BCD lift requirements and so many gear choice variables

[scuba_divo]

I have the bigger wing due to some specific requirements i have when sometimes working as a DM. I dont feel that the size of the wing gets in the way.

That's good to know. I guess if a giant wing is all I know starting out, it'll feel great because that's what gets me diving! hehehe

 

[scuba_divo]

SS backplate and a keel weight

By "keel weight", do you mean a weighted STA or the trim weights that attach at the bottom of the backplate?

 

[Aris Penaranda]

There are advantages in having a weight belt. You can distribute the weights. Any weight you put in the pockets transfers the weight to the BCD and to whoever will lift the BCD with the tank and weights all attached as one. When you need to ditch weights in an emergency, you don't need to drop all the 40 lbs that you carry. Not only because weights cost money but more importantly you don't want to shoot up like a SMB. You can select either to drop the belt or pull the ripcord for the pocket weights, or both.
If properly weighted, you don't need a big bladder. It makes you slower on the water. If you have all the weights in the BCD, you're forced to compensate with a higher lift bladder.

 

[kmarks]

I guess my concern with a belt would be, "where do all these belts go?" Don't the BP/W harness waist belt and the weight belt interfere with each other?

One of the pros of the BP/W for me is definitely the option to use a 6mm SS heavy plate and various trim weights on cam bands.

It's only two belts, but they work together pretty well. My harness waist strap, aka belt, sits a little higher, closer to my natural waist, right about at my belly button, and the weight belt sits a little lower, just above my hips - about where I wear my jeans. I have a pretty athletic build (which is why I sink), so my waist is considerably smaller than my hips and this works for me. It might not be the same for you, depending on how you carry your body weight.

Whether the weight belt or the crotch strap goes on first is a matter up for debate. Some people prefer the weight belt to be under the harness so that it doesn't interfere with freedom of movement or access to D-rings, but others prefer it be over the harness to make it easier to drop in an emergency. I still haven't figured out which I like better, and I usually end up putting the belt on when I remember to, whether that's before or after the harness straps. In the event of an emergency so dire than I can't release my harness buckle and crotch strap (it pretty much falls apart with an intentional flick of the wrist) before I ditch my belt, I'm probably already screwed anyway.

As Aris points out above, you don't need to be able to ditch all of your weight. Ideally, you want to ditch very little of it, just enough to become slightly buoyant so that you can get off the bottom, or even still be slightly negative but able to swim yourself up a little bit. As you come up, your suit is going to start expanding, increasing buoyancy. If you need 40 lbs to sink yourself with a full tank and halfway through your dive you drop all of your weight for some reason, you'll be dead by the time you hit the surface because you'll be coming up so fast. If you drop 10 lbs, you might gain enough buoyancy to get yourself out of whatever mess you got into (punctured wing, dry suit flood) and get to the surface under control.

Your body fat isn't going to compress much underwater, but your suit will, so your wing size is mostly to compensate for that. Your goal is to be neutral or very slightly negative at 15 ft with 500 PSI in your tank. Your wing needs to be able to compensate for the loss of buoyancy in your exposure suit as it compresses. I've had some spirited debates about whether wet suit compression follows a bubble model or if the elasticity of the wet suit dampens the compression and loss of buoyancy, but at 40 m, volume of air (and therefore buoyancy) in your suit will have compressed by about 80%. So if your suit was giving you 40 pounds of buoyancy on the surface, it will only be giving you about 8 pounds at depth. If you were carrying 40 lbs to get yourself underwater to start with, you'll need 32 lbs of displacement in your wing to become neutral at depth.

 

[jadairiii]

By "keel weight", do you mean a weighted STA or the trim weights that attach at the bottom of the backplate?

Yes, either a weighted STA or, now they have soft weights that can slide in the ridge of the back plate, between you and the BP. I have found, through personal experience, that weight pockets screw up your trim.

 

[scuba_divo]

Thanks for the responses. Some good things to think about and research.