Check my thinking on steel vs. alum buoyancy (not just weight)

[DivingLane]

Thought experiments are all fine, but this reminds me of "measure with a micrometer, mark with chalk, cut with an axe."

Any difference is going to be hidden and overcome by the volume and weight of boltsnaps and snap shackles. Why don't we ever have these discussions on the 2-3" of overlapping neoprene at the end of wetsuit legs where it covers the boots? Probably has more of an effect.

Don't lose the forest for the trees.

What is going on with the overlap of wetsuit and boots? Truly curious

 

[Wibble]

In reality, an aluminium backplate would be the same size and thickness of a stainless one, circa 3mm thick. The stainless backplate is considerably heavier than the ali backplate.

When you're diving in colder water you'll need lead to sink due to your wet/drysuit. May as well use the stainless backplate to add a couple of kg/pounds without lugging a lead brick.

Or, if travelling, the ali backplate is fine and you just add another couple of pounds in lead.


BTW you can get a thick 6mm stainless backplate that is heavy! Seen one once; never really caught on due to the cost as much as the weight.

 

[lowwall]

In reality, an aluminium backplate would be the same size and thickness of a stainless one, circa 3mm thick. The stainless backplate is considerably heavier than the ali backplate.

The OP was comparing a standard AL plate to this skeletonized SS plate:

If the OP were comparing AL applies to SS apples, then the steel plate would be 3 to 4 pounds heavier and let you drop about the same in lead.

FWIW, I like my steel plate even for travelling. It's nice having most of my needed weight built in and placed ideally for trim. If I lived in Singapore or somewhere where I could fly for short dive trips with just a carry on, I could see the appeal of ultralight gear, but flying longer international trips or SWA to FLL, I'm checking a bag anyway and the 3 to 4 pounds I'd save don't make a practical difference.

 

[sea_ledford]

Not hidden at all when those snaps & shackles are present in both cases. In this particular case, the difference isn't a big deal, but the OP didn't know that to start with -- thus the question. Other changes (AL to steel tanks for instance) can be significant. Not everyone has the luxury of jumping in the water to see the difference, especially when contemplating a new equipment purchase.

OP recognized that this might be an insignificant difference from the get go. Again, thought experiments are fine, I highly encourage them! But my point was to not get so bogged down in minutiae that it causes decision paralysis. There are plenty of examples of very expensive things to solve problems that end up being insignificant (carbon fiber backplate, I'm looking at you). I mean, I want a CF backplate. They look cool, but trying to convince myself that there is any functional difference from an AL plate would just be lying to myself.

 

[schuylerclark]

I'm quite aware that this approaches a high level of nit-pickery, but I am considering all the buoyancy aspects of my kit and this is one of those. I'm thinking to keep the plate to ~2lbs so it best meets a few dive scenarios that I typically find:

1. for local freshwater diving when it's warm I usually take 4-6 lbs total with an AL80 and I'll be getting a steel tank some point. This may make it so I don't need any additional weight for those dives. When I need more for colder dives I'm happy to carry more lead.
2. for warm ocean dives I'll need more weight but that's fine. I'm just trying to shave off a bit on the margins. I occasionally carry gear from the car over dunes to the shore, or dive from my kayak, so minimizing the "out of water" weight helps.
3. for flying, since my dive buddy and I are married, we're usually packing two sets of dive gear in 3 suitcases. Our current set up means two bags ~45 lbs and one ~ 35 lbs, so there's some allowance for more, but not lots. My current BCD is an AL Zuma (4.75 lbs), so any type of BPW is going to add a little weight to my setup.

I've got a background in mountaineering and backpacking, so I've spent lots of time finding ways to shave a few ounces from my pack. I've been considering all the small things I carry as well. I just enjoy optimizing this sort of thing. I'd prefer to have one kit to handle most of the diving I do, and this seems like a balanced choice that checks most of the boxes.

 

[The Chairman]

I'd appreciate it if someone more experienced than myself could check my thinking and make sure I have this right. In comparing a ~2 lb aluminum backplate to a ~2 lb steel backplate (the Dive Rite "lite" model), it seems to me that the steel would still be more negatively buoyant than the aluminum. Assuming they're the same weight, and since steel is typically ~3x denser than aluminum, it must also have less volume (hence the cutouts in the plate), thereby displacing less water and being more negatively buoyant, right?

If I had both plates on hand, I'm sure I could measure the volumes and calculate the difference but since I don't and I'm confined to a thought experiment, am I thinking of this correctly? If I am, then my follow up question would be is this buoyancy difference significant enough to reduce the additional weight needed for diving or is the real world effect negligible?

You're overthinking it. Underwater, weight is still weight. I doubt it would make more than a couple of ounces worth of weight in regard to the differing densities.