How much lift do I really need?

[joeabroad]

There’s a thread going around that begins with a post, possibly true, that recounts a divemaster telling his divers that they absolutely need a minimum of 80lbs. lift in their BC (www.scubaboard.com/showthread.php?t=149822).

I find that fairly ridiculous, but I am wondering why we always want so much lift--I’m beginning to shop around for a new BC or BP/W and I’m starting to wonder how much lift I really need for the kind of diving I do. I’m a recreational diver, and when I’m fully weighted, with no air in my BC or lungs and encased head to toe in thick neoprene, I hover just beneath the surface. And even at depth (say, 50 feet). I find that I don’t put all that much air in my BC anyway.

So, how much lift do I really need? Is there any reason I would want more than 30 pounds? I’m also looking for a unit that’s relatively light and compact to pack, so the smaller the bladder, the better.

 

[dherbman]

My thinking has evolved a bit on this topic. Currently, I believe that you should have enough lift to keep you safe and comfortable on the surface and permit your rig to float without you in it. Below the surface, minimal lift is required. 30lbs is more than enough, for diving a single in my case.

Some might argue that they need more when taking game or artifacts, but using your wing as a lift bag has risks associated. What happens if you drop that 40lb bag of scallops and instantly become 40lbs. positive?

 

[loosebits]

For singles, I use an 18# wing and that's with a steel tank and a SS backplate. 80#'s is rediculous. 30# would be more than enough.

 

[redacted]

As a wetsuit diver, you need enough lift to compensate for the weight you will lose due to air consumption (about 5 lb for an Al80), the lost bouyancy due to compression of your wetsuit at depth (Could be 15 to 20 lb with a xxl 7mm 2-piece) and any more you might want for comfort. The small wings (18 lb) should be fine in warm water with single tanks. 25 to 30 lb lift should do the trick in cooler water with single tanks and exposure protection. If you use largeer tanks with heavy exposure protection you might want to step up to 35 or so. 80 is damn near a protable life raft.

 

[victor]

My understanding of what other people have reported on this board, could the experts please correct my mistakes.

You need enough lift to float all of your gear including full tank/s plus your lead that you need to sink your wetsuit.
e.g. If your gear is 10lb neg and you need 15lbs of lead to sink your wetsuit then you would need 25lbs of lift.

To find out how much you need for your wetsuit, put it in the water and keep on adding lead until it sinks.
If you are using a back pack and wing you can discount the weight of the backplate and regs against the lead.
I would assume that a BCD is neutral so you only need to get the buoyancy of the tank you are using minus the weight of your regulators.

 

[divingjd]

I would say that you need some lift beyond the minimum to be able to establish positive buoyancy at the surface for you and your buddy in case your buddy has a catastrophic failure (either equipment or mental) and can not orally inflate the BC.

 

[loosebits]

I would say that you need some lift beyond the minimum to be able to establish positive buoyancy at the surface for you and your buddy in case your buddy has a catastrophic failure (either equipment or mental) and can not orally inflate the BC.

Of course let us not forget that's what ditchable weight is for.

 

[Quarrior]

One thing to keep in mind, when talking about floating something on the surface such as your gear, that is not including sufficient lift to keep your head and upper torso out of the water. It is only laying flat on the surface.

For me, I would suggest no less than enough lift to float your gear and lead, plus 5#. Also keep in mind if you will be using the same BC or wing for diving both wet and dry you will need to consider how much lead you will be using when diving dry because it will be more than when diving wet. Although you can add air to the drysuit on the surface, there is still that extra weight to be concerned with.

 

[loosebits]

Hmm, I've used a little 18# wing with my drysuit, 6# plate, 10# of lead and a LP95 just fine. I'll bet you'll need more of a wing with a thick wetsuit just because its buoyancy changes with depth, a drysuits buoyancy doesn't.

 

[cool_hardware52]

There’s a thread going around that begins with a post, possibly true, that recounts a divemaster telling his divers that they absolutely need a minimum of 80lbs. lift in their BC (www.scubaboard.com/showthread.php?t=149822).

I find that fairly ridiculous, but I am wondering why we always want so much lift--I’m beginning to shop around for a new BC or BP/W and I’m starting to wonder how much lift I really need for the kind of diving I do. I’m a recreational diver, and when I’m fully weighted, with no air in my BC or lungs and encased head to toe in thick neoprene, I hover just beneath the surface. And even at depth (say, 50 feet). I find that I don’t put all that much air in my BC anyway.

So, how much lift do I really need? Is there any reason I would want more than 30 pounds? I’m also looking for a unit that’s relatively light and compact to pack, so the smaller the bladder, the better.

Any BC, wings included, need to be able to do two things; Float your rig at the surface without you in it, and compensate for the compression of your wetsuit, or total flood of a Drysuit.

These two requirements are not additive, whichever one is greater determines the minimum lift needed.

To estimate the weight of your rig, without you and your buoyant exposure suit, just add up all the parts. Find the negative buoyancy of your tank full (many internet sources) and add the weight of your backplate, regs, harness, extra ballast attached to the rig, can light etc. For every thing other than the tank you can just use the "dry weights" because things like regs displace little water and are very dense (brass etc.) Just add up the values.

Example:

Tank LP85 full ~-7
Backplate and harness ~-6
Can light ~-4
Regs ~-3
Bolt on Weight Plates ~-8

Total rig 28 lbs.


The second value is essentially the initial buoyancy of your exposure suit. For a Wet suit just roll it up and throw it in the pool, add lead until it just sinks. This represents the max buoyancy your suit can loose from compression. A full 7 mil on a big guy can require in excess of 30 lbs, even more if you add jackets and vests etc.

For a Drysuit I like to see the diver get in the pool with just his undies and suit on, minimum gas. Add lead until the diver is just neutral. Here again this represents the max buoyancy that can be lost from a total flood, unlikely but possible.

In general in warm water it will be the weight of the rig that governs how much lift you need. In cold water it will be the exposure suit that governs.

As you get further from the equator wings get bigger......


Tobin