The role of height in on required weight to reach neutral buoyancy

[wKkaY]

If we are trying to be technical about diving, and quantify everything, why shouldn't we try to do the same thing with weighting? I don't believe it's more art than science, I think people just aren't willing to dig into the science enough to really understand what's going on.

Large factors in weighting can be modeled and estimated based on certain factors, but what matters ultimately is how one feels in the water. Fine tuning that is an art and it's not just about adjusting the amount of weight, but also the position of the weight/gear, or swapping out gear, changing technique, etc... all of that with personal preferences and environmental considerations in mind.

 

[NAM001]

the simple process is to get all suited up and weigh your self on a scale. . so you are now. 240# and you ad .35% as additional weight to comp from fresh to salt water. That works just ducky so long as all you are weighing is mass wise somewhat equal to the body. Wet suits screw that all up because you weigh ,, for the sake of argument,, the same with or without a wet suit on. The thicker the suit the more error you get in the comp weight numbers. Depending on what amount of lead you have on while on the scale that amount could and perhaps should negate the effects of the no weight high buoyancy wet suit. With out wet suit i add 1# for every 40 on the scale and with a suit I add 1 for every 30 and change.

 

[Jcp2]

Optimal Buoyancy Computer

Try playing around with diver height while keeping weight constant. The wetsuit buoyancy calculations are related to body surface area formulas.

 

[decompression]

Let’s try a real world question:

A student has perfect weighting on the first day of open water ocean dives. All the students are in drysuits which they have trained with in the pool. It’s cold here.

Student decides she was too cold the first day and adds a layer of undergarments. In this case a bright fuzzy yellow Pikachu onesie with a hood, ears and a tail. She is about 5 foot 6 inches tall and average weight.

How much lead do you add so she remains at perfect weighting?

Drysuit weighting is more art than science. And yes, the above scenario actually happened

if the onesie had ears and a tail, add 4 lbs.

 

[FreeFlyFreak]

There are too many variables.
Height, weight, body type, fat percentage, neoprene type, neoprene thickness, wetsuit construction, gear carried, tank size, brand and material, fin type buoyant negative or neutral.....on and on.
It the end it is only the total package that matters. Looking at just height and or weight..... M'eh, a waste of time IMO

Can it be figured out...?
Probably, but it would take a huge scientific experiment that no one would pay for because it can be done quickly and easily by suiting up and doing a weight check or two.

 

[Angelo Farina]

Some assumptions you made are wrong. Let's see:

First, lets consider the impact on total overall buoyancy of a wetsuit, for people of different heights. How much weight does it take to sink a wetsuit for someone who is 5'6" (66 inches) versus someone who is 6'7" (79 inches)? Just based on proportions, that's 20% more neoprene, not taking into account other factors, where a taller person likely, on average, requires a larger suit.

All this is unnecessarily complex, the calculation is much simpler. Just measure the weight of the suit on a scale, let's say it is 2 kg. As the relative density of neoprene foam is around 0.2 - 0.25, the positive force exerted by the suit is 3-4 times its weight, so it will be 6 to 8 kg. The fact that these 2 kg of suit are a thin suit of large size or a ticker suit of small size does not change the result... The type of neoprene foam instead changes the results (within the range shown here above), usually suits for free diving have lower density, hence they are slightly more buoyant.

Second, lets consider true "density" of a diver, taking into account body size. Buoyancy is a result of a force balance between gravity pulling down on an object, and a fluid pressing up on the object. The apparent weight in water can be defined by the weight of a person, minus the weight of displaced fluid, and the apparent weight of a diver must be negative for them to sink. For two people who are the same weight, and one is 5'6" and one is 6'7", the taller diver will displace much more water (again, at least 20%), making his density much lower. Therefore, the taller diver will need to carry more weight, EVEN FOR TWO DIVERS OF THE SAME WEIGHT! to be able to sink.

This is entirely wrong. The "form factor" or "total size" of an human body has little to do with its buoyancy. It is true that some people is more "positive", some other is strongly negative. Look at me and my wife. Both are around 1.75m tall (my wife is 1.73m), and both weight around 90 kg (quite fat!). I am positive by 4 kg, my wife is negative by 1 kg. The human body is usually very close to being perfectly neutral, and it is positive or negative a few kg, mostly depending and how much air is contained inside chest and intestine, and how much fat is under the skin. Skinny people are usually more negative, fat people are more buoyant.

My conclusion from this analysis (that I'm still chewing over and would like to get feedback from everyone else on) is that the common belief, that people who carry more body fat require more weight

This is correct.

and leaner people sink better,

correct again

hurts our guestimate at weighting more than it helps. Specifically, that guideline only works when comparing divers who are the same height.

That's wrong. height is not a factor.

And I think this is why we have such a wide range of answers when discussing weighting. I've often shocked people by saying that despite the fact I weight 205 pounds, I only need 9 pounds of lead to dive in my 8/7 mm semidry with a small (8 L) steel tank. This is likely due to being only 5'6" tall. If I was 6'3" 205 pounds (ah, one can dream...), I would likely require more weight to dive.

No one doubts of your buoyancy, everyone has his own constitution, determining the results. But you cannot affirm what would be your buoyancy if you had been taller or shorter, there is equal probability that you had been more positive or more negative, or perhaps nothing changes...
Only thing which is sure is that, nothing else changing, if YOU earn some fat, then YOU will be more buoyant. But, as you cannot earn more height (nor loose), all the rest is pure speculation.

 

[84CJ7]

FreeFlyFreak has fairly well summed up the whole discussion.

Like I said, just changing fin type totally screwed my weight up and unless you have the level of simulation they use to design and balance airplanes (which requires a ton of data to be entered initially for the simulation) you will never get a scientific way to calculate the distribution of weight for proper trim so calculating the total weight isn't worth the effort since it will require trial and error to get it set out anyways.

If you come to a dead stop in a level swimming position you should ideally stay level and not roll or sink head or feet first or you are incorrectly weighted between your front and back and head and feet. Maybe the correct weight total but the weighting is wrong because its not distributed right.

 

[Bob DBF]

And one has to find out if the diver is just giving just the lead weights he is using, or giving the entire ballast weight. Sometimes it sounds like someone uses little weight until you find they have a 6# plate, 6# stay, and tank that is -3#, instead of a poodle jacket and Al 80. Ballast is ballast, it all counts, even though one only has to add a little lead.



Bob

 

[Neilwood]

I really dislike the threads where everyone plays guess the weight required and comes up with a certain figure.

The only way to know the lead weight required is to perform a proper weight check with 500 PSI (or 50bar). Anything else is a guesstimate based on assumptions.

 

[The Chairman]

I also have found when it comes to weight, and equally importantly trim, science is a waste of time.

I teach the science of how trim, weight and propulsion affect your neutral buoyancy with every OW, AOW and my Trim, Buoyancy and Propulsion clinic. Once you understand the connection, your learning curve drops dramatically. You won't need a 100 dives to look and feel comfortable if you take a few moments and figure this out.

The only way to know the lead weight required is to perform a proper weight check with 500 PSI (or 50bar). Anything else is a guesstimate based on assumptions.

You can do it with a full tank. The check point will be the top of your head instead of eye level.