Wings - Stupid question of the day

[MRadke]

When a wing is given a lift rating, is it for freshwater or saltwater? I see every company advertising lift, but if a wing is rated for 24# in salt, it would be substantially less in freshwater. This would also throw the wing calculator for a loop. Does anyone know for sure?

[ianr33]

Don't know the answer to your question but,density difference between fresh and salt water is only around 3.5%.

A difference of +/- 3.5% of a stated lift capacity is not worth worrying about.

I think you may be getting confused by how much more lead a diver needs in saltwater compared to fresh??

[Bubble_Boy]

Don't know the answer to your question but,density difference between fresh and salt water is only around 3.5%.

A difference of +/- 3.5% of a stated lift capacity is not worth worrying about.

I think you may be getting confused by how much more lead a diver needs in saltwater compared to fresh??

3.5% might be the difference between holding in or letting out a fart.... Does anyone have statistics on this?

(stupid question of the day followed by stupider question of the day) :D

[Akimbo]

I come up with 2.89% difference, which is probably tighter than the manufacturing tolerance on the wing’s bladder, cover, and relief valve combination — higher inflation pressure = greater displacement. I would not be surprised to see a 5-10% variation in displacement between wings from the same manufacturing lot.

[CT Sean]

3.5% might be the difference between holding in or letting out a fart.... Does anyone have statistics on this?

(stupid question of the day followed by stupider question of the day) :D

Its not a matter of statistics - 3.5% is 3.5%. If you have a 30 lb wing (a pretty popular size) then that is only a 1 lb difference. If that 30 lbs is a "fresh water" measurement then that 30 lb wing will displace 31 lbs of salt water.

For the record I actually thought it was more like 2.5% difference between fresh and salt.

[DA Aquamaster]

As noted above the actual capacity is going to vary a bit - not so much from the bladder, but from the cover over the bladder.

The bladder itself is *probably* RF welded using a pre-made die so there will be very little variation from bladder to bladder. The sewn cover however could introduce some variation due to variation in seam locations, excess material inside, etc.

However the major issue is how the wing operates with the rest of your system.

For example, if your plate is a little flatter than the one the wing was tested with and/or your tanks are larger in diameter, have boots on them, or is perhaps you carry an inflation bottle or some other piece of hardware that prevents full inflation of the wing, then the wing will not carry the rated lift. Sometimes the hit you take is severe.

Sidemount adds another element as bungess over the top of the wing and any wing control devices will reduce the actual lift. For example the Nomad went from using a Trek wing to a Rec wing to it's own even larger wing to accomodate the losses due to bungees and wing control hardware.

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Looking at it differently, fresh water weighs 62.4 pounds per cubic foot and salt water weighs 64 pounds per cubic foot (on average - salinity varies).

So if you have a 55 pound wing (in salt water), the lift in fresh water would *only* be about 53.6 pounds - a loss of 1.4 pounds compared to salt water.

[ianr33]

For the record I actually thought it was more like 2.5% difference between fresh and salt.

Yep,seems you are correct. It's the salinity of ocean water that's 3.5%. My bad.

Seawater - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Seawater)

[Akimbo]

It is a squishy target, but most references I have seen show “average” surface sea water density at 64.1 Lbs/Ft³ and fresh water at 62.3 (62.2970625 Lbs at 0° C).

The 64.1 Lbs value also calculates back nicely to .445 PSI/FSW (0.4451388~ really) and 33 FSW (33.014299900156’)/Atmosphere (64.1 Lbs/144 square inches in a Ft² = PSI/FSW). I used to use the more common 64 Lbs/Ft³ but rounding kept generated unacceptable variations. When it comes to such a small and variable volume like a wing, it is irrelevant.

[MRadke]

Thanks for the responses. It sounds like the difference is negligible. I tried filling my wing with water and weighing it, but it difficult to get a good fill. I'll have to go out to the lake with it, if spring ever comes, fill it with air and see what it takes to sink it. It would just be nice to know what I have, not only for calculations, but also for a baseline for purchasing more equipment.

I haven't done a weight check yet in a 7mm suit. In a 2mm full suit in freshwater, I balance out nicely with 4lbs. I'm guessing in the 7mm, I'll be back up closer to 10 or 12lbs.