Tips on starting diving doubles

[inquis]

As for your suggested sidemount idea...I just don't know. It feels (emphasis on feels, I have not attempted to sidemount a tank, and would very much defer to more experienced divers on the matter) that manage the relatively loose sidemounted tank in the heavy current would be an added struggle.

You should understand many people use sidemounted cylinders in cave diving where the flow is severe enough to require a rope in some sections. A sidemounted AL40 would be quite innocuous. I would absolutely pick that over independent BM twins.

 

[inquis]

One question, to begin with, if I'm backmounting a single and sidemounting another single, would my 38 lb wing be sufficient for such a setup? Per my understanding, I'd need more lead to keep the AL 80 from getting floaty near the end of the dive.

You don't seem to understand the wing compensates for the *change* in gas weight (and wetsuit if that's being used). Yes, you'd need more lead, because an empty AL80 is a couple pounds positive -- but that lead makes you neutral at the end of the dive at reserve pressure. That's the exact same point you should be at at *any* dive.

A 30 lb wing would work just fine with BM single and slung (or sidemounted) AL80. (That's about 5 lbs per cylinder.) Honestly though, I'd say another 80 is overkill for your intended river dives.

 

[John C. Ratliff]

Cool rig. Noticed the quick release box at the chest. Reminds me of the Irvin GQ PX rig I initially trained on (static line, riser steered). Did you have steering toggles, or did you use the risers?

The S-10 parachute canopy used an oval hole in the rear, which was distorted using slip risers. Initially, when we first jumped, there were pins on each side that kept the risers from slipping. We pulled the pins (they were tethered) and could turn by pulling the front right riser down, and the back left riser down, distorting the air shooting out of the oval hole in the canopy to turn right. If you’ll look at this photo below, you can see that the jumper is holding the riser on the right with a slight asymetry in that riser group, indicating that he was turning toward the life raft target. This gave us about a 5 mph forward speed, unless we pulled a front slip, which brought it up to about 7 mph. However, if we wished, we could pull a full rear slip and actually collapse the canopy. I did this once on a parascuba jjump, and dropped about 50 feet before opening up again by righting the risers, and was able to better line up on the life raft target. But you only wanted to do this with a rear slip, as a front slip could endanger the jump by having the front go through that rear hole and not open again.

Parascuba Jump-Okinawa-1 by John Ratliff, on Flickr

Our later parachute, the S-17, had a 7TU cut in the back. It was steered with toggles that closed off the outside of the cut.

NCSB003 by John Ratliff, on Flickr

This above photo shows the S-17 parachute canopy with the 7TU cuts. (This is actually a photo I took as a smokejumper at the North Cascades Smokejumper Base (NCSB) in 1972 of a tree jump…long story that I’ve written up in my memoir.). Note the difference in the amount of air released backwards, giving us about a 15 mph forward speed. But the jumper could not collapse the canopy like with the S-10 with this setup; maybe that was good for jump safety though. I had been jumping this parachute for several years, but 1972 was the first year NCSB had jumped the ‘chute.

SeaRat

 

[John C. Ratliff]

@SelfDiver This is a valid set of points I had not considered. As you've said, in the event I have a freeflow going on behind my head with my current dive conditions, I'd put even money that I could be at the surface before the tank ran dry, but your point about reg freezing is a good consideration. Something to chew on, as far as failure scenarios go, and something to plan for as I design my current setup.

As for your suggested sidemount idea...I just don't know. It feels (emphasis on feels, I have not attempted to sidemount a tank, and would very much defer to more experienced divers on the matter) that manage the relatively loose sidemounted tank in the heavy current would be an added struggle. Today, for example, the current was pushing us hard enough I often had to press flat against the riverbed and fin like crazy, plus digging in with my screwdrivers just to advance forward. I worry an added AL 80 would represent an encumbrance that I simply would not be able to easily manage in such a situation, though I acknowledge I might be wrong about that fact.

I like backmount, generally speaking, because tanks in backmount are in a known position barring some extreme failure. If I'm on the bottom, and I encounter a logjam or other obstacle, like I often do, I can more easily move past it because I know that my major snag points are on my back (the tank valves and regs) and running back and to the sides (my HP and LP hoses). Sidemount I feel like adds extra variability to that equation, something else I have to manage for, while also dealing with current. Again, this may well be far less of an issue that I feel like it would be, as I've never done sidemount before, but it definitely feels like more of an acquired skill to manage.

If you have some tips on trying it, I'd certainly be down to mount an AL 80 on the side and take it out in the nearby lake for a low-risk test, then work up to more challenging dive conditions with it. One question, to begin with, if I'm backmounting a single and sidemounting another single, would my 38 lb wing be sufficient for such a setup? Per my understanding, I'd need more lead to keep the AL 80 from getting floaty near the end of the dive. I also have a pair of LP 72's, if those would be preferred for such a setup.

