Considerations around switching to a BP/W

[rombre]

Thanks for the detailed explanation Tobin.

That's why I said BP/Ws are a complicated topic. Again, for a local diver that's all very easy. For a holiday diver, my profile (different exposure suits and AL as well as steel tanks) is pretty common and comparable to the profile of most of my diving friends (all have jacket BCs though, so don't care about it).

We basically reached the conclusion that this "normal holiday diving" profile more or less requires 2 different backplates + a wing (+ a separate wing if I wanted to use it in colder water as well). That's a lot of different gear ...

 

[cool_hardware52]

We basically reached the conclusion that this "normal holiday diving" profile more or less requires 2 different backplates + a wing (+ a separate wing if I wanted to use it in colder water as well). That's a lot of different gear ...

No, you reached that conclusion. If diver is realistic about the range of application it's often possible to narrow down what is needed.

Let's start with plates. You want to dive cold water at home, and that quite clearly is best served with a SS plate.
How do you make a SS plate "work" while in warm water? 2 choices, stick with buoyant tanks, and or thicker suits when using negative steel tanks. Research what cylinders your dive ops are actually providing and find out what the buoyancy numbers are. Stay away from tank / plate / suit combos that over weight the diver. Usually not that hard to do. Often it involves wearing a bit more suit than you might otherwise. Easy to do if your leave it 1/2 zipped and pump a little water through it. Iff the dive op has al 80's use those.

Wings: Easy here too. Armed with the tank info I recommended you get you can easily determine the max negative for your rig, get wing bigger than that.

Done.

Tobin

 

[rombre]

Let's leave the cold water out (there I think it's obvious that a SS plate is better and more lift is needed - will look at a wing for that later), and only look at the warm water:

I go to Indonesia and dive there with a shorty, max. with a 3mm full wetsuit and a 12-15l AL. As you illustrated, this really asks for a SS plate.

Now I go to Malta where most dive ops only have steel tanks and given I'm a big guy, the wrecks are not shallow etc., it will be a 15l one. 5mm is the ideal wetsuit there, however this would mean that I am overweighted with a SS plate as per your calculation. Now I could use a 7mm wetsuit but a) that's not really ideal and b) the buoyancy change through compression might actually require a wing larger than 20 lbs there.

This would effectively limit my warm water diving to aluminium tanks which doesn't seem to be feasible.

 

[Lorenzoid]

. . .
The big - and crucial - question for me is, can I do 5mm wetsuit + 15l steel (which is a scenario that I had and will have again) with a steel backplate. . . .

A 5 mm suit, HP 120, steel DSS backplate, and DSS Torus 26 wing worked well enough for me on the occasions I have dived that combination. As those were tropical dives, I would have preferred a 3 mm suit, but I chose the 5 mm for the buoyancy to offset the heavy steel tank. More typically, I dive a 3/2 mm suit, aluminum 80, steel DSS backplate, and DSS Torus 26, and THAT combination is a joy to dive--I need essentially no lead at all.

 

[dumpsterDiver]

Although I do not refute the assumptions and calculations listed above, I find the whole exercise of trying to split hairs and encouraging a new diver to select the absolutely smallest wing possible is not something I would do.

How about other considerations, like having a little extra capacity to rescue an overweighted diver, or some extra lift to handle a surface emergency in a squall, how about handling a mistake where the diver accidentally wears a little too much lead? etc. etc. The drawbacks of having an extra 10 or 12 lbs of lift capacity are minimal at best.

I recently got a wing with 28 lbs of lift and it is extremely compact and I see very little benefit in trying to use a smaller wing. Actually i'm not sure it will be enough for me when wearing a 5 mm suit and when I catch a bag of heavy lobsters. If it doesn't work out this winter, I will use a different BC.

 

[runsongas]

a steel backplate and 30lb wing covers most single tank diving. AL backplate only really needed with big steel tanks in warm water.

 

[Bob DBF]

My choice of wing was much easier, I found out how much lift my jacket had and bought a wing with the same lift. The 40# oval wing might be a tad larger than the calculations for my cold water diving, but I had a lot of experience with that much lift and did not want to find out I miscalculated during an emergency. I don't have any problem with controlling buoyancy in a warm pool with no thermal protection, even though I'm carrying a lot of extra fabric and would rather have a smaller wing.

I guess this puts me in agreement with dumpsterDiver, there are other situations, aside from ones own personal comfort, that may come into play and should be considered. For me a bag of scallops, or finding a weightbelt, weights, anchor, or other treasure have kept me from looking at smaller wings. Like a lot of other choices in diving, a diver has to make his own choice on what he needs for his dive plan, and it is more than just a math problem.


