How much BC lift do I require?

[MikeS]

Leadweight,

I’m not trying to give you, or anyone else a hard time I’m just trying to understand the issue. If it appears that I’m being argumentative I apologize in advance.

leadweight once bubbled...
One rule of thumb for wing size is the amount of weight you need (including weight of the BP) plus 10 lbs.

What is the basis for this “rule of thumb?” It doesn't make sense to me based on the fact that lift is needed to overcome negative buoyancy not weight. It implies you would need the same amount of lift to lift a pound of lead as a pond of rubber. Would you please explain?

Mike

 

[leadweight]

MikeS, I said that your method was good. I saw that "rule of thumb" in Rodales. Diver magazine, has made statements to the effect that bouyancy of the wing need only exceed that of the weight attached.

I think that the extra 10# is to get your head above the water while waiting for a boat in not so flat salt water. Beyond that, I can't say.

At the moment I have a 30# travel wing on a TP2. At most I wear a 3 mil full with a 3 mil shorty over it, and an aluminum 80. The most lead that I have ever carried is 13# with this setup and I was nowhere near the capacity of my wing at depth.

What does this mean? Well, even though I don't use their style of gear, the statements in the Halcyon catalog are probaby correct. That is a 36# wing ought to be enough for just about anyone with a single tank. Or stated another way, there is no reason on this planet for BC's with 75# or 85# of lift.

So, the question is how much extra lift do you want to float on the surface at the end of the dive, is the diver weighted right in the first place, how fat are his wetsuits and for those that dive dry, to what extent do you want to rely on the drysuit inflation system to augment bouyancy. And then there are some divers that insist on redundant bladder wings. But, there are divers I know that insist on a pony bottle, even on a 40 ft reef dive.

A rather petite female I know has a BC with only 15# of lift and she usually has 11# on her belt. She has no trouble keeping her head above water in the ocean. And I know better than to say a pound of rubber is the same as a pound of lead, but I rather have a pound of ice cream

So the answer, is as little as possible.

 

[Dan Gibson]

Don't forget that the bladder should be able to float the rig at the surface without you in it with full tanks and all your non ditachable weight (including expensive light). If not, you might be saying goodbye to your gear.

When I dive dry in an al80, I am 22# positive from my suit and undergarments. I need to add approximately three more pounds of positive buoyancy when the tank is empty. Therefore I should use 25# of weight total. 6# is ditachable. 19# is on the rig (6# ss bp, 6# STA, 3# light, 4# trim pockets). My 27# wing works ok, but not by a lot.

When I have to add a little extra weight (3-4#) for dry gloves and a layer of polypro underwear, the 27# does not float. The 36# would have been a better choice where I do most of my diving. I am picking one up soon. When I started out, my old drysuit and undergarments were less buoyant. I got by with the 27#. I don't really think it actually has 27# of lift.

What sort of actual lift have you guys experienced with the 27# wing.

Dan

MikeS once bubbled...


An interesting question that I’ve never heard answered well. Enquiring minds want to know how much lift we need but also understand the principles involved and hopefully learn to calculate the lift requirements for any given situation. The following is the best explanation that I’ve been able to come up with. Anyone that can add anything please do so.

The reason that we need lift is to overcome any negative buoyancy to become neutrally buoyant. Why would we be negatively buoyant without air in the bladder? For the purpose of this discussion, let’s assume we are properly weighted. There are two reasons I can think of, equipment failure (i.e. drysuit puncture) and the negative buoyancy required to offset dynamic changes in buoyancy.

What are the dynamic buoyancy factors during a dive? The swing weight of our tank(s) as gas is consumed, the loss of lift resulting from compression of our exposure protection (i.e. wetsuit) and the buoyancy of anything we might detach (i.e. stage bottle).

When properly weighted we are neutrally buoyant with an empty tank just below the surface with anything detachable detached and no air in the bladder. At this point, we are as positive buoyant as we will ever be without adding air to the bladder and we’ve added enough weight to overcome the dynamic buoyancy factors other than air in the bladder and their most buoyant point.

We now need enough lift to overcome the negative buoyancy resulting when all of the dynamic factors are the most negative or worst case. This would be at the beginning of the dive (full tank), at maximum depth (maximum compression) with any detachable items with negative buoyancy still attached.

So to calculated the lift requirements we would add:
The swing weight of our tank(s)
Plus the swing in buoyancy of our exposure protection
Plus the negative buoyancy of any detachable objects

When purchasing a wing I calculated it as follows:

PS HP 120 swing weight 11 pounds
Swing in buoyancy of Xcel 7-6-8 wetsuit 7 pounds
Negative buoyancy of stage slung 30 cf pony 5 pounds
Total required 23 pounds

So to have a safety margin I went with the 36-pound wing.

