Is it the weight or something else?

[Mudd]

I have heard it mentioned casually and on the forums that being over-weighted causes divers to burn through their air at much quicker rates. At first I just accepted, but after some thought I wonder is it causation or correlation?

Thinking through it in my mind, someone overweights themselves and goes diving... they sink easily. They then take an air hit by having to put more air in their BCD to attain buoyancy at depth. But after that point in time, does the additional air compensate for the weight so that (should they practice appropriate skills and relaxation) no additional air be burned through over the course of the dive? Yes on dry land that additional weight is a pain, but in the water doesn't the neytral buoyancy negate the need to drag yourself around?

The diver returns to the surface with what should be a safe margin of gas so that the inflation of his or her BCD is a non-issue.

The diver did take that air hit to attain neutral buoyancy... but I am thinking this is a small excess compared to the overall dive...

So is it something in the over-weighting that I have missed that causes the over-use, or is it that instead divers who tend to overweight dramatically for a given situation also do things that burn through their air quicker. We as humans tend to blame the easiest culprit: too much weight instead of not enough skills / practice / etc.

Thoughts on this? BTW please do not use the example of someone diving with an engine block strapped to their BCD.

 

[dfx]

Inertia. More weight (more precisely: more mass) requires more force/effort to be moved.

 

[chrpai]

It's not just mass and inertia. The extra weight means more air in your BCD to off set it. This typically puts the diver in a poor trim position which increases drag. Drag kills inertia. In a worst case the diver is actually in such bad trim that he's actually negative buoyant when he thinks he's neutral. He's literally using thrust to maintain depth which is even more energy lost.

Other factors including being relaxed and water temp / proper exposure protection.

 

[lowviz]

posts clashed...

 

[rhwestfall]

add to it a possible struggling to attain neutral bouyancy (or position as stated above) may result in finning and or hand swimming, thus more air consumed in the active muscles...

 

[nimoh]

all good points. To a lesser extent the extra gas in your BC increases your volume (even if you have good trim) and it takes more energy to move through the water due to drag.

As I said, "to a lesser extent", since normally the bad trim is much more of an issue for drag.

 

[TSandM]

There is a minor increase in resistance from having air in the BC, and a minor increase in mass you have to move (but when you and gear weigh 200 lbs, 10 lbs more or less isn't going to make a difference). But honestly, technical divers dive massively "overweighted" all the time, because of all the gas they take underwater with them, and they don't burn through that gas as a result.

However, with overweighted NEW divers, there are several problems. One is that buoyancy is difficult for them, so they are up and down, constantly venting and refilling the BC AND, if they know how, using their breathing to correct buoyancy issues, and that is VERY inefficient use of breathing gas. Another is that that overweighting is often hung low on the BC or body, resulting in a head up/feet down swimming position. In that case, every kick drives the diver UPWARD in the water column, and as a result, he has to stay negative to stay at the same depth. This means all that kicking effort is being spent . . . going NOWHERE!

The final issue is that, with a huge amount of air in the bladder, any shift from side to side results in a lot of instability, and most divers have to correct that with hand-waving as well as kicking, and that's a lot of air usage.

An "overweighted" diver who is horizontal and stable and has good buoyancy control will use little, if any more gas than someone who is correctly weighted.

 

[Shasta_man]

The increased gas use is due mostly to having to keep screwing around to get the right buoyancy. You don't just go to depth, shoot some air and that's it. Being overweighted, you get the yo-yo going with you initially unable to maintain a neutral buoyancy (not up or down), so you add air. Then maybe you go to depth and now you add another shot of air to offset greater depth and a little suit squeeze. You are still falling when you stop moving so you add more air. You then might get a little shallower and start feeling light, so you dump a bit of air. However, now too much, so you have to add a little air back in because you are falling again. Back and forth trying to get the right balance. Then because you aren't well weighted it's not so easy to move. Compare that to a well weighted dive I would do where I jump in, go to depth, and add one blast of air. I might need one adjustment after that. Then usually nothing until I go up. I dump all my air in my BC and hover at the safety stop, and then very slowly go to the surface. Much less air use. An experienced diver can minimize the air use but it's still difficult to get it right and thus use more air, besides the increased stress fighting this problem throughout the dive.

Hmmm...should have just waited a few minutes and I could have just said "What TSandM said"

 

[GrumpyOldGuy]

Hmmm...should have just waited a few minutes and I could have just said "What TSandM said"

That is exactly what I was going to say, copy cat