Bob

 

[runsongas]

Now I go to Malta where most dive ops only have steel tanks and given I'm a big guy, the wrecks are not shallow etc., it will be a 15l one. 5mm is the ideal wetsuit there, however this would mean that I am overweighted with a SS plate as per your calculation.

only way you should be significantly overweighted would be if you sink like a rock or are diving a 15L heiser boat anchor. a 5mm full suit, regs, steel bp, and 15L steel tank should be close to neutral.

 

[halocline]

Let's leave the cold water out (there I think it's obvious that a SS plate is better and more lift is needed - will look at a wing for that later), and only look at the warm water:

I go to Indonesia and dive there with a shorty, max. with a 3mm full wetsuit and a 12-15l AL. As you illustrated, this really asks for a SS plate.

Now I go to Malta where most dive ops only have steel tanks and given I'm a big guy, the wrecks are not shallow etc., it will be a 15l one. 5mm is the ideal wetsuit there, however this would mean that I am overweighted with a SS plate as per your calculation. Now I could use a 7mm wetsuit but a) that's not really ideal and b) the buoyancy change through compression might actually require a wing larger than 20 lbs there.

This would effectively limit my warm water diving to aluminium tanks which doesn't seem to be feasible.

Okay, I think I have a simple answer for you, if what you need is a rig that will work with both AL and steel tanks in warm water. This is assuming you will be overweighted in a steel plate. Get the Kydex plate and use camband pockets for putting weights on the cambands. This will allow you to add weight up on the plate when you are diving AL tanks and also allow you to not be overweighted when you are using negative steel tanks. It's more versatile. Properly placed weights on the cambands approximate the effect of a steel plate. It's not quite the same but pretty close.

I believe you are confusing the issues of lift needed and ballast needed. You're thinking of a bigger wing for the steel tanks, but what you really need is less ballast with those tanks. With a given wetsuit, your lift needs should not change simply because you switch to a heavy steel tank.

For the wing, it doesn't really matter whether you get a 20 or 30 lb wing. It's nice diving with a very small wing, but you will not be disappointed with something like the 30 lb LCD wing. You won't notice much difference. They both vent very easily and are nice and streamlined.

 

[rombre]

A 5 mm suit, HP 120, steel DSS backplate, and DSS Torus 26 wing worked well enough for me on the occasions I have dived that combination. As those were tropical dives, I would have preferred a 3 mm suit, but I chose the 5 mm for the buoyancy to offset the heavy steel tank. More typically, I dive a 3/2 mm suit, aluminum 80, steel DSS backplate, and DSS Torus 26, and THAT combination is a joy to dive--I need essentially no lead at all.

The scenarios you are describing are pretty much exactly what I'm looking for (5mm + steel, 3mm/5mm + AL). Very helpful, thank you.

How about other considerations, like having a little extra capacity to rescue an overweighted diver, or some extra lift to handle a surface emergency in a squall, how about handling a mistake where the diver accidentally wears a little too much lead? etc. etc. The drawbacks of having an extra 10 or 12 lbs of lift capacity are minimal at best.

Good point! I thought about that briefly and then for whatever reason went back to the search for the smallest possible one. So I should probably have a look at the Torus 26 that @Lorenzoid also dives.

only way you should be significantly overweighted would be if you sink like a rock or are diving a 15L heiser boat anchor. a 5mm full suit, regs, steel bp, and 15L steel tank should be close to neutral.

That's what I calculated as well, I was correctly weighted with c.15 lbs with a large steel tank and a 5mm wetsuit when I was in Malta a few weeks ago. My calculation for that (retrospectively) is:
+4 (jacket BC) + 10 (5mm wetsuit) -2 (empty steel tank) -2 (reg) +5 (approx. inherent buoyancy - also matches with my rough calculations on other dives) = +15
Replacing the jacket BC with a SS BP/W should change it from +4 to -6, therefore I would still require 5 lbs. I actually tried with 10 lbs instead of 15 lbs - I was too positive, so I think the calculation wasn't too far off. But again, I'm far from an expert on this ...

Okay, I think I have a simple answer for you, if what you need is a rig that will work with both AL and steel tanks in warm water. This is assuming you will be overweighted in a steel plate. Get the Kydex plate and use camband pockets for putting weights on the cambands.

That's a good alternative. Best option would still be the SS one if it works, as this would also be the far better choice for cold water (at a later stage).

Summary (of my poor understanding - correct me if I'm wrong):

A) Issue of overweighting (BP)
- Will most certainly be an issue with SS BP, steel tank and a 3mm wetsuit (that's ok, it's a scenario that I can easily avoid)
- There are different opinions if SS BP, steel tank and 5mm wetsuit will lead to overweighting

B) Floating the rig (BP / wing)
- Tail end case is a full large steel tank (let's say -11), reg (-2) and the SS BP/W (-6), leading to -19
=> 18 / 20 lbs wing a bit on the small side, sth. like the Torus 26 should have more than enough lift

C) Loss of buoyancy (wing)
=> A 26 lbs wing should certainly cover all scenarios except a drysuit (and possibly a 7mm)