offset 3mm boyancy

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laivindil

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A couple fathoms underwater.
I'm looking to try some freediving sometime soon and was wondering how much weight you all wear to offset a 3mm shorty.
 
I used to wear 8lbs, but then I realized that I'm negative in freshwater with a 3mm full suit. Sure made it easy to get down.

Everyone's buoyancy will be different- you'll have to check- float eye level with a nearly empty snorkel. :D

I *heard* the rule of thumb is to be neutral at 20', although I don't know if that is so you can rest at the surface or so you might float to the surface if you black out on your way back? :11:
 
I do the bobbing plumb at eye level with an average breath thing, just like scuba and it works well for me. I can float and swim face down and snorkel breathe forever or back swim with my face out of the water. If I kick lightly I can rise enough to talk comfortably. That does seem to equate to esentially neutral at 20 feet. For me in a 3/2 full suit it means 3 pounds in fresh water, I'll add 5 if I ever get to skin dive in water warm enough for that little rubber.

Your need will vary with your body, gear and desire for surface buoyancy.

Pete
 
do it easy:
I used to wear 8lbs, but then I realized that I'm negative in freshwater with a 3mm full suit. Sure made it easy to get down.

Everyone's buoyancy will be different- you'll have to check- float eye level with a nearly empty snorkel. :D

I *heard* the rule of thumb is to be neutral at 20', although I don't know if that is so you can rest at the surface or so you might float to the surface if you black out on your way back? :11:
I'm curious about that "nearly empty snorkel" comment :wink:

Really, it depends upon your own body makeup, the buoyancy of the water you're in, and what you want to do. If by snorkeling, you mean simply paddling around on the surface, then no weight is needed (indeed, a 3mm short wet suit may not have much buoyancy). In the USAF years ago, we used to not wear weights with a 1/8 inch thick short wet suit top. Either of the checks mentioned above will work well to get you neutral at the surface. But if you are breathhold diving to 25+ feet, realize that you may want that buoyancy to assist you on your ascent too.

So experiment with it when you get to the site. If you are thin, you probably won't need much weight. If you are thicker, then get into a pool and check your buoyancy in fresh water.

If you know your displacement in the water (the number of cubic feet of water that you displace), you can calculate your buoyancy in salt wate and fresh water. Salt water weighs 64 pounds per cubic foot, whereas fresh water weighs 62.4 pound per cubic foot. To know your buoyancy, multiply the displacement by 64 pounds per cubic foot (or 62.4 if you know your weight in fresh water). Subtract your body weight from the weight of the water, and you'll know your buoyancy (or how fast you'll sink, which is my wife's case, if the number comes out negative). Once you have figured that out, you can then subtract the amount of weight to get you neutral in fresh water from the calculated weight for salt water, and that should work.

You can get your displacement (or close to it) by filling a bathtub (not completely full), and marking the side, then getting in and having only your nose above the water, and having someone mark the side. Be sure to wear what you would while snorkeling. By multiplying the width, length and depth (assuming a square tub), you can get the number of cubic inches, then divide by 144 cubic inches per cubic foot. Don't do like Archemedes though, as he arranged it to displace his mass with water going into another container, which he then weighed the water. But our bathtubs aren't made that way, so we would simply make big mess.

In the March 1970 US Navy Diving Manual, they used an example of a helmet diver in full dress, weighing 384 pounds. In a tank of water, he displaced 6.5 cubic feet of water, which in salt water is 416 pounds.

416 - 384 = 32 pounds buoyant, and so he needed to add 32 pounds of weight to become neutral. If we used this in for fresh water, it was:

405.6 pounds of fresh water - 384 pounds of diver = 21.6 pounds of weight to add.

So the formula is:

(Weight of water displaced with wet suit, either fresh or salt water) - (weight of diver) = amount of weight needed to be neutral.

I hope this helps.

SeaRat
 
https://www.shearwater.com/products/peregrine/

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