BP / Tank Question

[Goindrinkn]

I wanted to title this thread - For Sale: a jacket style BC!

I received my H.O.G. bp/w last week and had a chance to do some test dives on it and am absolutely amazed at the difference. A convert for sure. But I have one question as far as tank considerations.

I have a SS-BP and when I dove with my SS lp 95 Even though I was weighted perfectly for my 5mm full, I was trimmed way top heavy. As I would roll a little right or left the tank would pull me over. So on the second test, I switched to an AL 80 and added a bit of weight in the front pockets of a weightbelt. Problem solved.

If I were to use an AL plate with the SS 95, (about the same 4 lb shift) would it still be as easy to trim or is it the fact that the tank is our farther beyond my body -giving me the same tendency to roll?

We do a lot of reef diving - so in a single tank config I do spend some time upside down or sideways to to peer under ledges or take some photos. So it is important to be able to hold an angle. (At the same time, my wife uses less gas than I do, a 95 allows us to end dives about even.)

 

[The Kraken]

More than likely.

Imagine, if you can, your body suspended in water perfectly horizontal and perfectly trimmed. Remember, you will have a C of G, a center of gravity. Your CG will be predicated upon your thermal protection, your rig (where the tank is placed with respect to your longitudinal axis, the requisite weight required for your buoyancy and any other ancillary gear you're carrying.

A shift of weight in one direction is going to cause you to be heavy toward the end which the weight is shifted. That's both longitudinally and laterally. Basic physics.

The difference between tank lengths, materials and weights plays a very significant role in determining your longitudinal trim.

You're are taking all the right steps in deductively solving your trim issues.

As to whether you want to stand on your head or roll over on to your side is up to you.

But keep in mind, with a heavy tank, once you've gone past a certain point in rotating your body left or right, the tank is going to have a tendency to want to keep pulling you in that direction. That is, of course, unless your disposable weighting is placed on the opposite side of you horizontal line of gravity in such a place and position that it offsets the gravitational pull of the tank.

That's one of the things that I enjoy about diving, resolving all of these little tests!!!

the K

 

[b1gcountry]

The AL plate will probably help some if you take the weight and put in in front of you, but I've found that steel tanks always tend to want to roll you over. It is something that you can get used to in time; keep in mind that you have to 'muscle' steel tanks around more underwater than AL ones. In time you will gain enough control that you can feel comfortable going out of level in a steel tank, but you will also learn to get into places you want to without turning sideways as much.

At least that's what I've found.

Tom

 

[halocline]

The AL plate will help with the steel tank. Depending on what kind of 95 you have, it's 7-10 lbs negative when full. For 'most' or 'typical' divers (take that with a grain of salt) in moderate wetsuits, 15 lbs of weight on your back is quite a bit. If that's your total ballast, you're almost certainly going to be top heavy. OTOH, if you split that into 8lbs on your back (tank) and 8 lbs on your belt, you'll probably be in the ballpark trim wise. If you want to be picky about it, it all changes throughout the dive as the tank empties anyway. In the last scenario, at the end of the dive you'd have more like 2lbs on your back, depending on how far you breathe down the tank.

It will definitely help, and AL plates are not that expensive. You should be able to get one and harness materials for under $100, and just move your wing form plate to plate as you need to.

 

[Jim Lapenta]

I find that with a SS plate and a single 95 with anything more than a 3 mil I do not have these issues. I use a 30lb wing and may be a bit heavy in everything but my drysuit. The other thing is that you may find over time that this does not prove to be an issue. Been diving single 95's, 72's, HP80's and al80 with the same setup and other than needing a few extra pounds with an al tank there is no tendency to roll. I often wonder if that's why when I started using doubles there was no real learning curve so to speak when it came to handling them. I can do barrel rolls, flips etc with a set of 72's or 85's easily. May be a diferent story when I put the 95's together but I'll have 100 or more dives in steel doubles by that time. I;d suggest just diving more and be more aware of your trim and how the harness is adjusted.

 

[cool_hardware52]

I have a SS-BP and when I dove with my SS lp 95 Even though I was weighted perfectly for my 5mm full, I was trimmed way top heavy. As I would roll a little right or left the tank would pull me over.

If the SS plate and steel tank are appropriate in terms of total ballast you likely do not need any other gear.

Assuming your harness is properly adjusted, and you are reasonably well trimmed "fore and aft" the single most important thing to prevent "roll over" is fin position.

At rest keep your knees bent, fin blades horizontal, and well apart.

Thsi requires that your thighs are spread. Having your fins ~4 ft apart makes the fin blades much more effective in prevent roll over. Having your fins close together makes rolling easy.

Look at any of the photos of divers in doubles who are "hovering", their fins are always apart!

Weight positioning *might* help a little, but consider the typical diver in cold water with steel doubles; ~4 lbs of empty tanks, 4 lbs of bands and manifold, 4 lbs of dual regs, 6 lbs of SS plate and harness, 6-8 lbs of "Vweight" and 20 lbs of gas. Often without any other ballast.

How do they keep from flipping over? Practice and fin position.

Tobin

 

[TNdiver6387]

Not trying to be smart here at all, so please don't take it that way, but dont doubles usually sit several inches closer to the person due to the way a single tank usually sits ontop of the ridge down the middle of the plate? How much would moving them closer offset the added weight?

 

[cool_hardware52]

Not trying to be smart here at all, so please don't take it that way, but dont doubles usually sit several inches closer to the person due to the way a single tank usually sits ontop of the ridge down the middle of the plate? How much would moving them closer offset the added weight?

Doubles do sit considerably closer to the diver, and also have a greater polar moment of inertia than a single tank.

The fact remains however that most divers in doubles have a great deal of mass on their backs, and do not resort to hanging "keel" weight on their front to keep from rolling over.

Practice and technique is the answer.

Tobin

 

[Goindrinkn]

Thanks. I appreciate the input. I will test again with an AL plate and a wider fin spread. sounds reasonable. - - - If you are reading this and wondering if you should try a bp/w setup... It really is all they say. I am a well known skeptic. But for once I am happy to say I was wrong. Thanks to the educators out there!

 

[Goindrinkn]

I just wanted to post a follow up on this after 6 months for anyone who might be interested. So I have played around with a few variations and came up with of the same conclusions here, so kudos to those that pointed them out.

First - Tobin was dead on about fin position. I was using standard open water fins, and was used to keeping them right in line behind me. Moving to jets, and giving a little more lateral spread become much more stable.

Kraken - In hind sight you comment: "As to whether you want to stand on your head or roll over on to your side is up to you." now makes so much more sense. And I am not sure if it's initially what you meant, but Standing on my head and rolling over were ways to see places that I wanted to look. However, with a little Fundies help, I found that doing that is no longer necessary. Getting the harness dialed in. (It was a bit loose and allowed the tank to have some room to roll away from me and build momentem) and getting some idea of what good buoyancy and trim are, has allowed me to to now see those same things in a hovering position without disturbing anything around me.

So as Jim and Tom eluded to as well, get more time in the rig and become a better diver and these problems seem to go away.

--- and also as noted above, after my recent first time on doubles, I am amazed at how much more stable I felt. With a properly fitted harness and the fact that the tanks take a little more to get moving, I've found that this seems to be a much more stable platform.

So thanks again for your time. Good Stuff all around!