Still confused about lift

[ronscuba]

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.

Everyone's opinions and experiences are a little different. Easy to see why many new divers get confused.

He/she is a new diver and used 18 lbs with a rental BCD. As a new diver, most likely they could have dropped a few pounds. Rental BCD was probably a few pounds positive too. He did say he was negative in fresh water.

Like the thread starter, I am a warm water travel diver. My 1st wing was a Oxy 30. The fact that it was only a little bigger than the various travel wings and it is versatile enough to be used for cold water, is true and makes sense.

About a year later I bought a DSS 17. It packs much smaller than my Oxy 30. It vents way easier and it dives much more streamlined. For me, where diving is 90% warm water travel, it was money well spent. I frequently have conversations with dive staff at tropical locations. They either have a similar small wing or tell me they want one.

Go with the specialty travel wing or the versatile/compromise 30 wing ? He she could buy the travel wing now and the larger wing later if/when he decides to dive cold. Many people have more than 1 wing. Decisions, decisions.

He could give Tobin at DSS a call. He makes wings in a ton of sizes, 17,20,26,30,35 and he will make a recommendation. His wings also pack down small, which is great for travel and his package pricing is tough to beat.

 

[halocline]

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.

Honestly I'm a little surprised to read this from you, especially considering how quick you are to criticize other instructors. Assisting another diver to the surface should never require additional in your own BC; you would either drop the other diver's weights and/or use lift in the other diver's BC. That's right out of basic recreational rescue class. Over inflating your own BC to help a diver get to the surface puts you at risk of a runaway ascent.

That said, there really is little difference between a 20 and 30lb wing, as you mentioned. But not all 30lb wings are equal, and in general the smaller and more compact the wing is, the easier it is to vent. It's not that a slightly larger wing has significantly more drag in the water, it's that it's more cumbersome to manage with venting.

 

[cool_hardware52]

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

I speak with newer divers all the time concerning lift requirements. I'd be happy to walk you through the process, but I'll start with a comment I frequently make to those who are trying to understand "lift"

Today with every dive shop offering a "delux" BC with 50+ lbs of lift the physics of buoyancy has become a lost topic.

Remember that not all that many years ago scuba divers used no BC's. Some still dive that way today. Thinish suits, modest sized tanks, careful weighting and good water skills.

Keep that in mind when contemplating if a 17~20 wing seems like it's too small, they would have seemed *HUGE* to the sports pioneers….


Tobin

 

[mi000ke]

Thanks for all the input. This is starting to make a lot more sense.

So here is my analysis – check me out on it (just for my approach, because if I have that right then I can adjust for actual buoyancy numbers, various equipment, lighter weight, etc.):

ASSUMPTIONS:

1. My body is slightly negatively buoyant – let’s call it -1 lb
2. Let’s assume my BCD is +1
3. I dive a 5mm full suit, so let’s assume that’s +15 at surface and +1 at depth (won’t be going deeper than 100&#8217
4. Tank is an AL80, so assume -2 at beginning of dive and +3 at end of dive
5. Assume regulator is -2 (and assume no other equipment)
6. Assume 18# weight on my BDC as I’ve been using (but I will revisit that later in this post)

AT SURFACE/START OF DIVE
Wetsuit +15
Me -1
BCD +1
Tank -2
Reg -2
Weight -18
Net buoyancy = -7

AT DEPTH/START OF DIVE
Wetsuit +1
Me -1
BCD +1
Tank -2
Reg -2
Weight -18
Net buoyancy = -21

AT DEPTH/END OF DIVE
Wetsuit +1
Me -1
BCD +1
Tank +3
Reg -2
Weight -18
Net buoyancy = -16

AT SURFACE/END OF DIVE
Wetsuit +15
Me -1
BCD +1
Tank +3
Reg -2
Weight -18
Net buoyancy = -2

So given this, the min lift I would need (assuming I do not take of the BCD) would be 21 lb, and if I needed to float the rig at its point of minimum buoyancy, then about the same:
BCD +1
Tank -2
Reg -2
Weights -18
Total = -21

NOW… it looks like I could probably reduce my weight to around 14 lbs, which would change the net buoyancy as follows:
AT SURFACE/START OF DIVE: -3
AT DEPTH/START OF DIVE: -17
AT DEPTH/END OF DIVE: -14
AT SURFACE/END OF DIVE: +2
And therefore I would only need 17 lb lift (plus maybe a bit more for safety factor)

So taking the three factors into consideration of 1) offsetting max negative buoyancy 2) need to float BCD, and 3) need to help a dive buddy

1) Minimum lift (at depth at stat of dive) = 17 lb
2) Floating BCD = 17 lb (-14 weight, -2 reg, -2 tank)
3) Help a buddy = 30 lb ?

