DPV training books

[DA Aquamaster]

Depending of course on when you lose the scooter and or if you lose gas too makes the "math" vastly different, so I have so far been diving thirds disregarding the scooter, that is I go no further than I could swim out of on 1/3 of my starting gas, in other words doing it this way means the scooter doesn't get you any further than swimming will, so it's an awfully conservative approach and you don't really get much out of having a scooter except being lazy.
I have a back up scooter that I haven't really used yet, and it has been brought up to me that a back up scoot only solves one problem, where extra gas may solve many problems, of course having both solves a lot of problems, but is there much if any advantage with driving around with four bottles attached to you as opposed to dropping stages?

A single stage breathed to 1/2 plus 200 will take you a long way in a 50'-100' deep cave and two gets you far enough in that you'll want a very healthy reserve. In that regard, I'd argue it makes sense to only use the stages and keep all the back gas in reserve.

Assuming a diver scooters at 150 fpm and swims at 50 fpm, it's obvious they'll need at least three times the time to exit via fin power. What is slightly less obvious is that the swimming SAC is going to be quite a bit higher than the scootering SAC, so you'll need a lot more than 3 times the penetration gas to safely exit. Assuming you use *only* 66 cu ft of penetration gas from 2 stages, and have a scooter SAC of .4 compared to a swimming SAC of .6, you're looking at around 300 cu ft of gas needed to exit if the scooter fails at max penetration in a no flow cave.

If you consider the gas in a pair of 130s as well as 80 cu ft of gas in the two half empty stages, you've only got 340 cu ft in reserve and you'll need 300 cu ft to swim out even if no other failures or delays occur. That's also assuming you don't rack up a significant deco obligation on the 3 times longer exit.

Consequently, it might make sense to drop the first stage (or not depending on the depth, flow, scooter, cave, etc) but the second one will stay on you as you'll turn the dive when you hit 1700 psi on it, as you probably don't want to touch the back gas on a two stage scooter dive.

Add a faster scooter in the 200-250 fpm range and the gas requirements for exit go up yet again as you are now swimming at only 1/4th to 1/5th the scooter speed and the reserve needed to exit becomes 400 and 500 cu ft respectively.

Where it gets even more complicated is when you scooter into the cave and then swim farther past the point you drop the scooters. You then have to figure the impact on your reserve gas as well as the rock bottom needed to swim out in the event of a scooter failure plus the potential deco obligation.

[a64pilot]

While I don't scooter that fast, I don't think my Magnus would drag me and four bottles that fast, I can see the prudent thing seems to be to carry two 130's and two stages, using only the stages.
Two 80's exiting the cave W/500 ea. at a scooter SAC rate is as long as I want to stay in the water anyway. I'm a wimp, I get cold after a couple of hours.

So,
When do you bring the back-up scooter?

[a64pilot]

I would say this-- a stage at the first T will largely go unused even during an emergency situation as even with an elevated SAC you're using 700-900psi from that point. If it was further back (closer to max penetration) you'd have the opportunity to use all of the gas if you needed it. In this case the full stage won't really be of any more help to you than a half empty stage dropped there because you used it on the way in.

Your right, I had thought first T might be a little early, but was the thought was, how many times have you read about the guy that ran out of gas, close to getting out? The thinking was that the point of max penetration wouldn't be much if any further than the stoplight, drop a bottle there and I know I could swim from the stoplight to the first T. All of this is my thinking out loud so to speak.

[jj1987]

Your right, I had thought first T might be a little early, but was the thought was, how many times have you read about the guy that ran out of gas, close to getting out? The thinking was that the point of max penetration wouldn't be much if any further than the stoplight, drop a bottle there and I know I could swim from the stoplight to the first T. All of this is my thinking out loud so to speak.

Fyvie and Berman each died close to their next stage bottle. As far as running out of gas near the entrance, that's got nothing to do with placement of gas-- that's just not taking enough.

[Tegg]

So,
When do you bring the back-up scooter?

