HOW MUCH LIFT FOR COLD WATER

[drbill]

I carry 32# of lead (sometimes 38#) mainly to keep myself stable when filming on the bottom in surge. My wings have all been in the 28-32# range in coldwater. I dive with about 20-25# lead in warm water and use an 18-20# wing there.

Years ago I was on a 200 fsw dive and my 32# wing developed a significant leak. I had to swim up to the surface carrying my camera and a spear gun I found at depth. Not fun.

 

[cool_hardware52]

I carry 32# of lead (sometimes 38#) mainly to keep myself stable when filming on the bottom in surge. My wings have all been in the 28-32# range in coldwater. I dive with about 20-25# lead in warm water and use an 18-20# wing there.

Ballast carried and required wing capacity have some relationship, but it's not direct. It is a mistake IMO to base wing capacity on ballast required.

Buoyancy Compensators are just just a device to compensate for things that change in buoyancy with respect to depth.

For divers the single element that loses buoyancy as they descend is their exposure suit. Cylinders become less negative as the contents are exhausted, and a diver's personal buoyancy does not change WRT to depth as long as they are breathing compressed gas.

A diver's personal buoyancy can effect required ballast, but it need not be compensated for with a BC.

That leaves the two criteria I have noted for a properly weighted diver

1) Float the rig with a full tank

2) Compensate for the maximum possible change in buoyancy of the diver's suit.

Tobin

 

[StefinSB]

Why don't you just call Tobin? He manufactures wings and has a huge selection of sizes to guide you to the correct one. For wetsuit diving in Cali I am good with a 26# wing from DSS which is very nicely streamlined.

 

[BenjaminF]

For 8mm (6.5mm and a 1.5mm shorty) I used a 24lb wing with a soft backplate and 12lb of lead for a long time. I only stopped using it when moving to a dry-suit. There I needed more weight, so I got a metal backplate and a 30lb wing.

 

[sarahlee]

I carry 32# of lead (sometimes 38#) mainly to keep myself stable when filming on the bottom in surge. My wings have all been in the 28-32# range in coldwater. I dive with about 20-25# lead in warm water and use an 18-20# wing there.

Years ago I was on a 200 fsw dive and my 32# wing developed a significant leak. I had to swim up to the surface carrying my camera and a spear gun I found at depth. Not fun.

super cool info - thanks kelp lorax

 

[Boaty McBoatface]

A slight departure, but @sarahlee seems to have her question answered.

I'm still new (and so is my wetsuit). I'm in the process of working down the weight I carry. One definition of having enough weight is having just enough to become slightly negative at the start of a dive. On a recent dive, the DM had me drop a couple more pounds. Once on the water, I burped the wetsuit thoroughly, but I wasn't sinking enough to start the dive, so she had me swim downward. My suit compressed, I became negative, and I ended up not being too buoyant either on the dive or on the safety stop -- at least with the variables at play on that particular dive.

Q: Is it normally accepted practice to swim down like that to get negative enough? What if I and/or my wetsuit weren't new?
Thx

 

[Trace Malinowski]

About 1/18 of a Boeing 747's weight to escape cold water diving.

 

[rhwestfall]

Q: Is it normally accepted practice to swim down like that to get negative enough?

Yes, often referred to as a "duck dive"..

What if I and/or my wetsuit weren't new?

I will loose "some" buoyancy over time, but not a bunch.

You will likely reduce a bit of your lead requirements as you become more comfortable. Sometimes, getting off the surface is challenged with not completely exhaling, or even inhaling as you start to go under. Common response, especially with a new diver..

 

[drbill]

Ballast carried and required wing capacity have some relationship, but it's not direct. It is a mistake IMO to base wing capacity on ballast required.

I agree. I merely posted what I personally use. It wasn't intended as a general rule

 

[Buoy_A]

Q: Is it normally accepted practice to swim down like that to get negative enough? What if I and/or my wetsuit weren't new?
Thx

Air has weight. Compressed air has even more weight (by volume). Since the air in your tank has weight, it adds to your negative buoyancy. Let's say that an 80cf aluminum tank is filled to 3000 psi. It has appx 6lbs of weight (1lb per 500psi). So it adds *about* 6lbs of negative buoyancy to your whole rig. But, importantly, this is at the beginning of your dive.

So let's say it's the beginning of your dive, and you've just jumped off the boat. You're "carrying" 6lbs of air on your rig. And you're saying someone told you it's OK to have to swim down at this point to "get negative"? OK. So somehow you manage to swim down (Not sure how this is done as I would imagine your fins would pop out of the water and you wouldn't get any traction...but anyways...). So you do your dive. Let's say you use 1000 psi going away from the boat. That's 2lbs of negative buoyancy you've lost. And then let's say you use another 1000 psi meandering back to the boat. You're now 4lbs "lighter" than at the beginning of the dive. You've lost 4lbs of air "weight". So you ascend, and in the process you use another 500 psi. And you're at your Safety Stop, 15 ft below the surface. So how are you going to hold your safety stop? You're now 5lbs "lighter" than when you started the dive. It's as if you've ditched 5 lbs of lead during the dive. And to start the dive, you had to swim down to get negative? You were 5 lbs heavier when you had to swim down.

rwestfall stated that this was a known technique. But it doesn't sound like a good idea and I've never seen anyone practice it. That's just my $0.02 cents on the subject.

Regarding an old wetsuit vs a new wetsuit, yes, old wetsuits definitely lose buoyancy over time. The neoprene compresses. No one can say how much, only that it will.

If people are interested in the subject of how much lift is required for a wing, and to understand buoyancy in general, I'd highly recommend reading the "Ultimate Wing Lift Calculator" sticky thread at the top of the Buoyancy Compensator forum. Try to understand the nuances, such as why a person's head sticking out of the water is 100% negative weight vs when a person's head is under water. Pick apart the calculator and understand why the author calculated the things the way he did.