Overweighting - Impact on SAC?

[miketsp]

I've just been reading the thread on "are you a hoover?" and there are at least 10 posts that imply that overweighting automatically increases SAC.

I find this a little strange since in general it hasn't been my experience.
It just means some extra air in the BC and some more care in the shallow part of the dive, but unless I'm fighting a current where the extra BC volume is significant I've never seen any SAC alteration as calculated and logged by my computer.

I checked back in my log to a couple of dives where I recovered weights or even a complete weight belt near the beginning of the dive and then carried it with me throughout the dive. No change.

Last week, I did 11 dives which varied from 1kg underweighted to about 3kg overweighted - every dive was done with a different weight belt and there was no way to know the weight of each individual piece as they were from varying materials.
The only dive where my SAC altered was the one where I was underweighted and had to swim downwards for 45 mins.

 

[jlyle]

You will see many new divers who are overweighted swimming vertically instead of horizontally - they will be constantly kicking, yoyoing in the water column and fiddling with their low pressure inflators. All of this action increases air consumption rate. Of course if you are sitting on the bottom it doesn't make any difference.

 

[miketsp]

You will see many new divers who are overweighted swimming vertically instead of horizontally - they will be constantly kicking, yoyoing in the water column and fiddling with their low pressure inflators. All of this action increases air consumption rate. Of course if you are sitting on the bottom it doesn't make any difference.

I agree that it makes it more difficult to control as the larger air volume in the BC is more sensitive to pressure changes.
The point I want to make is that an experienced diver with good trim and buoyancy control is not really affected to any significant degree by overweighting.

 

[jonnythan]

It will mean more adding and removing air from the BC. If you're wearing an extra 8 pounds, that's an extra gallon of air in the BC at depth. That gallon of air will expand as you ascend a few feet, and force you to dump air sooner than if you didn't have that gallon of air.. and force you to add gas faster as you descend. It will also, as you noted, make the entire diver system more sensitive to depth changes and probably cause more task loading since you have to focus that much more energy on maintaining your buoyancy. Of course this will increase your SAC

 

[jonnythan]

1 kg underweighted, and had to swim down for 45 minutes.... that's an impressive SAC, to only use about 30 cf of air on a 45 minute dive

 

[The Kraken]

Hey, John, where did the 30 cf come from?

 

[jonnythan]

30 cf of gas is about 1 kg, roughly, I think.. if you were underweighted by 1 kg, you'd get positive when you have about 30 cf of gas left.

 

[The Kraken]

OK, sorry, just didn't make the correlation. Brain fart, old age, whatever . . . .

 

[scubanimal]

Having a few pounds of extra weight can be minor or major depending on where you put it. If the weight is properly placed and does not affect your attitude in the water column then the extra displacement and resulting drag is not significant. However, many new drivers place the extra weight on their belts or the dump pockets of their BCs. This can lead to inproper trim attitude for the conditions, thereby requiring extra effort move through the water. Of course any up-current work is increased even more. Like MikeTSP said, the most noticable effect of overweighting is the increased sensitivity in bouyancy control. While a good diver can cruise along a couple feet from the bottom just using breath control to ascent and descend, an overweighted diver is going to find it much harder to be as smooth. If you are into photography this can cause you to lose shots as adjusting your BC to stay off the reef which can frighten some critters.

Next time you're diving with a group, watch for the divers that seem to effortlessly cruise along, never touching the bottom, and seemingly hardly ever having to adjust the BC. You'll find that they use the minimum weight and have a very light touch on the BC controls.

 

[Uncle Pug]

The point I want to make is that an experienced diver with good trim and buoyancy control is not really affected to any significant degree by overweighting.

Mike, your point is the exception to the rule. There are always exceptions that can be pointed out... and you are correct in the observation you make above. However when dealing with n00bs wanting to improve their consumption rate, especially in cold water diving where the amount of weight carried is significantly more that in warm water, proper weighting can make a huge difference.