Still confused about lift

[mi000ke]

I am a relatively novice diver in the market for a travel BCD. I have done and will be doing all of my diving in warm water or in Florida springs (72 degrees year round), and need to fly to the dive sites. My concern is about the minimum lift I will need, especially given how little lift some of the travel BCDs offer. (And for those of you who will recommend a BP/W – which I am open to - I still need to determine the minimum lift).

I’ve read just about every post and article on lift and buoyancy, but still not sure I have it completely figured out. The factor that gets mentioned the most and which I find the most confusing is whether diving in warm water or not. Is that because of the thickness/buoyancy of the wetsuit one would typically use in warm water, or is there something about water temperature itself that affects buoyancy and lift?

In my case I use a 5mm full suit (with 5mm hood for the spring dives and without hood for the 80+ degree ocean dives) and that is perfect for me. I’ve used 14 lbs weight in fresh water and 18 in salt water with different rental BCDs and regulators, and that’s worked pretty well. I carry very little equipment (safety sausage and maybe a small dive light in the springs). My body is slightly negatively buoyant in fresh water (a strapping 148 lbs on a 5’ 10” frame).

So for salt water I figure 10 lb for the wetsuit/hood, 5 lb for tank/air, I assume the BCD and my natural buoyancy are a wash, and maybe a pound or two for regulator and equipment. So about 16 lb. (and I weight at 18). In that case would 20# lift be adequate? What am I missing?

Assuming I dove with AL80s, can anyone ballpark what minimum lift I might get away with? I plan to take a peak buoyancy course to refine my weighting, but I want to have my equipment in hand for the course so we’re fine tuning the equipment I will actually be using.

Thanks

 

[ronscuba]

A small travel wing will be ideal. They range 17-25 lbs. of lift.

I would look at a DSS BCD package with their kydex plate and 17 or 20 lb wing or same wings with an Apex or Oxy softplate. Both priced about the same, around $460. Softplate will be a little lighter and pack smaller, kydex plate will feel more stable diving. The DSS package price with the Kydex plate is a very good deal.

My wife and I dive with the DSS 17 and Oxy softplate. She has small XS Scuba weight pockets on the waist and cam bands. Great for diving and travel.

 

[victorzamora]

You'll need between 17 and 25 pounds of lift. With your 5mm, I'll err on the side of caution. When I was in Mexico last, I used nearly an empty wing. With that 5mm compressing, you're likely to need 7# at depth plus a few for gas. Properly weighted, you could ALMOST get away without a wing at all in a 3mm suit. In my 3mm shorty I have no issues diving with no wing at all. On the surface, wings are nice. But that's it. Get a small, light travel BPW setup. There's almost nothing out there with too little lift for you

 

[halocline]

I’ve read just about every post and article on lift and buoyancy, but still not sure I have it completely figured out. The factor that gets mentioned the most and which I find the most confusing is whether diving in warm water or not. Is that because of the thickness/buoyancy of the wetsuit one would typically use in warm water, or is there something about water temperature itself that affects buoyancy and lift?

The only reason cold water makes a difference is because you wear a thicker wetsuit, which then compresses more and has a greater net loss of buoyancy at depth. So, you need to add more air in your BC to offset that buoyancy loss.

The easiest way to determine your minimum lift requirement is to find out how much positive buoyancy your wetsuit has. You can do this by putting the wetsuit in a mesh bag, floating it, and adding weights to the bag until it sinks. The wetsuit can't lose any more buoyancy than it inherently has. Then you add 5 lbs for the weight of the air in your tank and that's a good safe minimum lift requirement. The weight of the tank, your regulator, and any other non-compressible gear affects your weighting but not your lift, because it does not change at depth.

You'd probably be fine with 20 lbs lift but a 5 mil suit could take 15 lbs to sink, add 5 lbs for air and you're fairly close. Note that this shouldn't affect you on the surface, when your suit is providing plenty of buoyancy, but at maximum depth with your suit compressed, and a full tank of air. Realistically speaking, your suit is not going to compress to zero-buoyancy (you'd be pretty cold if it did!) so there is a margin of safety built in.

The other thing is if you are carrying 18 lbs of lead, you should consider splitting it up between your rig (ballast mounted on the BC) and your body, like with a weight belt, so that ll the ballast is not on your rig. You need the wing to be able to float the fully-weighted rig without you in it. More importantly, it's better for trim and safety to have the weight split up. If all of it were in weight pockets and those pockets accidentally emptied, you'd be heading straight for the surface. And if you ever needed to remove your BC at depth, it would sink and you'd be hanging on upside down, clinging to it like an anchor.

