drk5036
Contributor
This is a topic I've been considering a lot, and feel like has not been significantly discussed. What is the role of height when determining the required lead / weight check for diving?
To achieve neutral buoyancy, one needs to add enough lead (lets include tank weight for simplicity) to counteract the buoyancy of ones body. I think that height, and projected body size with limbs extended, likely plays a huge role in determining person-to-person weighting, and has primarily been ignored. To discuss this further, I want to look at two points: 1) the difference in buoyancy of thermal protection with varying size, and 2) the variance in true "density" in the water, when calculating volume based on projected area.
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. I think this number being 33% greater is not out of the question.Given that this is a percentage, this effect likely isn't very noticeable for thin wetsuits. If it takes 4 pounds or so to sink a 3 mm wetsuit for someone who is 5'6", it might take 5 pounds for someone who is 6'7". However, if you're wearing a 7 mm wetsuit, or a drysuit that requires 25 pounds to sink, that would increase the required lead up to 32 pounds, a significantly noticeable increase.
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.
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, and leaner people sink better, hurts our guestimate at weighting more than it helps. Specifically, that guideline only works when comparing divers who are the same height. 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.
Any thoughts?
To achieve neutral buoyancy, one needs to add enough lead (lets include tank weight for simplicity) to counteract the buoyancy of ones body. I think that height, and projected body size with limbs extended, likely plays a huge role in determining person-to-person weighting, and has primarily been ignored. To discuss this further, I want to look at two points: 1) the difference in buoyancy of thermal protection with varying size, and 2) the variance in true "density" in the water, when calculating volume based on projected area.
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. I think this number being 33% greater is not out of the question.Given that this is a percentage, this effect likely isn't very noticeable for thin wetsuits. If it takes 4 pounds or so to sink a 3 mm wetsuit for someone who is 5'6", it might take 5 pounds for someone who is 6'7". However, if you're wearing a 7 mm wetsuit, or a drysuit that requires 25 pounds to sink, that would increase the required lead up to 32 pounds, a significantly noticeable increase.
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.
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, and leaner people sink better, hurts our guestimate at weighting more than it helps. Specifically, that guideline only works when comparing divers who are the same height. 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.
Any thoughts?
