OP
The role of height in on required weight to reach neutral buoyancy
- Thread starter drk5036
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He was not as comfortable in the water and was not mentally willing to empty his lungs completely when descending. I found that as I got more comfortable, I needed less weight. I reduced my requirements by at least 4 lbs just due to that comfort level and trust of my gear. The divemaster on my last LOB was probably 5'10" to my 6'1" and maybe weighed 20 lbs more than me. He doesn't require any lead vs my 8 lbs. It took him a large number of dives to get down to that number, yet his body build and gear had not changed.
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Over 10 years I went from 42 pounds to 37 pounds. Figured it's like most will say, due to improved diving ability. Until I dumped the old used farmer john wetsuit for a much newer used one. Back to 42 pounds. Figured I'd just continue with the 37 and was amazed that there was no chance in Hell of descending with that.
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
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
B
Buoyancy, is related to both volume and density. Just because you weigh the same dose not mean you both have the same density. It also dosen't mean that you displace the same amount of water. I'm not sure if it's possible to have the same muscle to fat ratio and weigh the same as a diver at 6'7". It may be possible but seems suspect.
Your idea about the suit may have some bearing. More neoprene means more weight. Does his suit have more material because of his extra height? Who knows? Different types of neoprene have different buoyancy characteristics too. Especially after they are compressed. Other factors could be a BC that was not fully deflated or his general comfort in the water.
Also for practical applications, divers have different lung volumes and different breathing "styles" for lack of a better word, that have an effect on their real world weighting requirements. Deeper breaths increase volume and buoyancy. Deeper breaths also seem to last longer, giving that buoyancy more time to affect you. Smaller breaths still increase it but to a smaller extent and over a shorter time so the change has less time to affect trim.
All things being equal, physics always wins. A set volume creates a set amount of buoyancy that has to be offset with weight or extra buoyancy to remain neutral depending on the density of the object displacing the water. The trick is to find out what's different. His density, yours, suit material, breathing volume ect, ect, ect.
For me, I'm 5'11" and 205 +/- lbs. In salt water, diving an old school plastic back plate and a steel 72 with no BC bladder, 2mm socks, Large SP Jet Fins, shorts, and no wetsuit, I need 10 lbs on my belt. Any less and I'm ok at the beginning of the dive but at the end I get floaty with every breath.
Add a 3mm 1 piece with a 3 mm hooded vest to that setup and the weight almost doubles. Not quite but close.
Buoyancy, is related to both volume and density. Just because you weigh the same dose not mean you both have the same density. It also dosen't mean that you displace the same amount of water. I'm not sure if it's possible to have the same muscle to fat ratio and weigh the same as a diver at 6'7". It may be possible but seems suspect.
Your idea about the suit may have some bearing. More neoprene means more weight. Does his suit have more material because of his extra height? Who knows? Different types of neoprene have different buoyancy characteristics too. Especially after they are compressed. Other factors could be a BC that was not fully deflated or his general comfort in the water.
Also for practical applications, divers have different lung volumes and different breathing "styles" for lack of a better word, that have an effect on their real world weighting requirements. Deeper breaths increase volume and buoyancy. Deeper breaths also seem to last longer, giving that buoyancy more time to affect you. Smaller breaths still increase it but to a smaller extent and over a shorter time so the change has less time to affect trim.
All things being equal, physics always wins. A set volume creates a set amount of buoyancy that has to be offset with weight or extra buoyancy to remain neutral depending on the density of the object displacing the water. The trick is to find out what's different. His density, yours, suit material, breathing volume ect, ect, ect.
For me, I'm 5'11" and 205 +/- lbs. In salt water, diving an old school plastic back plate and a steel 72 with no BC bladder, 2mm socks, Large SP Jet Fins, shorts, and no wetsuit, I need 10 lbs on my belt. Any less and I'm ok at the beginning of the dive but at the end I get floaty with every breath.
Add a 3mm 1 piece with a 3 mm hooded vest to that setup and the weight almost doubles. Not quite but close.
OP
drk5036
Contributor
I see this statement a lot. The first few posts in this thread also make similar statements. It bothers me.
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.
OP
drk5036
Contributor
I wish there was a way for us to try some experiments given we are the same weight.
For me, I wear a 3 mm full suit, with a skin underneath. 5 mm boots. Mares Quattro fins. Small Freedom backplate with 23 lbs VDH wing. 0 extra lead.
It's just so interesting to me that the amount of weight needed varies so much. I'm trying to get my mind around it. I feel like there must be something we are missing.
What tank are you using? You using a Scubapro MK7 regulator by chance?
My 205 lbs probably takes up a bit more volume than yours as at least some of it is in extra "insulation" around my middle. Without the extra padding I'd probably be 185 and not need any lead unless I was in a wetsuit.
In just a swimsuit, in a freshwater pool, if i hold my breath I float vertically with the tip of my nose touching the water. If I exhale, I sink. Don't know if that means anything but you can compare that to what happens to you in a freshwater pool.
I think one issue is that body type is a big factor. It probably plays more of a role than you realize. Also the comfort of a diver plays a big role. If a diver is not used to their gear they may not properly deflate the BC and then flail around fighting that extra lift.
You can always try putting your gear in a pool to see if it floats or sinks. If it floats, verify that all the air is out and then add weight to sink it. That is the amount of weight that your gear requires and it should be pretty constant. Adjust that weight for salt water. Everything else is you. Do you float or sink in the pool? How much weight do you need to sink.
Keep in mind the change in buoyancy change in the tank as it gets empty.
My 205 lbs probably takes up a bit more volume than yours as at least some of it is in extra "insulation" around my middle. Without the extra padding I'd probably be 185 and not need any lead unless I was in a wetsuit.
In just a swimsuit, in a freshwater pool, if i hold my breath I float vertically with the tip of my nose touching the water. If I exhale, I sink. Don't know if that means anything but you can compare that to what happens to you in a freshwater pool.
I think one issue is that body type is a big factor. It probably plays more of a role than you realize. Also the comfort of a diver plays a big role. If a diver is not used to their gear they may not properly deflate the BC and then flail around fighting that extra lift.
You can always try putting your gear in a pool to see if it floats or sinks. If it floats, verify that all the air is out and then add weight to sink it. That is the amount of weight that your gear requires and it should be pretty constant. Adjust that weight for salt water. Everything else is you. Do you float or sink in the pool? How much weight do you need to sink.
Keep in mind the change in buoyancy change in the tank as it gets empty.
