Wing lift requirements .. Help..

[Andy692]

Hi guys , I'm sure this has been done to death , but cant find much , thought id ask ..

working out my daughters weight , and wing size for her in a 7mm wetsuit ..

At the surface she needs 6kg / 13lbs to be neutral with an empty wing ..

As she descends, her wetsuit compresses and losses buoyancy, so when she reaches , say, 30m / 100 feet , she needs to compensate with adding lift to her wing ..

How much buoyancy is lost roughly ?? , I did read that a 7mm wetsuit has the characteristics of a 3mm wetsuit , if that was the case,.. when she dives a 3mm wetsuit she needs she needs 2kg / 4.4lbs , so my thinking would be that she would need to add 4kg / 8.8lbs of life to her wing...

But I read another post which mentions a 7mm wetsuit at 30m / 100 feet , actually weighs more , meaning that she would need 7kg / 15lbs to be neutrally buoyant ??

Im guessing there is no definitive answer , what I need to know is her 22lb /10kg wing going to be enough, or am I going to have to buy her something new ...

 

[lowwall]

You want enough lift to become neutral in the worst case scenario, meaning a tank full of gas at the max depth you'll dive to. If a diver is neutral with no air in the BCD/wing at the end of the dive, then this is easy to calculate. You take the difference between the start and end weight of the gas in the tank: for example, around 2kilos for a "tropical" AL80 or 3kilos for a 232bar 12L. To this number, you add the amount of lift required to offset the loss of buoyancy in the wetsuit at depth. A decent rule of thumb for a wetsuit is 1 kilo of buoyancy per mm of suit at 1 atmosphere (sea level). At 4 atmospheres (30m/100') it will be around 1/4 of that (neoprene gets its buoyancy from gas bubbles trapped in the material which obviously compress as the pressure increases). 7 kilos - 1.75 kilos = 5.25 kilos.

So the worst case is you need to need to replace 5.25 kilos of lost buoyancy and be able to overcome 3 kilos of extra gas weight = 8.25 kilos of lift needed. Round up a kilo or two for uncertainties in weighting and neoprene thickness and you can see that a 22lb/10kilo wing is spot on.

Note that If she starts out overweighted, for example if she's diving a steel twinset, you'll need to add in lift to compensate.

 

[keeperguy64]

Generally, my calculations assume that the worst case is your exposure suit losing all provided buoyancy. So, the wing needs to provide enough lift to float her along with all of her equipment. Most people float, so if we just calculate her equipment::

Full AL80 (12L): -2lbs
Weights: -13lbs
Regulator: -2lbs
BCD: 2lbs (typically the padding makes them positive but it depends)

She would need 15lbs of lift to offset the buoyancy of her equipment, so 22lbs provides you a bit of a cushion.

 

[johndiver999]

You don't want to skimp on lift for daughter in a 7 mm suit. A lot depends on how big she is. A very small person wears a lot less neoprene than a 250 lb person. I would determine how much lead it takes to sink her entire wetsuit in a pool, add about 5 lbs for the air in the tank and maybe another 4-5 lbs for a little buffer. I bet you will find that 22 lbs is too little.

 

[lowwall]

keeperguy64 and my estimates are reasonably close. My number is a bit higher mainly because I assumed a higher capacity cylinder.

 

[-JD-]

working out my daughters weight , and wing size for her in a 7mm wetsuit ..

At the surface she needs 6kg / 13lbs to be neutral with an empty wing ..

Questions:
- is that with a full or "empty" tank
- what size and type of tank
- does she carry the weights on a belt or on the rig

With wing sizing, you should also consider the ability to float the rig on the surface with a full tank.

 

[lowwall]

I would determine how much lead it takes to sink her entire wetsuit in a pool, add about 5 lbs for the air in the tank and maybe another 4-5 lbs for a little buffer.

This assumes there will be no buoyancy left in the suit at depth, which means you've already built in around a couple of pounds (1 kilo) buffer.

And note we are assuming a theoretical worst case. In reality, you've had to breathe some of that air to get to your max depth. And even in the worst case, you can easily overcome a small amount of negative buoyancy with breath control and swimming.

 

[Julius SCHMIDT]

Wing size/lift calculation questions.


There are decades of serious diving experience in here.

 

[Curious_George]

Andy, welcome to the forum. Here’s the best calculator - it will take out all the guesswork. Optimal Buoyancy Computer

 

[rjack321]

22lbs is on the small side in a 7mm wetsuit, although its hard to tell how much neoprene that actually is. Only 13lbs of lead suggests that she's not a very large person.

30-34lbs has enough lift to compensate for compression of that suit and help get her higher on the surface