Help with wing lift calculation

[49north]

My wife and I would like to get smaller wings for warm water travel. We currently use 40lb wings for cold water drysuit diving. We were thinking of the VDH 23lb wings and I just wanted to confirm that I am looking at this correctly. She wears more weight than I do and has a more buoyant wetsuit, so here are her numbers:

SS plate: -6lbs
Regs: -2lbs
Full AL80: -2lbs
Light: -1lb
Total = -11lb

Her wetsuit and hood take 11lb to sink in fresh water, so things are pretty much balanced. Typically she uses 10lbs of weight in her weight pockets to sink and maintain a safety stop. Any less and she can't sink and has trouble with her safety stop. I realize that some of this required weight may be due to the 40lb wing trapping some gas.

It seems to me that the 23lb wing would float the rig at the surface with a full tank and have more than enough capacity to compensate for wetsuit compression.

Have I overlooked anything?

Thanks.

 

[runsongas]

if the wetsuit and hood planned for use are 11lbs in freshwater, most likely would be a few pounds more in salt water. you would have enough to cover suit compression with the vdh 23, but the margin wouldn't be huge. 16 pounds suit compression + 6 pounds gas in al80 = 22 pounds of lift possibly needed.

 

[rsingler]

Take a look at this spreadsheet to help you decide:
Advice on lift capacity for BP&W

Based on the data you supplied, I calculate her to be -9# at dive start and -4# at dive end.
With wetsuit compression, she might be as negative as -16.5# at depth.
Therefore she needs wing lift of 16.5# minimum.
With a torn wing, and dropping her 10# of weight, she would need to be able to swim up 6.5# at the beginning of the dive, or 1.5# toward the end of the dive, but would be buoyant at the surface.

Put the data in and see if you agree.

 

[2airishuman]

You've overlooked the compensation for gas to be used in the dive, and the need to float the lead if it's on the kit, but that doesn't change the conclusion. 13 cf of gas is one pound. Usable gas in an AL80 is 5 pounds.

For someone who is ideally weighted at the end of the dive, to maintain neutral buoyancy at depth at the beginning of the dive, you need a BC that can provide lift equal to wetsuit compression plus gas to be used during the dive. Figure 75% of the 11 pounds required to sink the suit, =8 pounds, plus 5 pounds for the gas, 13 pounds of lift.

To float the kit at the surface, you need the total of 11 pounds in your OP, plus 10 pounds of lead in weight pockets attached to the kit, for 21 pounds of buoyancy. If she is using 10 pounds of lead in fresh water, she will need at least several pounds more in the ocean, which may push the buoyancy requirement up above 23 pounds, unless she has the weight on a belt or harness that is separate from her kit.

 

[2airishuman]

if the wetsuit and hood planned for use are 11lbs in freshwater, most likely would be a few pounds more in salt water. you would have enough to cover suit compression with the vdh 23, but the margin wouldn't be huge. 16 pounds suit compression + 6 pounds gas in al80 = 22 pounds of lift possibly needed.

Salt water is 102.5% as buoyant as fresh water, so the difference in wetsuit buoyancy is 2.5% x 11 pounds, or about a quarter pound. Wetsuits do not compress completely to the point of zero buoyancy even on a deep dive, I use 75% for dives within recreational limits but it's probably less than that for most wetsuits.

 

[49north]

You've overlooked the compensation for gas to be used in the dive, and the need to float the lead if it's on the kit, but that doesn't change the conclusion. 13 cf of gas is one pound. Usable gas in an AL80 is 5 pounds.

For someone who is ideally weighted at the end of the dive, to maintain neutral buoyancy at depth at the beginning of the dive, you need a BC that can provide lift equal to wetsuit compression plus gas to be used during the dive. Figure 75% of the 11 pounds required to sink the suit, =8 pounds, plus 5 pounds for the gas, 13 pounds of lift.

