The Science of Weight + Neutral Buoyancy

[Mrjoemorgan]

The title of this post makes it sound a lot more exciting than it's going to be, but I'm curious about body composition and how it impacts the weights you dive with to achieve neutral buoyancy.

Diver Profile:
100+ Dives
36 years old
190lbs
20% Body Fat (Acceptable)
58% Body Water (Standard)

I'm a calm diver, always relaxed in the water. I usually end Dives with plenty of air left (the same as the dive masters who are leading the tours). I share this because one theory is poor breathing technique and/or muscle stiffness, which can cause the need to carry more weight.

But with a 5mm shorty, and 5mm boots, I need 13lbs of weight. (5.89kgs)

And every dive shop I dive with is always adamant that I am wrong, that I should only need 4lbs - that I'm too stressed or breathing incorrectly etc.

At the end of the day, I don't really care what others think or say about the weights that I use.

But I am curious if there is something to do with body science that impacts all of this.

Appreciate any thoughts or theories from the community.

Cheers,

Joe

 

[johndiver999]

yes the less muscle, more fat and decreased bone density a diver has, the more weight they will need. Lung volume might affect things a little too. However, I have seen at least one young man who was not fat, was reasonably fit and simply required considerably more lead than you would expect.

13 lbs is not a whole lot of lead for saltwater and an aluminum tank and some wetsuit.

 

[inquis]

You won't get much info, since equipment dictates required lead far more than body composition. At the end of the day, to be neutral the negative things must balance the positive things. Common negative things: lead, backplate, steel tank, regulator, fins, reserve air.
Common positive things: AL80 tank, wetsuit/drysuit, many padded BCs.
Some depends on the water: most (all?) people are positive in the ocean, but I would guess about half are slightly negative in fresh water.

You need what you need. Can you hold a safety stop with nearly empty BCD, nearly empty tank, and breath normally? If so, then you're correctly weighted for that equipment combination.

 

[grantmac]

What is your buoyancy when swimming in the ocean in shorts?

 

[Rilelen]

13 pounds in saltwater is not a ton, esp in a 5mm shorty. I (petite but curvy lady) use 8 pounds in a 3mm full suit, and 12 pounds in a 5mm full in saltwater (with a jacket BC, AL80, etc).

The only time I'd get away with 4 pounds is in a 3mm in freshwater.

 

[lexvil]

The actual science is; you need what you need.

 

[The Chairman]

The often overlooked variable in the science of weight is trim. I can't overstate this enough. If you aren't flat, you'll have to have more weight to compensate for a small upwards thrust. Of course, once you stop kicking, you'll have to add air to compensate for the lack of upward thrust. Once underway again, you'll have to vent air to maintain the same level.

Trim first. Make sure your thrust from kicking is only horizontal. Then, it is what it is. I've posted weight titration elsewhere, but if you aren't familiar with it, I'll be glad to repost it here.

 

[boulderjohn]

I agree with the Chairman. Trim first.

But to your question, I have known outstanding divers who needed far more weight than people think they should.

 

[The Chairman]

I have known outstanding divers who needed far more weight than people think they should.

I'm an above-average diver, with a very buoyant personality. I usually need 14 lbs without a wetsuit on an AL80. I have been known to dive successfully without even a bladder.