When you've had a scooter failure that made you swim one time. (once is all it takes)

Anything past about 3-3.5K ft in <=150ft is when I make sure to have a good buddy or a backup dpv.

[PfcAJ]

While I don't scooter that fast, I don't think my Magnus would drag me and four bottles that fast, I can see the prudent thing seems to be to carry two 130's and two stages, using only the stages.
Two 80's exiting the cave W/500 ea. at a scooter SAC rate is as long as I want to stay in the water anyway. I'm a wimp, I get cold after a couple of hours.

So,
When do you bring the back-up scooter?

How fast do you think you're going? While I've never rode a magnus, I can get ~150/min with my Gavins.

[a64pilot]

When you've had a scooter failure that made you swim one time. (once is all it takes)

Anything past about 3-3.5K ft in <=150ft is when I make sure to have a good buddy or a backup dpv.

I have, it was a bad battery cell, but apparently that is all it takes. Luckily it was one of the few times I was with people and other scooters so it wasn't nearly the hassle it could have been.
Actually if you clip it off behind you, you almost can't even tell it's there swimming, as long as it's slightly positive that is.

[a64pilot]

How fast do you think you're going? While I've never rode a magnus, I can get ~150/min with my Gavins.

I almost never use speed three, it's for ripping flow in my opinion like JB used to be right before the first T, and I usually use pitch 6, so I think me with a dry suit, two 130's and two stages probably won't be moving much more than 100 FPM on speed two, it's a nice easy comfortable speed, I usually come out on speed one, I'm in no hurry and not trying to see how far I can go, rather trying to see everything.
I've spent whole swim dives never having gotten past first breakdown

[LiteHedded]

I have, it was a bad battery cell, but apparently that is all it takes. Luckily it was one of the few times I was with people and other scooters so it wasn't nearly the hassle it could have been.
Actually if you clip it off behind you, you almost can't even tell it's there swimming, as long as it's slightly positive that is.

that must be nice

[RN]

OK, I'm a simple kind of tourist diver, no exploration, nothing exciting, nothing deep as of yet, The JB kind of cave, actually usually JB, for some reason I really like JB and I know that most of it I haven't seen, and by that I mean the first 1,000 ft. or so. I could spend a long time just looking around JB.
Remember all of this is purely hypothetical, pure theory, just in the thinking stage at this point.
Why not drop a stage at say the first Y and another at the stoplight as insurance? Plan your dive as if they aren't there so that your exiting the cave with min. 1/3 of your back gas remaining. Do the surface interval / lunch thing and "clean up" your stages at the end of the second dive or maybe the next day if it's a weekend diving, never having breathed off of them except to check the regs. These stages are outside of any gas plan if that makes sense, they are there, just you don't count them in any plan
Swimming out of JB isn't that bad, when the flow is up, I often basically just "drift" out kicking a little every now and again, I'm in no hurry.
I guess what I'm asking is why not carry a pretty big reserve with you, but also stage some pretty big reserves along the way?
Or, is dragging a back-up scooter a better plan?
This hypothetical dive is a solo dive BTW.

What you're talking about here are what most people call safeties, not stages. A stage is typically thought of as a cylinder you count in your gas plan and breathe from during the dive. A safety is a cylinder you place in the system because you're planning a big dive and want the added "safety" in case something goes wrong. But you still don't count that in the gas plan. It's just there as an added buffer.

Even with a backup scooter I still plan on having enough gas to swim out and decompress with a scooter failure. I borrowed a scooter for backup once and when I got back to the cavern I was going to take it for a quick spin. When I hit the trigger all the props flew off. Someone had put them back on incorrectly. Fortunately I didn't need that scooter during the dive. This wouldn't likely happen to me if it was my own scooter but, unfortunately, I only have one scooter (the Makos don't count) so any backup scooter right now would have to be borrowed.

Oh, and for solo scooter dives you have to gas plan very conservatively. I've done quite a few solo scooter/swim dives in the 3000-4000' penetration range and I always come out with tons of gas left.