How much lift.

[kingprawn]

The age-old question. How much lift do I need in my wing.

OK, I have been searching and reading through threads much of the afternoon. I know there may not be a definitive answer, but I am looking for a few points of view.

I have a back inflation BC that is great, but I am adding a BP and wings to my gear (long list or reasons). My LDS told me I need a 40# wing. I believe this is a bit high. My BC has 54 lbs. It is usually empty.

Question, What are the criteria for determining lift in a wing?
My understanding is:
1) You and the rig should be neutral at safety (empty tank).
2) You and the rig should float comfortably on the surface.
3) The rig must float by itself (full tank).
4) You must float by your self (with weights).
5) Flooded dry suit and dropped weights you should be able to get to the surface.

Are there any criteria I am missing?

A few numbers / assumptions

I am 6’6” 220lb.
I am 5 lb positive. (In the tropics I use 10 lb of lead, with no air in the BC and no buoyancy in the suit. Assume 5lb for the empty AL 80 and 5 for me.)
With my BC I dive dry with 35 lb of weight (I may be able to drop some in the future, but now any less and I pop to the surface).
I dive steel singles (don’t do the doubles math yet).
Yes, always have a lift bag (50 lb).

Here is my math

+ 5 Me
+25 Dry suit
-15 BP, STA, Reg, Reel, pony …
-10/-0.5 HP 120 steel

1 At the end of the dive I will need (35 – 10) 25 lb of lead on my belt to be neutral.
2 I should need 10# of lift to be neutral at the beginning of the dive.
3 The rig will need 25# of lift to float (full tank).
4 I will float like a log by my self. ( +5 )
5 I will need 20# of lift to get to the surface if my dry suit looses all buoyancy.

Ok, What am I missing? It sounds like I need about 27-30# of lift.

Can someone check me on this?

 

[jonnythan]

kingprawn once bubbled...
The age-old question. How much lift do I need in my wing.

OK, I have been searching and reading through threads much of the afternoon. I know there may not be a definitive answer, but I am looking for a few points of view.

I have a back inflation BC that is great, but I am adding a BP and wings to my gear (long list or reasons). My LDS told me I need a 40# wing. I believe this is a bit high. My BC has 54 lbs. It is usually empty.

Question, What are the criteria for determining lift in a wing?
My understanding is:
1) You and the rig should be neutral at safety (empty tank).
2) You and the rig should float comfortably on the surface.
3) The rig must float by itself (full tank).
4) You must float by your self (with weights).
5) Flooded dry suit and dropped weights you should be able to get to the surface.

Are there any criteria I am missing?

A few numbers / assumptions

I am 6’6” 220lb.
I am 5 lb positive. (In the tropics I use 10 lb of lead, with no air in the BC and no buoyancy in the suit. Assume 5lb for the empty AL 80 and 5 for me.)
With my BC I dive dry with 35 lb of weight (I may be able to drop some in the future, but now any less and I pop to the surface).
I dive steel singles (don’t do the doubles math yet).
Yes, always have a lift bag (50 lb).

Here is my math

+ 5 Me
+25 Dry suit
-15 BP, STA, Reg, Reel, pony …
-10/-0.5 HP 120 steel

1 At the end of the dive I will need (35 – 10) 25 lb of lead on my belt to be neutral.
2 I should need 10# of lift to be neutral at the beginning of the dive.
3 The rig will need 25# of lift to float (full tank).
4 I will float like a log by my self. ( +5 )
5 I will need 20# of lift to get to the surface if my dry suit looses all buoyancy.

Ok, What am I missing? It sounds like I need about 27-30# of lift.

Can someone check me on this?

You're right. Get the 36 lbs Pioneer.

 

[Genesis]

For a wetsuit, you need enough lift to compensate for the loss of buoyancy in the suit due to depth (figure nearly ALL of the suit's surface buoyancy, plus the shift of the tank's gas)

For a drysuit you need enough lift to cover the shift of gas in the tank, and you assume that the suit has ZERO lift (its punctured and flooded), assuming a shell suit. For a neoprene suit, its the same calc as a wetsuit.

Paradoxially, you need LESS lift for a shell drysuit, since in the normal case (its working) your lead and the suit are equilibrium (there is no, or little, shift in the suit.)

Now, you ALSO need enough lift to float the RIG, since you might want to doff it at the surface and having it sink would be expensive.

So let's assume you have a HP100 (8 lbs of gas, roughly) and a 3 mil wetsuit (10lbs positive.)

The RIG (tank + BP + STA) is about -20 full, assuming a SS BP and steel STA, so a 27lb wing is more than enough (the dynamic need is only for 18lbs of lift; the static need is greater)

The 36 is more than you need, but its ok too.

Anything bigger is way more than you need.

Grossly more lift than required adds to the risk of a runaway ascent if things ever get out of hand on you (e.g. stuck inflator) and thus adds somewhat to the danger of your kit.

 

[Braunbehrens]

Here is a url describing how to pick the wing size.

http://baue.org/faq/index.html

 

[kingprawn]

Braunbehrens once bubbled...
Here is a url describing how to pick the wing size.

http://baue.org/faq/index.html

Good link. I found one of the numbers I was looking for:
10# of lift to be comfortable on the surface. More in rough seas.


Thanks