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

[Silt Life]

Went to test a regulator repair at my LDS and discovered I suddenly need no weight at all to descend..... Then I noticed I had grabbed an old steel tank~!

 

[W W Meixner]

OK, I want to propose a hypothetical question. If I, at 5'6" 205 pounds, and another diver at 6'7" 205 pounds are wearing the same exact gear, including wetsuit, will approximately the same amount of weight work for both of us? If not, why not?

I'm not trying to prevent the need for a weight check, so you don't need to post "just do an in water weight check". I'm more academically interested in why the tall skinny guy I was diving with in Komodo could not stay submerged with 5 kg or weight, needing at least 6 kg, and I can dive without any extra lead, when we were both wearing new 3 mm wetsuits.

Drk...

Could be a number of factors...an incompletely evacuated BCD...air trapped suit...inexperience...put all three together... the difference is obvious...

W.W...

 

[Angelo Farina]

And also the lung capacity, and how much gas is trapped inside the digestive apparatus (which strongly depend on diet and bacterial activity).
It is really impossible to compare the buoyancy of different humans (just the naked body) without measuring it.
Add to this the equipment, and you see as the task you are attempting is simply impossible.
I am a scientist, I do this as my job (teaching applied physics at the University).
One of the first thing you learn in scientific training is that you can get sensible results only when you control (by measuring or by getting reliable input data) all the variables of a problem. When there is missing info, you cannot estimate anything!

 

[drk5036]

And also the lung capacity, and how much gas is trapped inside the digestive apparatus (which strongly depend on diet and bacterial activity).
It is really impossible to compare the buoyancy of different humans (just the naked body) without measuring it.
Add to this the equipment, and you see as the task you are attempting is simply impossible.
I am a scientist, I do this as my job (teaching applied physics at the University).
One of the first thing you learn in scientific training is that you can get sensible results only when you control (by measuring or by getting reliable input data) all the variables of a problem. When there is missing info, you cannot estimate anything!

I’m also a professor. Which was was I was trying to wrap my head around the variables, and I felt that one really important one towards making a “better” approximation was being overlooked.

I understand what you’re saying about true effective density. But I think height should play a role here. Obviously a “bigger boat” holding the same mass ends up with lower density. It displaces more water.

 

[Ghost95]

But does a tall skinny guy really displace more water than a short rounder guy? Sounds like you need a bathtub with a way to catch the water spilled over the edge.

Eureka!!!

 

[tursiops]

Obviously a “bigger boat” holding the same mass ends up with lower density. It displaces more water.

If i understand what you are saying, you have Archimedes Principle wrong, as I implied in pist #3.

 

[Angelo Farina]

If i understand what you are saying, you have Archimedes Principle wrong, as I implied in pist #3.

Exactly!

 

[drk5036]

Exactly!

okay, could one of you explain what I have wrong?

 

[Angelo Farina]

I understand what you’re saying about true effective density. But I think height should play a role here. Obviously a “bigger boat” holding the same mass ends up with lower density. It displaces more water.

I suppose you mean a "longer boat, more narrow and streamlined", as the two boats have the same weight, so if its longer it must be narrower. In practice, it has a different form factor, expressed, for example, as the ratio between the volume V and the surface S.
Such a form factor affects effectively a number of physical phenomena, for example heat loss. The larger the ratio V/S, the smaller will be the heat loss, as human body produces a given heat per unit volume, but dissipates in proportion to body surface. This explain why "round, fat" humans are less prone to being cold than "skinny, tall" humans.
But when it comes to buoyancy, the V/S ratio has no relevance, as buoyancy is determined by the difference between the weight of the body (which is strictly proportional to its mass: Wb = M*g, where g is the heart's gravity acceleration) and the weight of water displaced, which is strictly proportional to the body's volume V: Ww = V * rho * g, where rho is the water density (typically around 1000-1030 kg/m3).
So what matters is effectively the ratio between the body mass and the body volume. The body's form factor has no effect.

 

[drk5036]

I suppose you mean a "longer boat, more narrow and streamlined", as the two boats have the same weight, so if its longer it must be narrower. In practice, it has a different form factor, expressed, for example, as the ratio between the volume V and the surface S.
Such a form factor affects effectively a number of physical phenomena, for example heat loss. The larger the ratio V/S, the smaller will be the heat loss, as human body produces a given heat per unit volume, but dissipates in proportion to body surface. This explain why "round, fat" humans are less prone to being cold than "skinny, tall" humans.
But when it comes to buoyancy, the V/S ratio has no relevance, as buoyancy is determined by the difference between the weight of the body (which is strictly proportional to its mass: Wb = M*g, where g is the heart's gravity acceleration) and the weight of water displaced, which is strictly proportional to the body's volume V: Ww = V * rho * g, where rho is the water density (typically around 1000-1030 kg/m3).
So what matters is effectively the ratio between the body mass and the body volume. The body's form factor has no effect.

I think maybe I was making a mistake based on the difference between floating and buoyancy.

The point I was trying to make was thus: "Consider a 1-ton block of solid iron. As iron is nearly eight times as dense as water, it displaces only 1/8 ton of water when submerged, which is not enough to keep it afloat. Suppose the same iron block is reshaped into a bowl. It still weighs 1 ton, but when it is put in water, it displaces a greater volume of water than when it was a block." But I also realize (now) that when that bowl finally gets submerged, and water flows above it, it will sink the same as a 1 ton block of iron. So a tall person who ways the same as a small person will FLOAT better than the smaller person, but once under the surface, only density matters, not body shape.