To float the kit at the surface, you need the total of 11 pounds in your OP, plus 10 pounds of lead in weight pockets attached to the kit, for 21 pounds of buoyancy. If she is using 10 pounds of lead in fresh water, she will need at least several pounds more in the ocean, which may push the buoyancy requirement up above 23 pounds, unless she has the weight on a belt or harness that is separate from her kit.

Sorry, I wasn’t clear. The 10lbs of lead required is in salt water. The buoyancy of the suit was tested at home in fresh water. If the difference between salt and fresh is about 2.5%, then I wouldn’t think it would change the number for the suit significantly.

 

[49north]

Take a look at this spreadsheet to help you decide:
Advice on lift capacity for BP&W

Based on the data you supplied, I calculate her to be -9# at dive start and -4# at dive end.
With wetsuit compression, she might be as negative as -16.5# at depth.
Therefore she needs wing lift of 16.5# minimum.
With a torn wing, and dropping her 10# of weight, she would need to be able to swim up 6.5# at the beginning of the dive, or 1.5# toward the end of the dive, but would be buoyant at the surface.

Put the data in and see if you agree.

Thanks. That’s what I got working it out manually.

I figured, worst case, if you assumed 0 suit buoyancy at depth then you would be -21 lbs with a full tank. If you dropped all 10 lbs, would it be reasonable to think that you could swim up 11lbs to start your ascent and then be at least neutral at the surface? In this scenario, I don’t think wing size would matter would it?

 

[2airishuman]

Sorry, I wasn’t clear. The 10lbs of lead required is in salt water. The buoyancy of the suit was tested at home in fresh water. If the difference between salt and fresh is about 2.5%, then I wouldn’t think it would change the number for the suit significantly.

Then you should, by the numbers, be OK.

I dive a 17# and a 30# singles wing. You do have to realize that the rig is going to be marginally buoyant with a smaller wing and be prepared for that in situations where it matters. For example, if you're going to gear up in the water, as I do when diving from a canoe or other really small boat, be prepared for the kit to be just barely buoyant.

I do not find that a 17# wing is any better in the water than a 30# wing. It does pack down somewhat smaller for air travel.

Thanks. That’s what I got working it out manually.

I figured, worst case, if you assumed 0 suit buoyancy at depth then you would be -21 lbs with a full tank. If you dropped all 10 lbs, would it be reasonable to think that you could swim up 11lbs to start your ascent and then be at least neutral at the surface? In this scenario, I don’t think wing size would matter would it?

In a loss of buoyancy scenario, wing size doesn't matter. Most people can swim up 10 pounds but it's a good exercise to try if you're unsure. I often dive with zero lead and would have to ditch my kit to become buoyant in the event of a total wing failure.

 

[rsingler]

Thanks. That’s what I got working it out manually.

I figured, worst case, if you assumed 0 suit buoyancy at depth then you would be -21 lbs with a full tank. If you dropped all 10 lbs, would it be reasonable to think that you could swim up 11lbs to start your ascent and then be at least neutral at the surface? In this scenario, I don’t think wing size would matter would it?

You're right, wing size wouldn't matter there. But swimming up 11# until suit compression eases off is a bit of a stretch. One more reason to carry an SMB for redundant flotation.
On the other hand, assuming 0 suit buoyancy at depth is a bit much. My spreadsheet uses about 60% loss of lift at depth, which is about right for all but the softest neoprenes, down to 80 feet or so. Outside that, an attentive buddy, or some tech considerations might begin to apply, regarding redundant lift and air.

 

[rsingler]

My wife and I would like to get smaller wings for warm water travel. We currently use 40lb wings for cold water drysuit diving. We were thinking of the VDH 23lb wings and I just wanted to confirm that I am looking at this correctly. .

I have the VDH 18# wing and it is fantastic for warm water diving! Tiny and streamlined, and very well made. I LOVE it. That should work for her too, if she's comfortable with 4# on a belt instead of pockets on the wing.
The only thing I changed on the VDH was the length of the corrugated hose, to make manual inflation (should I need it) a bit easier. Talk to @Bryan@Vintage Double Hose and see if he's gotten in some longer hoses since I bought mine. A great wing!