s are working fine for me now.

I have to admit to being hooked on (what I'm un-professional-qualifiedly) calling good trim, even on shallow reef OW dives. I just love that "even puppet string on each arm and leg" feel - it's part of what makes me keep wanting to dive again

Some of this advice sure would have saved me some frustration and experimentation though! OTOH, I kind of like to experiment, as long as I can just change one variable at a time

Aquaregia, I will see what I can dig up. I'm pretty sure I have a photo from last summer, with my rental weight belt ends flourescently flapping all over the place I have to say that my diving enjoyment goes up linearly with each piece of gear I'm not renting :cool2:

 

[TSandM]

I just love that "even puppet string on each arm and leg" feel - it's part of what makes me keep wanting to dive again

Isn't it addictive? It's one of the things that keeps me going out and mentoring new divers -- I want everybody to know that that is POSSIBLE, because it is so much fun!

 

[Blue Sparkle]

Nice to know that I described that in a way such that you knew what I meant I mentioned it to someone else diving and he gave me a blank look, so I wasn't sure it would "translate." Yes, it's diving crack!

 

[Charlie99]

I think that I'm a little more clear on this also. How about:

When your tank goes buoyant there is a trim shift so you rotate in the water. Maybe you sensed this as a buoyancy change and were compensating by adjusting your buoyancy. (Your buoyancy didn't need any adjustment.)

-fun thread, lots to think about...

Think about this .....

even with a steel tank, when you consume the air, you have changed the weight and therefore your trim. Just as with overall weighting, where the change in buoyancy does not depend upon the absolute buoyancy, neither does the trim.

I find that the position of the tank is what counts. More specifically, the position of the center of the tank.

If you have trouble with trim changing as the tank (steel or aluminum) empties, try changing the height of the tank on your back. Changing lead positioning appropriately also, of course.

Charlie

 

[lowviz]

.....................I find that the position of the tank is what counts. More specifically, the position of the center of the tank................................

Yes, indeed. And once you trim out the tank you can drain it without altering trim -but only if the tank is centered over the point you would rotate around if someone spun you underwater. Torque, resolution of forces and all that stuff. Was trying not to go there. Would make a hell of a video, get someone horizontal, neutral and stationary then gently push down on their head while filming from the side. The center of rotation should show itself before your flotation (BCD wing suit) messes things up....


edit:
Like dumpsterDiver, I tend to be pragmatic about my trim. However, if I ever get the chance, I may try this in a pool (close to the side so I can use the bricks as a reference grid)...