Formula for estimating wing size?

[Bob DBF]

And even if one had to remove a rig in an emergency, even the smallest BC has the lift capacity to keep most setups from sinking, (My 11lb Outlaw bladder on a twin steel setup full of air and weights bolted to the back plate notwithstanding). I will reiterate the following and if I am wrong, please show me where...

The Heiser 45 at -2.575# , the most buoyant steel tank I found, doubled with a -5# backpack runs -10# not counting your integrated weight and regs.

A pair of 100's is -16# without any rig.

Others have covered the rest.


Bob

 

[NAM001]

Full disclosure... I was an instructor, just not scuba. I used to teach college physics

So maybe we are just talking over each other, but maybe not. Whether we are talking about steel tanks, aluminum tanks, double tanks, h-valves, ball valves, .. whatever... it makes no matter... and unless I'm not aware of some super secret aspect of recreational diving, there is no need whatsoever to 'float a rig' in the water. And even if one had to remove a rig in an emergency, even the smallest BC has the lift capacity to keep most setups from sinking, (My 11lb Outlaw bladder on a twin steel setup full of air and weights bolted to the back plate notwithstanding). I will reiterate the following and if I am wrong, please show me where...

For a diver that is PROPERLY WEIGHTED, that is, neutrally buoyant at the surface at 500psi with zero air in the wing or BCD, the only lift an air bladder is required to provide is to:

1. Counter the weight of the unconsumed gas at the start of the dive
2. Counter the effect of the loss of buoyancy of a diver's wet suit.
3. Allow the diver to bob comfortably at the surface.

Note even that there is no circumstance where all three of these conditions are met at the same time.

Now if you take extra gear with you without reducing your lead, then of course, you will need more lift. If you pick up a gold bar from the bottom, of course you will need more lift.

I have been on small boats diving and unable to get out of hte water witht he rig and location of the ladder at the engines. I sometimes have to remove the rig get in the boat, and then lift the kit into the boat. got to float the rig till then

 

[cool_hardware52]

The max negative buoyancy for a diver's "rig" can certainly be the determining factor for wing capacity.

Consider a diver using *no* exposure suit. Floating the rig with a full cylinder is the *only* criteria.

Tobin

 

[Epsylon]

Hello all,

Just for the sake of getting a confirmation of my understanding of the lift needed for my future wing...

I dive in salt water, without a suit. I'm not planning to go into a twin setup, so I compromised with a main tank and a pony.

Here's my future gear:

Gear	Weight in kg	Weight in pounds (1kg = 2.20462 lbs)
Pony tank	6.25​	13.78
Pony air (6.8L at 300 bar)	2.4​	5.29
Pony reg	1.5​	3.31
Tank	16.4​	36.16
Tank air (100 cu)	5.3​	11.68
Fins	1.5​	3.31
Regulator	1.5​	3.31
Transmitter scubapro	0.09​	0.2
Transmitter Shearwater	0.1​	0.22
Plate	0.8​	1.76
Comp. scuba pro	0.2​	0.44
Comp. shearwater	0.19​	0.42
Torch	1.37​	3.02
Total	37.6​	82.9

Do I understand correctly that I need to find a wing able to lift 41.36 kg or 91.19 lbs (I add 10% for safety)?

 

[gmerick]

See

Optimal Buoyancy Computer​

At the beginning of the dive, the rig should float itself. Rescue dive course teaches being able to remove ones own rig if needed to assist an unresponsive diver.
Initial rig weight must also account for what is needed to conduct a safety stop.
Add to the wing floatation requirement the buoyancy needed to be neutral at planned depth. Account for wetsuit compression or a flooded drysuit.
My calculation
Buoyancy of my person - +6lbs
Wet - 7mm farmer john, hood, gloves, boots. compression delta (10ft vice 130ft) 16 lbs
Dry - CF200 - normal buoyancy +14 lbs, flooded -2lbs (16lbs buoyancy needed)
Air consumption delta (2x Faber 117) 18 lbs

It works out that I need 24 lbs of weight to comfortably hold a safety stop, and a minimum 28 lbs wing capacity to float the rig.
I need an additional 34lbs of lift to maintain neutral buoyancy at depth at the beginning of the dive when diving wet, or to asceend with a flooded dry suit.

Needed wing capacity - 28 lbs (rig) plus 34 lbs (suit) minus 6 lbs (me) = 56 lbs. Add a safety margin.

