Do I have enough buoyancy with this 25lb donut

[Seville]

I have been using a sea quest malibu BCD and it has been working well. I am 210 lbs, usually dive with a steel 120 tank, 7mm wetsuit with 3mm gloves, booties and hood. Usually, I need 10 lbs of trim weight on the back and 12 lbs of weight in the front. Making it a total of 22lbs of extra lead. I am not sure what the buoyancy is for the sea quest Malibu.

I recently got a backplate setup. The donut said it has 30lbs of buoyancy but checking it, it has more like 25-26lbs of buoyancy. I have a 5 lb ss backplate and a 2 lb single adapter. Do you think this setup does not has enough buoyancy to keep me at the surface? Trying to do the math, it looks like it would be close to natural buoyancy, not sure how much buoyancy my wetsuit would add to it. What do you guys think?

 

[runsongas]

jacket bcd is usually a few pounds positive, so you may be able to run a few pounds less with the bpw. if it has 26lbs of lift in freshwater, you likely will be ok in the ocean. for freshwater, you would have to test how it holds up at the beginning of a deep dive with the full 120, it likely will be fairly close.

 

[AfterDark]

Only you can answer that question. I'm about 190lb use a 7MM farmer John wet suit, and steel tank(s) my wing has much more lift than that but that's for me.

 

[scubadada]

Can you float your rig?

 

[James79]

Wing lift is needed for 3 major reasons:

1) Offset weight of gas at start of dive (1200 cf of air is about 9.5 lb, but you will have some still in the tank at end of dive, so figure a 8 lb buoyancy swing from start to end of dive)

2) Offset loss of buoyancy from suit compression at depth.

3) Be able to float the rig independent of you if needed (taking it off to get back on a boat, etc.)

So, rule of thumb on wetsuit buoyancy is approx 2 lb. per mm. So a 7mm suit is approx 14lb lift. Your steel 120 can be as little as 11 to as much as 16 lb negative when full (depending on brand) and your plate and STA are 7lb, so in terms of Reason 3, your 26 lb wing is fine... if you have little weight on the rig or you have one of the less negative tanks.

For reasons 1 and 2, 32 lb is plenty to offset gas weight and wetsuit compression.

So, the real question is weight of tanks, and distribution of lead (how much is on you vs. how much is on the rig).

Respectfully,

James

 

[johndiver999]

Too small for a man using a 7 mm suit. Sell it and get a bigger one, it will be popular for people diving little or no wet suit.

 

[inquis]

My experience is the 2 lbs per mm is on the low end. Fortunately, it's pretty easy to just measure it by sinking it in a pool, hot tub, trash can, or similar.

However, floating the rig without you is probably the more stressing condition. I use a weight belt in cold water because my wing is not large enough to float my rig with that weight (integrated). In warm water, I put the (smaller amount of) weight on the rig.

 

[Seville]

Wing lift is needed for 3 major reasons:

1) Offset weight of gas at start of dive (1200 cf of air is about 9.5 lb, but you will have some still in the tank at end of dive, so figure a 8 lb buoyancy swing from start to end of dive)

2) Offset loss of buoyancy from suit compression at depth.

3) Be able to float the rig independent of you if needed (taking it off to get back on a boat, etc.)

So, rule of thumb on wetsuit buoyancy is approx 2 lb. per mm. So a 7mm suit is approx 14lb lift. Your steel 120 can be as little as 11 to as much as 16 lb negative when full (depending on brand) and your plate and STA are 7lb, so in terms of Reason 3, your 26 lb wing is fine... if you have little weight on the rig or you have one of the less negative tanks.

For reasons 1 and 2, 32 lb is plenty to offset gas weight and wetsuit compression.

So, the real question is weight of tanks, and distribution of lead (how much is on you vs. how much is on the rig).

Respectfully,

James

Thanks for the breakdown! I just couldn’t believe how small the donut looked compared to my older BC. I was hoping to have all the weights on my rig so I so not have to have a separate weight belt

 

[Scared Silly]

BP 5 STA 2 Lead 10 Cylinder 4-12

So depending on the cylinder (called it 8) you have 25 pounds on your rig right there which is right in line with what you found with the wing fully inflated. A bit tight because the wing may not be able to fully inflate while kitted up. Further, one really does not want their rig floating just under the surface but above the water. That means some of the wing will be out of the water.

Note: though you mention having all of weight on your rig that is probably not going feasible even with the most buoyant cylinder

BP 5 STA 2 Lead 10 Lead 5 Cylinder 4

26 pounds. So in that case yer rig is probably gonna be calling Davy Jones.

Next lets see if you can prevent being a dirt dart at the beginning of a dive. Here add your additional 5lbs, but remember your wetsuit at depth has little to no positive buoyancy.

BP 5 STA 2 Lead 10 Lead 5 Cylinder 8

So at depth you have 30 pounds. Does your body have enough positive buoyancy to offset the 5 lbs??? Probably. But if not yer dropping your weights or yer gonna be a dirt dart.

At the end of the day, IMHO the wing does not provide you enough lift. Keep it and use it for warm water diving which where I used mine.

 

[lowwall]

I have been using a sea quest malibu BCD and it has been working well. I am 210 lbs, usually dive with a steel 120 tank, 7mm wetsuit with 3mm gloves, booties and hood. Usually, I need 10 lbs of trim weight on the back and 12 lbs of weight in the front. Making it a total of 22lbs of extra lead. I am not sure what the buoyancy is for the sea quest Malibu.

I recently got a backplate setup. The donut said it has 30lbs of buoyancy but checking it, it has more like 25-26lbs of buoyancy. I have a 5 lb ss backplate and a 2 lb single adapter. Do you think this setup does not has enough buoyancy to keep me at the surface? Trying to do the math, it looks like it would be close to natural buoyancy, not sure how much buoyancy my wetsuit would add to it. What do you guys think?

I think there is a lot of overcomplication going on here. This is not a case of figuring out your weight from scratch. You need 22lbs of negative buoyancy (all lead) with your current setup. Worst case is you'll need 22lb with new setup. So yes, 25lbs of lift is enough to float you.

It's actually a little better than that. I found an old review that says your BCD has 2 pounds positive buoyancy. So that's 2 pounds you should be able to drop from the total, leaving you with a need for 20. Maybe even less since you are probably using a larger size than they tested with. Ok, you need 20 total. Take off 7 for the steel plate and STA and that leaves 13 in needed lead.

The next question is if you hang all that on plate or harness, will the rig float with you out of it?

Well, the answer is that it's close. A full 120 is around 9lbs negative. Combined with the 20 between your rig and lead and it will float if the wing can reach its rated lift. Or it will sink if it can only reach your estimated 25 pounds of lift.

The only way to be sure is to set it up and see what happens. If it does sink, I'd transfer at least some of your lead to a weight belt or harness. It's probably a good idea anyway with a 7mm. If you ever have to remove your rig underwater, it will be a lot easier to keep everything under control if some of the weight stays on you.