Given all of this, if I don’t take into account helping a buddy (putting morality aside) then I could probably get away with 14 lb of weight and closer to 17 lb BCD lift (or say 20 to have a safety factor).

Did I do this analysis correctly? If so, then I can determine the actual buoyancy of my wetsuit and rig to fine tune my weighting and lift requirements. If not, please point out where I went wrong (other than my assumptions, which are just assumptions).

Thanks again

 

[halocline]

So taking the three factors into consideration of 1) offsetting max negative buoyancy 2) need to float BCD, and 3) need to help a dive buddy

1) Minimum lift (at depth at stat of dive) = 17 lb
2) Floating BCD = 17 lb (-14 weight, -2 reg, -2 tank)
3) Help a buddy = 30 lb ?

Given all of this, if I don’t take into account helping a buddy (putting morality aside) then I could probably get away with 14 lb of weight and closer to 17 lb BCD lift (or say 20 to have a safety factor).

Did I do this analysis correctly? If so, then I can determine the actual buoyancy of my wetsuit and rig to fine tune my weighting and lift requirements. If not, please point out where I went wrong (other than my assumptions, which are just assumptions).

Thanks again

A couple of ideas:

1. The only things that require lift are things that change buoyancy at depth. The weight of your tank, regulator, etc...are irrelevant for lift purposes, because you account for those things by being properly weighted at the beginning of the dive. If you are negatively buoyant on the surface at the end of a dive with a near-empty tank, you're overweighted.

2. Please do not increase your lift requirement in an attempt to account for lifting another diver to the surface. I don't know why Jim, who is a serious and knowledgeable professional, made that comment. If you get to the point where you are studying rescue, you will learn that you do not ever put yourself in danger to assist a diver, in fact that's the first big thing you learn. You always use other means (dropping the other diver's weights, inflating his/her BC) to get an unconscious or disabled diver to the surface. That way you don't put yourself at risk of an uncontrolled ascent, and the diver in your care can actually stay on the surface without your lift.

3. As I mentioned before, if you really do need 18 lbs of ballast, consider splitting it between the rig and your body. This would be a safer and likely more comfortable way to dive. It also substantially reduces the lift needed to float your rig. The typical jacket style BCs with 20 lbs+ in weight pockets are very cumbersome both in and out of the water IMO.

Otherwise you're definitely zeroing in on it!

 

[mi000ke]

If you are negatively buoyant on the surface at the end of a dive with a near-empty tank, you're overweighted.

So that would support my plan to reduce my weight to about 14 lbs, as that would leave me at about +2 at the end of the dive (vs -2) if my assumptions are anywhere close. Or I could at least set my weights closer to the actual buoyancy of my wetsuit, which I intend to calibrate by weighting it in water.

Other than a wetsuit, what else loses buoyancy at depth?

 

[halocline]

So that would support my plan to reduce my weight to about 14 lbs, as that would leave me at about +2 at the end of the dive (vs -2) if my assumptions are anywhere close. Or I could at least set my weights closer to the actual buoyancy of my wetsuit, which I intend to calibrate by weighting it in water.

Other than a wetsuit, what else loses buoyancy at depth?

The only other thing that changes is the weight of the air in the tank. That's why you do a buoyancy check with a near-empty tank.

You don't need to sink your wetsuit in order to determine your weighting (ballast) requirement. You just do a weight check in the wetsuit you're going to dive with. You sink the wetsuit to get an idea of the maximum amount of buoyancy it could lose, and use that to help choose your wing size.

