Buoyancy changes from BCD to BPW

[Rickk]

I am currently diving with a BC and am considering switching to a BPW set up. I dive warm salt water, no neoprene just board shorts and a rashie. I now use 4.5 kg weight and am thinking of dropping a half kg. I am currently on a fat loss/muscle gain quest and expect that by the time I hit my targets, if I do lol, I should be able to drop a second half kilo.



My concern is that if I go to a stainless steal plate, I will not be able to add any trim weight without being over weighted. I have no problems now being neutral at 50 bar as I can pass between positive and negative with my lung capacity with my BCD empty. Currently I need to be lungs fuller to get positive so I am thinking of dropping the half kg so I return to being neutral at half full lungs.



How much buoyancy does an empty BCD have over an empty BPW? I am thinking that there is at least a couple kgs difference simply due to the size of the BCD over a BPW. That would mean that without any trim weight I would already be over-weighted with a steel BPW.



Travel weight is not a consideration for me. I live in a dive destination and will leave my gear at the dive shop between dives as they have secure storage and are only a few steps from the boat/shore entry point.When I go to a different dive destination, I will most likely drive and again leave my gear at the shop between dives.



I am thinking that if I go to 4 kg lead now with my BCD, that switching to an aluminum BP, I would be able to drop another 1.5 or 2 kgs for the difference in the buoyancy between the two rigs and just have 2 to 2.5 kgs lead.

 

[lowwall]

When I went from a jacket BC to steel BP, I dropped 3kg.

Can you try a steel backplate? The weight distribution is nice, you might not actually need any trim weights beyond the kilo or two you will still require.

 

[Kupu]

After long time use of a compact back inflate BCD (Zeagle scout) and then making the switch to BPW, I found my lead needs to be practically the same when using aluminum back plate.

When using steel back plate, I remove lead equal to the weight of the plate.

 

[KaosMick]

I switched from a SP Hydros Pro, to a Halcyon BP/W with 6lb SS plate and 30lb wing. I typically use my steel Faber 12.2 cylinders, and wear a 1.5mm wetsuit just for a bit of protection from the wrecks we dive here. With the Hydros, I needed 6lb weight. When I changed to the BP/W, I needed no weight, and was actually a few (maybe 2-3) lb negative. I had no issues with that, as I was confident I could swim the rig up in the event of wing failure...
Since then, I have gone a bit overboard frankensteining, and now run a 3lb SS Diverite XT Lite plate and a 22.5lb Mares XR wing, and I am pretty much spot on neutral at 30bar in the steel tank... But why stop there, I have ordered yet another type of SS backplate with an STA to try, along with a P weight pouch to try and get dialled in for AL80s that we use on liveaboard trips... It never ends, but that's why we love it right?

 

[Rickk]

When I went from a jacket BC to steel BP, I dropped 3kg.
......

If I drop a half kilo now because of my weight loss, then drop 3 more because of the gear change, I will only have one kilo for trim. If I hit my fitness targets, I may be dropping another half kg so only 500 g for trim. I should be able to get most of the trim adjustment by placing teh trim weight as high up on my body as possible, shoulder height. Wrist mount so I can dive arms out in front? Get a really big DC? lol

I can always play with the placement of the rig and tank within the rig to get trim. I will be at zero ditch-able weight but that is OK because I can always use my DSMB for alternate buoyancy in an emergency.

 

[NothingClever]

Keep burning calories until your fitness-nutrition ratio is in a sustainable balance and then go for a bold update in your equipment configuration. Trying to make end-game adjustments as you’re losing weight will probably yield limited satisfaction and only temporary solutions.

 

[lowwall]

If I drop a half kilo now because of my weight loss, then drop 3 more because of the gear change, I will only have one kilo for trim. If I hit my fitness targets, I may be dropping another half kg so only 500 g for trim. I should be able to get most of the trim adjustment by placing teh trim weight as high up on my body as possible, shoulder height. Wrist mount so I can dive arms out in front? Get a really big DC? lol

I can always play with the placement of the rig and tank within the rig to get trim. I will be at zero ditch-able weight but that is OK because I can always use my DSMB for alternate buoyancy in an emergency.

You probably won't need to add weight up high. A steel plate already has plenty of weight forward of your center of balance (when you are horizontal).

