BCD toss-up...again!

[jonnythan]

Jonny,

The Scout was available with an 18 or 24 lb bladder - but came STANDARD with a 35lb. It is possible that your friend had one of the smaller bladders!!

Scott

Ahhh, that was most likely it.

Lesson: be sure the BC you buy has sufficient lift if you're going to be wearing it in cold water with lots of exposure protection.

 

[Kim]

Keep in mind that if you're diving Dutch Springs and wearing a nice thick wet suit, the Travel Wings or Scout may not have enough lift for you.

How does that work? My wet suit is positively buoyant - aren't they all?
In any event I dive a large steel tank with a 5mm suit and have never encountered any problems with buoyancy using the travel wing. Most of the time it has almost no air in it (or none at all). The only times I use a lot is at the surface just to provide a lot of buoyancy - again, no problems.

 

[H2Andy]

How does that work? My wet suit is positively buoyant - aren't they all?

at the surface, yes.

but neoprene compresses with depth, and you lose a lot of positive bouyancy
as you descend.

 

[Kim]

at the surface, yes.

but neoprene compresses with depth, and you lose a lot of positive bouyancy
as you descend.

Yes I know that. Even taking that into account though if I think how much air I need in my wing at 34 meters it still isn't a lot. Maybe using a 7mm would take a little more I suppose.
Does anyone have any hard figures as to the effective weight swing due to compression of a wetsuit for different thicknesses and depths?

 

[H2Andy]

well, basically, as i understand it, a wing is rated for the amount of weight it can
float at the surface.

so... if you get to depth and you lose lift from your wetsuit, your wing many not
be enough to offset that loss, since a wing that can offset 30 lbs at the surface
is not going to be nearly that efficient at depth.

now, granted, i think this is a rather extreme situation. you'll probably be ok so long
as your wing doesn't completely fail, as you'll probably be able to offset up to about
15 (?) lbs of lift by swimming up.

i understand that having to overcome more than 15 lbs by swimming up gets dicey.

 

[mxracer19]

A wing that provides 35 lbs of lift must displace .56 cubic feet of water. This is because an objects bouyancy is directly proportional to the amount of water in pounds that it displaces. water weighs roughly 62.5 lbs per cubic foot. so infact, whether or not you are at the surface, your bcd will still provide 35 lbs of lift, because fully inflated it will still displace 35 pounds of water with in this case, a relatively weightless gas. the difference will be that the net downward force on your bcd will increase due to a loss of flotation by yout wetsuit as it experiance the effects of water pressure. In short, your bcd isnt the only thing making you float at the surface, but a combination of the fluid your body displaces, as well as the flotation of your wetsuit, and every other small feature attached to you.


lol I wouldnt wanna swim with 15 lbs of downforce on me

-Matt

 

[H2Andy]

so infact, wether or not you are at the surface, your bcd will still provide 35 lbs of lift.

ok, i'm having trouble grasping this.. help?

a wing rated to 35 lbs will not lift 35 lbs of weight at depth. but it will float
35 lbs of weight at the surface.

this is correct, right?

 

[mxracer19]

no it will, however you will need to overcome the loss of flotation due to the fact that your wetsuit is being compressed by waterpressure as you come to depth. this is the reason why you will have to inflate your bcd more and more as you go farther down.

 

[Kim]

Actually I would have thought that it would be reasonably close in lift at the surface or at depth. Whichever it is it'll be the same volume of water that is displaced, just the water at depth will be under a higher pressure. However water doesn't actually compress right? So the weight of displaced water will be the same in both cases and the weight of the displaced water will be the weight in lift that occurs. (disclaimer: this is simply what I think according to my school level understanding of archimedes principle - if I'm wrong - blame my teachers!!!!!)

 

[Chad Carney]

Here's another way of looking at it, from personal experience.

Before hitching my wagon solely to Z products, I loved a 16 lbs lift BC (no longer made) from one of those S companies.

Rarely did I ever dive deep in a two piece 6.5mm wetsuit and almost never in fresh water, but we went to dive Ward Sink some years ago and I thought it just might max out the BC. I dove it anyway as a buoyancy test to about 155', and that's where the BC bladder was full and I was only neutral, even with a high lung volume. Any deeper and I would become negative. I took out 4lbs of weight and set it on a rock, then continued the dive to about 180'. I picked my weights back up on the way out and ascended.

I think the S company stopped making that BC because people were buying them that could get themselves into trouble they didn't expect. Many divers on this board are wearing more weight than their BCs will float at the surface. They need to be careful not to take the BC off. (Well, actually they just need to get weaned off the extra lead.)

Chad