Trim stability with BP/W

[JanK]

Hello all!

Since this is my first post here, some introduction before the question. I've been a SCUBA diver for three years, have 57 dives, of those 32 this season and plan to do around 10 more (I'm hooked ) I got CMAS P1 two years ago, PADI AOWD last year and will be completing deep diver and rescue diver this month. I plan to dive for a bit to see where my ambitions will lead me before taking further classes.

For the last few dives I've been trying to set my trim. My goal is to be as stable as possible in any position. With the standard BCD I can be stationary in a lot of positions but as soon as my front faces up I'm pulled into face up position. If I got this right it's because the air flows to the front part of the bladder and the centre of buoyant force shifts towards the belly, thus increasing the distance between centre of gravity and so increasing the stability of the position - an undesired effect.

As I understand the working of a BP/W this should not be such an issue as the bladder does not move as much - the smaller the bladder the less of movement it makes. Am I correct?

If so, is it possible (and how) to trim a BP/W the way that I'd like it? I will be trying out a plastic BP/W this weekend and would appreciate some hints about the position of the 10kg of weights.

TIA,
Jan

 

[StreetDoctor]

if you're wearing 10kg of weights you should use as stainless steel bp. Any ditchable weight you have to wear should be on a belt closest to your center of gravity with the weights over your lower back.

 

[fire_diver]

While I love my BP/W, neutral bouyancy can be achieved in a jacket BC, BUT you need to have almost no air in your BC for it to work. This means that you must be almost perfectly weighted so that you are never more than a few pounds negative even at the start of your dive. This removes your air bubble that changes position and changes your center of gravity.

 

[ligersandtions]

I agree with StreetDoctor -- if you need that much weight, you should be using an SS backplate as it will take some of that weight off of your belt. For example, my plate weighs around 3.5 pounds; in addition, I have weight plates that attach to the plate that add about 6.5 pounds. This means that 10 pounds that used to be on my belt are now on my plate, directly above my lungs. However, if you're just trying out (borrowing) the plate to see how it feels, that's cool....but I'd recommend buying stainless if you choose to go that way!

As for how trimming out works with the bp/w set-up, it's pretty much the same. You'll need to play around with your weights and tank to see what your optimal set-up is. You can trim out a jacket BC if you need to....likewise, you can trim out a bp/w....but neither is going to be easy if you don't have your weights set up properly. Unfortunately, I can't tell you how to set them up -- it's something you'll have to play with yourself.

I find that my HP130 trims out almost effortlessly, while diving an Al80 requires me playing around with it a lot more. Play around with it and report back to let us know what you think. Have fun!

 

[StreetDoctor]

To further add one Ligersandtions said, I have a DSS SS plate that weighs 6lbs. When I dive dry with an AL80 I need a fair amount of weight. I then also use weight plates that add 8lbs. If you did this you'd only have to wear 8lbs (roughly 4kg) on your belt. If you are diving wet though there's no way you should need that much weight. Shop around when you decide to purchase, different companies make different weighing plates.

 

[JanK]

Yesterday I tried the BCD that I borrowed from my diving club, so now I have something to write about (instead of just saying thanks). Unfortunately it was not a true BP/W, rather it was a "tech" wing BCD, a Poseidon Besea W50 Wing & Platform Harness (can't post a link yet, but you can Google it).

It was a mixed experience. I could only do one dive with it as the first dive of the day was an orientation dive that I needed to do to finish my deep diver course (I was part of a group that took the course three weeks ago and we all failed miserably, so each of us needed a repeat orientation dive to complete the course). So the experience is limited, but still partly valuable.

My usual setup is a basic BCD, a Sepadiver Nautilus, with an added crotch strap for stability (otherwise unless I tighten the belly straps so much that I can hardly breathe, the BCD rides up under my armpits when vertical, making me feet heavy). The 10kg weights are split among the weight pockets (3kg in each pocket with 1kg lead at the back and 2kg lead at the front of the pocket) and the belt (with weights at the front). The 15 liter 200bar steel is positioned as high as possible. In fact I'm thinking of mounting the HP hose for the pressure gauge on the opposite port, so that I can mount the first upside down, since my head is banging against the barrel (ScubaPro Mk25). As written in the original post, with this setup neither the BCD itself nor the cylinder move around, I got the trim sorted out so that I can hover in the horizontal position (head back, fins horizontal above knees) and the only problem is that if I pitch or roll too much I get pulled face up.

The effect was even more pronounced with the wing. I know that the 25kg lift and thus volume were too high - my wet suit doesn't loose that much buoyancy at 30m - and I suppose the right lift would be 15kg. This probably moved the bubble and with it the CoB in the horizontal position too high, so the CoB moved quite a bit when rolled over.

The size also meant that the dumping of air through the back exhaust valve required a relatively large pitch down before the bubble would reach it (I drained more water from the wing after the than I usually do from my BCD).

The BCD wasn't as stable on the back as I thought it would be. I suppose this is because of geometry - the narrow width of the plastic plate - and that the "real" size BP would be more stable.

I did like very much the absence of bladders between the arms and the torso.

I am sure that the real BP/W would improve on the one that I had. But I suspect that it would not be a drastic improvement on the current BCD, at least for the sort of diving I currently do.

So instead of spending the funds on this, I will start trying out drysuits. The usual temperature at depths in our parts is 14C and I'm usually cold. So the increase in warmth will probably increase my enjoyment of diving more (albeit at double the price :depressed.

Anyway, thanks for the advice!

 

[sibermike7]

Your Besea is basically a BP/W with a fancy harness! The plate is made from a composite material. We use them here in Estonia as well as the Poseidon BP/W platform. There is virtually no difference in establishing trim with these rig. You may still be a little overweight, but not knowing what you are wearing for undergarments, you could be right on. What I can tell you is dive that rig a number of times to really give it a good evaluation. the Besea is a pretty nice set-up. I found that I needed to put some more weight a little higher up on cam bands around the tank. This gave me a more balanced feel and I was able to stay horizontal quite easily. I now have very little ditchable weight, preferring to keep most of it on the rig.

I don't think that you notice any difference between the Besea and a BP/W except for the difference that a SS plate would make (as far as trim goes).

 

[JanK]

Oh, thanks for reminding me of something I forgot to write. Unfortunately I won''t be able to play with it as the next few dives will be deep at around 40m and I don't want to add unnecessary workload at the nitrogen narcosis depths. After that it's the rescue diver course and the BCD is more comfortable at the surface and again I know its behaviour, which will make the water exercises easier. Later I hope I will be able to dive but, if I do, it will be too cold for the wetsuit, so I won't be able to dive without a drysuit anyway.