 

[James79]

Hello all,

Just for the sake of getting a confirmation of my understanding of the lift needed for my future wing...

I dive in salt water, without a suit. I'm not planning to go into a twin setup, so I compromised with a main tank and a pony.

Here's my future gear:

Gear	Weight in kg	Weight in pounds (1kg = 2.20462 lbs)
Pony tank	6.25​	13.78
Pony air (6.8L at 300 bar)	2.4​	5.29
Pony reg	1.5​	3.31
Tank	16.4​	36.16
Tank air (100 cu)	5.3​	11.68
Fins	1.5​	3.31
Regulator	1.5​	3.31
Transmitter scubapro	0.09​	0.2
Transmitter Shearwater	0.1​	0.22
Plate	0.8​	1.76
Comp. scuba pro	0.2​	0.44
Comp. shearwater	0.19​	0.42
Torch	1.37​	3.02
Total	37.6​	82.9

Do I understand correctly that I need to find a wing able to lift 41.36 kg or 91.19 lbs (I add 10% for safety)?

You understand incorrectly... it isn't the weight of things you need to worry about, it is the buoyancy. For example, that 100cf tank that weighs 36 lb on land, is only going to be a little negative (depending on the tank type/brand). The Optimal Buoyancy Calculator that @gmerick linked to above breaks it out wonderfully. I recommend you download it, play with it a bit, and then ask questions in a new thread if you need further clarification. One great thing about it is that once you get it dialed in, it does a great job of figuring out lead requirements when you make changes in your gear or conditions (salt to freshwater, different exposure protection, different tanks).

As another example of it being about buoyancy (not weight), I dive a 3mm full suit in freshwater with an aluminum BPW and steel 72cf tank. The whole rig weighs 35 lb or so... and I use a homemade wing that has 12 lb lift. It addresses buoyancy swing from air usage and wetsuit compression, gets my head out of water even with a full tank. And even though the rig WEIGHS 35 lb, it is only about 5 lb negatively buoyant... so that 12 lb wing floats it at the surface just fine.

Respectfully,

James

 

[BlueTrin]

@Epsylon another way to see it easily is to compare how buoyant you are vs your own weight. I weight 95kg but I surely don’t need a buoyancy aid of 95kg to float. I am pretty floaty naturally, thanks to my diet

You only need to determine the quantity of negative buoyancy to offset to make it float (and maybe add a bit of safety margin)

 

[lowwall]

Do I understand correctly that I need to find a wing able to lift 41.36 kg or 91.19 lbs (I add 10% for safety)?

Yes that is correct if you are looking for the lift of a hot air balloon to fly your gear into the sky.

But as a scuba diver you are more interested in flying in water in which case you need to subtract the weight of the water your gear displaces. What's left is your gear's net bouyancy. But since the gear is normally attached to you, you also need to figure in the net bouyancy of your body and your exposure protection.

That's all kind of tricky, but luckily you should have done this already via a weight check at the end of one of your dives. The amount of lead you need is your system net bouyancy.

Of course, if you need lead at all then you can fly without the wing (in water). So what is the wing for? The weight check amount is actually your most bouyant state while diving. The wing is there to compensate for the difference between that and your least bouyant state. That difference is determined by 2 things, 1) the weight of the gas in your tank at the beginning vs. the end of the dive and 2) any difference in the bouyancy of your exposure protection from the deepest part of your dive to the surface.

The first is easy, 2kg (4.5lbs) difference between full of gas and as empty as you want to be at your safety stop for an AL80. 3kg (6.5lbs) for an HP100 (13L steel).

The second depends on the thickness of your wetsuit. Most of the volume of neoprene fabric is gas in the form of tiny little pockets trapped in the fabric. This compresses at depth which leaves you less bouyant (and colder). How much bouyancy you lose is highly variable, but a reasonable approximation for a medium to large fullsuit is 2 pounds/1kg per mm of thickness.

Thus for tropical diving with an AL80 and a 3mm, your wing only needs to compensate for 5-6kg (12-14lbs) of negative bouyancy. Which is why they make 18-22 pound wings.

For temperate to moderately cold diving with an HP100 and 7mm, you need around 27 pounds. Add in accessories like a hood, gloves and thick booties and you see why 30-35 pound wings are standard.