So in your case, a steel plate plus maybe 2-2lb weights on the cambands, then 8 lbs on a belt might work well. Something along those lines, anyhow.

 

[ams511]

As a general rule of thumb, for warm water diving 20 to 30 lbs of lift. For cold water 30 to 40 lbs of lift. You are neither warm water or cold water so I would recommend something in the 25-30lb range.

 

[Jim Lapenta]

I always tell people that they need to consider one other thing. How much extra would you like to have should you need to assist another diver? For me I could get by with a 23 lb wing as long as I decided I was not going to ever help someone. Problem is that is not in my nature. Past events have proven that out and so I was glad I had that 32 lb wing. Size wise it's not much bigger than the 23 and it pack just as small. I also do not see diving as a race. So if there is any increase in drag (which there isn't if everything else is configured right, you are properly weighted, and in trim) is a non issue.

 

[mi000ke]

I always tell people that they need to consider one other thing. How much extra would you like to have should you need to assist another diver? For me I could get by with a 23 lb wing as long as I decided I was not going to ever help someone. Problem is that is not in my nature. Past events have proven that out and so I was glad I had that 32 lb wing. Size wise it's not much bigger than the 23 and it pack just as small. I also do not see diving as a race. So if there is any increase in drag (which there isn't if everything else is configured right, you are properly weighted, and in trim) is a non issue.

Good point, thanks

 

[Peter Guy]

OP -- I've written before that it is my belief a BP/W setup is the least worst travel BCD (I don't think there is a "best" one!) due to being able to take it apart and pack so easily. For what it's worth, the weight/volume difference between a 17 pound and a 25 pound wing is pr7obably insignificant. Were I you, I'd seriously look at getting a 25 pound wing which should be "good enough" for just about all the diving you are currently planning to do.

The good thing is that IF you do expand your diving situation, adding a bigger wing is easy and you won't need a smaller one. (And yes, I dive a 25 pound wing with a 5mm or dry suit in warm (75 and up) water.

 

[TSandM]

Lift needs to do two things: It needs to float your gear on the surface with you not in it, and it needs to compensate for whatever buoyancy your exposure protection loses (or can lose) at depth.

If you are diving in warm water, you use little or no exposure protection. You don't have to wear very much lead to sink yourself or your gear, so even if you put all that lead on your BC, it still isn't going to be very negative by itself, even at the beginning of the dive, when the tank is full. If you are diving an Al80, for example, it's about 2 pounds negative when full. If you need, say, 6 pounds of lead to sink, and you put it in your BC, your BC could be maybe 8 to 10 pounds negative at the very beginning of the dive. So 17 pounds of lift will keep it on the surface very nicely.

Similarly, if it takes 8 pounds of lead to sink you, then you are only 8 pounds positive, so that's all the lift you could lose at depth. Assume you go down to 100 feet at the very beginning of your dive, and your wetsuit compresses, and your tank is full. Your tank is -2, and you've lost that 8 pounds of neoprene lift through compression, so you're ten pounds negative. Again, 17 pounds of lift is going to be just fine.

In cold water, it looks kind of different. Using steel tanks, the tank is much more negative when full -- my HP 100s are about 8 pounds negative at the beginning of a dive. My dry suit is 26 pounds positive, so I could lose a lot of that through a flood -- say, at least 20 pounds of it and maybe more. So I could be 28 pounds or more negative if my suit flooded at the beginning of a dive, and I need at least that much lift to hold me on the surface. So cold water divers need much bigger air bladders to be safe.

Jim Lapenta makes the point that you might have to hold yourself up AND another diver; in that case, honestly, both of you should drop your weights, so the total lift needed would be minimal.

 

[eelnoraa]

It also depends on where you want to carry your 18lb.. If you have all 18 on your rig, then 20lb isn't enough. -18 lead + (-2 full AL80) + (-2 reg), is already more than -20lb. if you ditch your rig, it will sink.

A lot of people recommend getting the smallest wing possible. I respectfully don't agree. I would say get a like 30lb wing in general. It is large enough for cold water drysuit. It is stll small that doesn't create significant drag over al 18-20lb wing. It is just an overall more flexible wing with little compromise.