Hope this helps,
Mike

 

[PnL]

If you add the ACB's with 30lbs and the stainless BP w/ 36# Pioneer wings and Al80 your (or I guess I should say 'my' ) rig barely floats with the bladder full - it might be slightly negative, I never let it settle so I'm not sure. I definitely won't be taking off my rig until I get back in the boat

 

[leadweight]

before you get back to the boat?

 

[PnL]

Given the type of diving that we do the only time I would have to remove my rig would be in an emergency. We giant stride off the boat and climb the ladder back on and take of our gear at our stations. If I'm beach diving my rig doesn't come off until I'm back at the car. I don't dive off kayaks or small boats, where I could see the benefit/necessity of taking off your rig before boarding.

And, of course, if there is an emergency situation ditching my gear is not the primary concern. My rig doesn't necessarily sink. My best estimate w/o having testing the bouyancy of my BP/wings setup is that it slightly negative but may be neutral. I only know this from removing the rig at a pool when I was testing out some other pieces of equipment - it didn't sink so I wasn't really concerned.

Anyway, my point in reporting this in this thread was to let the original poster know what the characteristics of the 36# Pioneer would be with the ACB loaded with 30lb to help him out in deciding what he needed. Best bet, I still think, would be get recommendations from Halcyon, EE or FifthD.

 

[MikeS]

Leadweight,

Please believe me when I say that my post was not meant to be derogatory towards you. Nor did I mean to imply that the “rule of thumb” was without value or needed defending, rather I was hoping that someone could explain the reasoning behind it.

As you pointed out the Halcyon website suggests a relationship between tank/exposure protection configurations and required lift, for example “the 36 lbs. wing is designed for use in cold water where heavy exposure protection with steel cylinders is the norm.” But it doesn’t explain the rational behind these recommendations. That is what I am attempting to understand.

Dan Gibson once bubbled...
Don't forget that the bladder should be able to float the rig at the surface without you in it with full tanks and all your non ditachable weight (including expensive light). If not, you might be saying goodbye to your gear.

Dan,

“Good point” he said as he sadly watched his gear sink into the depths.

ScubaKims once bubbled...
I definitely won't be taking off my rig until I get back in the boat

ScubaKims,

Let me introduce you to everyone’s silent dive buddy. ScubaKims this is Murphy, Murphy this is ScubaKims. He may have a different dive plan that requires you to take off your rig!

Mike

 

[PnL]

MikeS once bubbled...

Let me introduce you to everyone’s silent dive buddy. ScubaKims this is Murphy, Murphy this is ScubaKims. He may have a different dive plan that requires you to take off your rig!

Mike

Yes, I know you ... you bastard!

Then as it may be. If I have to remove my rig for some reason then it will be removed. And if it does get removed it will not plummet ... as I mentioned it might be neutral so it'll float around, maybe not as high and proud as a MC w/o weights, but it'll be around

Again .... I didn't bring this up to begin some discussion about whether your rig should float or not. I was just trying to give some information to the fella who asked "How much lift?". So based on my experience with my rig (and if he finds himself similarly configured) a 27# Pioneer would probably be not be enough. It was my hope that my input would help him in his decision.

Nonetheless, Murphy, you've shown me time and time again that you will befuddle me at the most inopportune times so I’ll heed your thoughts and opinions.

Now get out of my face Murphy

 

[leadweight]

I think that the 36# wing and Halcyon's catalog statements are based on what they expect to be maximum wetsuit compression on an average to slightly larger than average size man (two piece 7 mil @ 20#) plus the weight of air in a big single tank (10#) with a little left over for safety. Diving dry is a completely different story as the drysuit can be inflated to keep its bouyancy nearly constant.

Ultimately the BC must do three things:
1. Compensate for exposure suit compression and weight of gas.
2. Float the diver at the surface.
3. Float itself.

Estimating how much lift is required is tricky as some weight is carried to offset variable bouyancy and some is to offset the fixed bouyancy of body fat. Some men have a lung capacity of as much 8 liters which means significant bouyancy changes are possible with only breath control.

The answer is pretty much a trial and error process, and wings can only be made in so many sizes. Then throw in the marketing guys who think that bigger is better.

 

[BillAllbritten]

I know it's a worst case scenario and probably won't ever happen to you but consider what would be necessary to keep a disabled buddy's head clear of the water, with or without gear that may or may not function properly, in a seaway for a period of time before the boat could get to you.

My .02