Really stupid question... Why dive thirds with doubles?

[MXGratefulDiver]

and I get the most distance for the least amount of gas when I swim as fast as I can and still control my breathing.

Just curious how that works ... I mean, more exertion leads to more CO2 production ... and hypercapnia is either going to lead to increased breathing or some really bad side-effects ...

... Bob (Grateful Diver)

 

[LiteHedded]

a lot of that is going to depend on your fitness level as well.
and I have heard legends about Marchand's sac rate so I tend to believe him here

 

[ucfdiver]

Just curious how that works ... I mean, more exertion leads to more CO2 production ... and hypercapnia is either going to lead to increased breathing or some really bad side-effects ...

... Bob (Grateful Diver)

Kicking lightly once every 5 minutes is going to get you less distance per unit of gas than an all out sprint. Once we accept that, we know there's some happy medium. I find that a swim rate of about 60-70fpm is the most efficient use of my gas, however for contingency planning I assume a 50fpm exit speed, unless it's high flow such as Manatee or Ginnie, then I'll use 75fpm for the exit.

I've personally seen Marchand make it over 2000ft in Ginnie before coming off of his 2nd stage, swimming both of them into the cave (no setup dive). He's not BS'ing you.

 

[MXGratefulDiver]

Kicking lightly once every 5 minutes is going to get you less distance per unit of gas than an all out sprint. Once we accept that, we know there's some happy medium. I find that a swim rate of about 60-70fpm is the most efficient use of my gas, however for contingency planning I assume a 50fpm exit speed.

I've personally seen Marchand make it over 2000ft in Ginnie before coming off of his 2nd stage, swimming both of them into the cave (no setup dive). He's not BS'ing you.

I agree with that ... just like a car has a "sweet spot" where you'll get the best gas mileage. Slow down or speed up and you get fewer miles-per-gallon.

My cave instructor stressed the need to not dawdle on an emergency exit, and for good reason. What I was wondering about was the "swim as fast as I can" comment. I can swim pretty darn fast when I want to ... but it sends my breathing rate through the roof, and for sure would get me less feet-per-cubic-foot than swimming at a more moderate pace.

... Bob (Grateful Diver)

 

[ucfdiver]

I agree with that ... just like a car has a "sweet spot" where you'll get the best gas mileage. Slow down or speed up and you get fewer miles-per-gallon.

My cave instructor stressed the need to not dawdle on an emergency exit, and for good reason. What I was wondering about was the "swim as fast as I can" comment. I can swim pretty darn fast when I want to ... but it sends my breathing rate through the roof, and for sure would get me less feet-per-cubic-foot than swimming at a more moderate pace.

... Bob (Grateful Diver)

I think he was just saying to swim as fast as you can until you notice that your SAC has increased, and tone it down a notch from there. I can usually tell really easily when my SAC is substantially increasing, and don't see the need to do an exercise to prove that. Realistically, I don't think that the most efficient pace when you're stressed is the same as the most efficient when you're relaxed, so I don't think the drill mentioned is really useful.

 

[boulderjohn]

My cave instructor stressed the need to not dawdle on an emergency exit, ...

And the same guy stressed the need for me to slow down on entrance. No question about it--I was working way too hard.

 

[marchand]

swim as fast as you can while maintaining your normal breathing rate is what I meant to say.

 

[Doc Harry]

A better way to increase BOTH divers safety by providing more gas without the added drag of a pony bottle is to shave a mere 200psi off your backgas turn pressure (in 104s, for this example).

That gives each diver 600 additional psi at the end of the dive. Thats roughly 48 cubic feet each, for a total of nearly 96cuft. Its in a format that can be used independently or shared.

Just 200psi off your turn pressure...

That's what I like to do.

 

[MXGratefulDiver]

And the same guy stressed the need for me to slow down on entrance. No question about it--I was working way too hard.

Probably entering one of those Devil's body parts, eh? They kicked my butt going in ...

... Bob (Grateful Diver)

 

[Doc Harry]

Kicking lightly once every 5 minutes is going to get you less distance per unit of gas than an all out sprint. Once we accept that, we know there's some happy medium. I find that a swim rate of about 60-70fpm is the most efficient use of my gas, however for contingency planning I assume a 50fpm exit speed, unless it's high flow such as Manatee or Ginnie, then I'll use 75fpm for the exit.

I've personally seen Marchand make it over 2000ft in Ginnie before coming off of his 2nd stage, swimming both of them into the cave (no setup dive). He's not BS'ing you.

When I did my distance/air consumption tests, I got the best mileage (feet traveled per cubic foor of gas) by kicking ridiculously slowly. 50% better than my "normal" pace. But I agree with you in principle. Idling in park gets you zero miles per gallon.