BC lift

[jk47]

Hello - I have a BC lift question. I get that differently sized BC's or wings have different pounds of lift associated with them, but how do you convert 35 pound lift, 45 pound lift, 60 pound lift etc. into what you actually need based on your size, weighting and equipment?

 

[Nomad]

Are you talking about replacing the bladder on your BCD or are you talking about upgrading to a larger wing?

...or is this more of an academic question?

 

[jk47]

Academic and practical both. I am buying my first BC. But I also want to understand what's going on.

 

[Nomad]

Regarding BCD, you will want to make sure that the the bladder can be removed, and that a replacement bladder (in differing sizes) is available. BCD bladders are NOT universally interchangable. Not all models are built to allow user replacement.

Although I do not use a BP/W, it is my understanding that they are much easier to customize. Simply buy a new wing. If the holes on your backplate are not in the right place, drill new holes and attach the new wing.

Anybody with a BP/W wanna add to this?

 

[Kray_Z]

thats right its a lot easier.. really no need for the hole drilling being that everything is made for that kind of use with standard 11" holes... as far as lift goes.. what type of diving are you doing.. like what kind of exp. suit.... single or doubles? that kind of stuff...

 

[ChrisM]

In determining how much lift you need, you need to take two general factors into consideration, and strike a balance between them:

1. You need to have enough to float you and your gear comfortably on the surface. In my opinion, you need a bigger wing to float you than you would need if it was just for buoyancy UW.

2. Theoretically, you should be weighted such that you are neutrally buoyant underwater. Meaning you could, if required, kick yourself up to the surface without inflating the BC. In other words, just because you wear 30# of lead does not mean you need 30# of lift. Ideally, your wing is not inflated underwater. But ideal conditions rarely exist.

Of course, there are a lot of variables - i.e., exposure suit (wet, how thick? dry?), size of tank, amount of air in the tank....etc -- that play into this, but let's assume diving wet, you will generally be heaviest at the beginning of the dive. This is due to (1) fuller tank = heavier tank, and (2) assuming you are not diving a square profile but a multilevel, your wetsuit will compress at deeper depths, expand as you come up.

Thus, assuming a theoretical ideal weighitng you need sufficient lift to compensate for wetsuit compression and weight of the gas, the two things that most affect your buoyancy, and which are greatest at the beginning of the dive. Ironically, you want to be neutral at the END of the dive.

As an example, when I dive California with a 7 mil with an 80 steel tank, I wear a 6# backplate and a 4# weightbelt. My total weighting is 6# for the BP, 6# for the steel 80, 4# for the belt and 2# for the STA, total of 18# to be neutral (at 500 psi). I use a 36# wing,

Theoretically, I could get by underwater with a 15 pound wing or so, but that wouldn't float me. So the 36 is a balance between what is available on the market, what I need to float, and what I need UW. When I travel to warm water, I bring an aluminum backplate and the same wing (could get a smaller wing). BP/W is a mix and match system, depending on conditions.

I see a lot of newer (and older) divers with really big oversized wings. Bigger is not always better. A primary reason you don't want a wing that is too big is that if your inflator button sticks, you will have a LOT of air to deal with and try to dump to prevent a rocket ride to the surface.

It's my opinion that single tank diving requires nothing more than a 36 or so pound wing. In tropical waters I'd say you could go with a 20 pound wing, depending on particulars. I don't dive doubles, but I can't see a 45 or more pound wing being used for anything but doubles

Chris

 

[Fun Diver]

I think it's easier to consider the lift required just based on the tank size/configuration (i.e. single/double) and diving wet or dry. In general <40 is going to be Ok for diving a single tank wet or dry, and go up considering doubles & tank size. As mentioned buy for what you primarily will dive with, not what you think you need down the road.. if your needs change, buy another wing.

 

[jk47]

SO the poundage of lift is just offsetting tank +weights? Or more? I mean I obviously weigh more than 30 pounds so it's not supporting my whole body weight, but it's lifting more than just the gear... is there just some complex mathematical formula to do the 'conversion'? Obviously in practice it seems like there's a pretty easy rule of thumb.

To answer the questions above - recreational diving, typically 7mm suit, steel backplate, aluminum or steel 80 single tank (24lbs weight with the alumnium 80 and no backplate). My LDS suggested a 45 lb wing for me (or maybe she said 40) as opposed to 35. I weigh about 170 - does this seem like a pretty big wing? I don't want to have a huge non-hydrodynamic silhouette.