I don't think you should wait. It takes a lot of added muscle or subtracted fat to equal the negative buoyancy of 1 kilo of lead. If you get to the point where you are overweighted with just the plate, then treat yourself to an aluminum plate. They are not expensive. Or drill out some holes in your steel plate for an even cheaper option.

 

[stepfen]

Too many unknown factors (eg buoyancy of current BCD, buoyancy of BPW, tanks used, personal buoyancy, weight etc etc) to get any specific advice.

The following tool was designed to help people get answers to such questions
Optimal Buoyancy Computer

Give it a try...

In any way keep in mind that all these are rough estimates. Although it is general true, it is not a universal law eg that all BPW are less buoyant than all BCDs. It depends for example on how padded and (ab)used your old BCD is and/or how padded you will make your BPW.

Also since you dive warm water with no suit you probably have to consider aluminium plate (about 1kg weight vs 3kg for steel). In summer I dive with a thin full body wetsuit (it is more like a rash guard material than neoprene) and I need zero lead with my aluminium plate and 15lt steel tanks. Luckily I am ok-ish with "natural" trim. If I had steel plate I would be 2kg negative and no way to trim other than make myself even more overweight...

 

[Brett Hatch]

For what it's worth, I usually dive in cold salt water (started with 8mm wetsuit, now using a drysuit). Earlier this year I spent some time in warm salt water, where I wore a 3mm full wetsuit, no hood or gloves, used single AL80 tanks. I brought along my stainless BP/W. With a pair of 1kg lead blocks, my weighting was good -- 1 or 1.5kg might have been better, but those increments were not available to experiment with. I brought along two small weight pockets, since a BP/W harness has so much standard 5cm nylon webbing available, I was able to dial in the trim very easily. I weigh 75kg and am 180cm tall.

I didn't try it, but would guess that my 3mm wetsuit is a few kg's buoyant. If I were diving without any exposure gear, I would have been neutral or maybe slightly negative with the plate alone, and would not want to add lead for trim. That being said, the backplate is a great place to put ballast for most divers, so there's a good chance that I would not have needed any. I saw a lot of the local staff wearing SS backplates, AL80s, and either no exposure gear, just a rash guard, or a 1-3mm wetsuit. All seemed fine with no lead, as far as I could tell.

+1 to lowwall's point: gear up for today. You have to lose 10kg of fat to drop 1kg of lead. Or convert 6kg of fat into 6kg of muscle for the same effect. Both aluminum or stainless are likely to work well today, and if you decide that the other one would be better, well a new plate only costs like $75 or so, so don't sweat it. If you know anyone in your area who would trade BC's with you for a dive, or just let you borrow theirs, that would probably help a lot.

 

[jjardin]

Perhaps someone can loan you a BP to try out. If you're gonna buy, I agree with Iowwall--buy a used steel plate. If you end up going somewhere a little chilly you can throw on a 3mm without having to add too much additional weight (if you start with an alum plate with additional weight diving naked, you will be hanging all sorts of additional weight to dive w/ neoprene). If it is too heavy sell it and get an aluminum. Or buy both and sell the one you like least. Keep in mind that the backplate has webbing and d-rings which are also negative--but you can switch them back and forth --so you're really only buying just the plate. One note on accessories--I like using a STA with my bp--I find it makes the tank rock solid which for me more than compensates for the 1/2 it moves the tank off my back. So that is an additional weight and STAs also allows you to add weight on the spine...so perhaps you can get the aluminum and dial in a weight in an STA to get it right.

FWIW I use a halcyon steel backplate & STA and 18lb oxycheq when naked or w/ a 3mm (and use that mainly just on the surface--I don't really inflate the BC ever while diving--after over a decade in the 3mm I'm not sure it is still 3mm thick and doesn't really compress all that much).

I really don't mess with my weights too much and despite sometimes carrying all sorts of crap with me (e.g. an extra reel or strobe or extra weight for the kids or a ***poopy *** dive partner) it really makes no big difference. To round out the weight--I typically carry a short 3 ft smb, a few spools, a small strobe, and signal mirror, dive alert and whistle in my pockets glued on my legs or dangling from the butt ring and additional belt pocket. I'll typically have at least 2 lights at night. I carry a 6' smb in the mc pack thing which is the only padding. Sometimes I'll have a go-pro.

This leads me to question whether you can actually really feel a 500g difference--perhaps you should have someone add or subtract a few pounds in a blind test to see if you can actually tell or if you are just overthinking.

***Edit----- I guess SB filters out "bad" words these days lol--