Let's assume for example that you could actually get away with 16 lbs in a jacket BC, AL80, and current wetsuit. If you bought a SS plate, you might be able to get to 8 lbs on a belt pretty easily. You'd lose the inherent buoyancy of the jacket, replacing it with 6lbs of ballast. Then you'd have a nice light weight belt with a couple of 4 lb weights, your rig would be very easy to float, you'd still be positive on the surface without your rig, and your ballast would be well distributed between your back and hips. And, if you ever need to remove your rig at depth (like if you got tangled up in something) you and your rig would be somewhat close to neutral so you wouldn't go flying to the surface while your BC headed for the bottom.

 

[ams511]

Given all of this, if I don’t take into account helping a buddy (putting morality aside) then I could probably get away with 14 lb of weight and closer to 17 lb BCD lift (or say 20 to have a safety factor).

As Tobin mentioned, most commercially available BCs have more lift that most divers will ever need. That is why lift capacity is not really talked about much in courses. You only really need to worry about it when choosing a backplate and wing.

On Scubaboard there are two "camps" regarding lift requirements. The first group recommends using the smallest size wing possible. They claim a smaller wing is easier to vent and more streamline. The second camp recommends having a larger wing because it is more adaptable and has a safety margin. The extra drag in negligible and most find larger wings are no problem to vent. Also there is no standardized way to measure wing lift and some divers complain that when assembled the wings do not have the advertised lift capacity.

You could get away with a 17 lb lift wing BUT what happens if you want to carry the weight in pockets rather than on a belt? You need a bigger wing. What happens if you need more weight? You need a bigger wing. What happens if you wear a thicker wetsuit? You need a bigger wing. Most manufacturers do not make wings with 20 lbs of lift or less. Scubapro and Halcyon's minimum size is 30 lb, Dive Rite and Hollis 25 lb, Hog's is 23.

 

[cool_hardware52]

So that would support my plan to reduce my weight to about 14 lbs, as that would leave me at about +2 at the end of the dive (vs -2) if my assumptions are anywhere close. Or I could at least set my weights closer to the actual buoyancy of my wetsuit, which I intend to calibrate by weighting it in water.

Other than a wetsuit, what else loses buoyancy at depth?

Mike,

You are getting some, er, well, "interesting" advice concerning required lift, some of it sound and some of it less so. I'll again extend my offer to walk you though the process I use to recommend a wing for given application.

626-799-5078. I'm around most week days.

Tobin

 

[Jim Lapenta]

Honestly I'm a little surprised to read this from you, especially considering how quick you are to criticize other instructors. Assisting another diver to the surface should never require additional in your own BC; you would either drop the other diver's weights and/or use lift in the other diver's BC. That's right out of basic recreational rescue class. Over inflating your own BC to help a diver get to the surface puts you at risk of a runaway ascent.

That said, there really is little difference between a 20 and 30lb wing, as you mentioned. But not all 30lb wings are equal, and in general the smaller and more compact the wing is, the easier it is to vent. It's not that a slightly larger wing has significantly more drag in the water, it's that it's more cumbersome to manage with venting.

Never said anything about getting another diver to the surface. Its correct you use other means to get them there whenever possible. On the surface it is nice having a little extra lift, a little mind you as 8-9 lbs (23 vs 32) is not a lot. It is possible for a diver whose weights have been dumped to still be negative. Steel tanks, doubles, etc.

While we are ditching their rig, that they may not be able to inflate, I personally want that little extra lift. You can't always know what another diver you come across needing assistance is using. Or where they have weight stashed or how much they are using. A newer diver does not need to be struggling to stay afloat by using a wing that has just enough lift for them when helping someone out.

A 40 or 50 lb would be overkill. Unless you're raising a guy in doubles. A 30-32 is not.

Being able to have that little extra lift in my wing is nice. If they are tired they don't have to kick while we are getting their BC off. I can better support them with that little extra lift. If a non responsive diver is ooa however and I can't inflate their BC from behind them in a rescue position I may use my wing to get us started up. I may have to use my wing for buoyancy unless I'm sure they are dead. I'm not going to drop a belt from them with 25 lbs of lead on them at depth. Sure way to send us both on an express ride up. Rather use all the lift I have and drop theirs where its safer to do so at the surface.