 

[Don Burke]

SO the poundage of lift is just offsetting tank +weights?

No

The lift you will need underwater is the weight of the gas in your cylinder(s) plus the amount of buoyancy change due to compression of your neoprene.

Or more?

perhaps

I mean I obviously weigh more than 30 pounds so it's not supporting my whole body weight, but it's lifting more than just the gear... is there just some complex mathematical formula to do the 'conversion'?

A Type I PFD is about 32 pounds positive. I believe that is a reasonable minimum for the purpose of staying on the surface unless you are a beach diver or quarry diver, in which case less may be more convenient for you.

Obviously in practice it seems like there's a pretty easy rule of thumb.

fairly easy

To answer the questions above - recreational diving, typically 7mm suit, steel backplate, aluminum or steel 80 single tank (24lbs weight with the alumnium 80 and no backplate).

Figure about six pounds for the gas in the 80 cubic foot cylinder. The wetsuit is probably something on the order of 25-30 pounds positive. The material the cylinder is made of, how much lead you wear, and the material the backplate is made of are not issues when figuring how much lift you need.

My LDS suggested a 45 lb wing for me (or maybe she said 40) as opposed to 35.

With a 7mm suit, I would not argue with 40 or 45. 35 would be cutting it a bit close for my taste.

I weigh about 170 - does this seem like a pretty big wing?

Your weight is not an issue in this case.

I don't want to have a huge non-hydrodynamic silhouette.

good thought

Since you touched on weight, I would like to say that there is a misconception that the amount of weight on the belt and the amount of BC lift should move in lockstep.

The two are related in a secondary way. A BC is there to provide lift in normal conditions and in most emergencies. Ditchable weight is ditchable to provide lift when the BC is not hacking it in abnormal conditions. Many people wear too much of their weight as ditchable. Some types of diving are safer with no ditchable weight at all.

 

[ChrisM]

SO the poundage of lift is just offsetting tank +weights? Or more? I mean I obviously weigh more than 30 pounds so it's not supporting my whole body weight, but it's lifting more than just the gear... is there just some complex mathematical formula to do the 'conversion'? Obviously in practice it seems like there's a pretty easy rule of thumb.

Do you mean offset on the surface or underwater? On the surface, you are correct (aided by the 7 mil and body fat). Underwater, a BC really only offsets westuit compression and tank weight, which are greatest (most negative) at the beginning of a multilevel dive.

Assuming you are perfectly neutral, you really shouldn't need a BC underwater, right? But the equation changes when your suit compresses, you are no longer, neutral, but negative, so you have to offset that. You are also negative because the air in the tank, so you need to offset that. Neither of those is very great (say maybe 6# for a steel 80 and 10 # positive for a 7 mil, that's a rough guesstimate - edit - Don's probably right, noit sure a 7 mil is 25 #, but let's go with 20). So a theoretical 26# wing would be in the area of the minimum range to offset the weight and get you to the surface if you had an emergency at the beginning of a dive. Of course, that wouldn't work practically if once you get there you can't float. You'd also want a bit of a safety margin over that.

To answer the questions above - recreational diving, typically 7mm suit, steel backplate, aluminum or steel 80 single tank (24lbs weight with the alumnium 80 and no backplate). My LDS suggested a 45 lb wing for me (or maybe she said 40) as opposed to 35. I weigh about 170 - does this seem like a pretty big wing? I don't want to have a huge non-hydrodynamic silhouette.

Yup, a 45 is a big wing for singles and is also typical of a lot of dive shops that don't really know how wings even work. They probably said, "well, you wear 24# + BP plus tank, that's 36#, you need a 40#". But how much your rig weighs on land has zip to do with the amount of lift needed. All they think is more is better. But as you've said, creates a horrible drag in the water (not to mention the stuck inflator issue I mentioned earlier).

A 40 is still on the high end.

How much it weighs in the water, with you in it, does make a difference since you want to float. Your 7 mil and your body provide additional buoyancy so you don't necessarily take into account the gross weight.

I am 6'2", 240#, 7 mil suit, steel 80, steel backplate, 4# weightbelt, I use a 36 and it's plenty.

Hope some of that made sense