A Free flow from a valve O-ring will not produce a frozen regulator. The regulator will continue to function normally until the tank runs down to about 250 psi, then will get hard to breath. The most likely failure point is the valve’s O-ring between the parts of the valve, and not at the tank. The tank/valve interface has an enclosed, or “captured” O-ring and if it’s going to fail, it is most probable that this will occur upon pressurization and be due to the valve not being screwed all the way into the tank.

But a freeflow from a regulator second stage will probably freeze up the regulator, either at the first stage or the second stage, or possibly both, depending upon the temperature of the surrounding water too. A free flow from a burst O-ring on the piston of a first stage will definately freeze up the first stage. (Diaphragm first stages are almost immune from this failure.)

SeaRat

 

[old frogman]

The S-10 parachute canopy used an oval hole in the rear, which was distorted using slip risers. Initially, when we first jumped, there were pins on each side that kept the risers from slipping. We pulled the pins (they were tethered) and could turn by pulling the front right riser down, and the back left riser down, distorting the air shooting out of the oval hole in the canopy to turn right. If you’ll look at this photo below, you can see that the jumper is holding the riser on the right with a slight asymetry in that riser group, indicating that he was turning toward the life raft target. This gave us about a 5 mph forward speed, unless we pulled a front slip, which brought it up to about 7 mph. However, if we wished, we could pull a full rear slip and actually collapse the canopy. I did this once on a parascuba jjump, and dropped about 50 feet before opening up again by righting the risers, and was able to better line up on the life raft target. But you only wanted to do this with a rear slip, as a front slip could endanger the jump by having the front go through that rear hole and not open again.

Parascuba Jump-Okinawa-1 by John Ratliff, on Flickr

Our later parachute, the S-17, had a 7TU cut in the back. It was steered with toggles that closed off the outside of the cut.

NCSB003 by John Ratliff, on Flickr

This above photo shows the S-17 parachute canopy with the 7TU cuts. (This is actually a photo I took as a smokejumper at the North Cascades Smokejumper Base (NCSB) in 1972 of a tree jump…long story that I’ve written up in my memoir.). Note the difference in the amount of air released backwards, giving us about a 15 mph forward speed. But the jumper could not collapse the canopy like with the S-10 with this setup; maybe that was good for jump safety though. I had been jumping this parachute for several years, but 1972 was the first year NCSB had jumped the ‘chute.

SeaRat

Super cool stuff. Did you ride the chute into the water and then climb out of the harness, or did you hit the QRB at very low height above the water in prep for sliding out of the harness on water entry?

I am not sure if I can see capewell cutaways in the photograph. Were they fitted and occasionally used i.e. strong wind water entries?

 

[Celt]

My independent heavy steel double 7ltr,s with cross bar attached. Pillar valves make them easy to shut down but it’s not necessary as a leaking tank can simply be used till empty and then switch. Thinking about the OP,s diving I can’t see any reason to use them other than sport. Working in a fast river with poor visibility I would be looking for a very streamlined minimalist rig. A single steel tank on a back pack, no bcd, in a well fitting wetsuit. Ditchable weight on a belt with small powerful fins. The system diving with 2 divers I would use, is one in and the other tethered standby on the bank or in a moored tender. The 2 most important things I would consider for the diver in the water, is clean and heavy.

 

[SelfDiver]

@SelfDiver This is a valid set of points I had not considered. As you've said, in the event I have a freeflow going on behind my head with my current dive conditions, I'd put even money that I could be at the surface before the tank ran dry, but your point about reg freezing is a good consideration. Something to chew on, as far as failure scenarios go, and something to plan for as I design my current setup.

As for your suggested sidemount idea...I just don't know. It feels (emphasis on feels, I have not attempted to sidemount a tank, and would very much defer to more experienced divers on the matter) that manage the relatively loose sidemounted tank in the heavy current would be an added struggle. Today, for example, the current was pushing us hard enough I often had to press flat against the riverbed and fin like crazy, plus digging in with my screwdrivers just to advance forward. I worry an added AL 80 would represent an encumbrance that I simply would not be able to easily manage in such a situation, though I acknowledge I might be wrong about that fact.

I like backmount, generally speaking, because tanks in backmount are in a known position barring some extreme failure. If I'm on the bottom, and I encounter a logjam or other obstacle, like I often do, I can more easily move past it because I know that my major snag points are on my back (the tank valves and regs) and running back and to the sides (my HP and LP hoses). Sidemount I feel like adds extra variability to that equation, something else I have to manage for, while also dealing with current. Again, this may well be far less of an issue that I feel like it would be, as I've never done sidemount before, but it definitely feels like more of an acquired skill to manage.

If you have some tips on trying it, I'd certainly be down to mount an AL 80 on the side and take it out in the nearby lake for a low-risk test, then work up to more challenging dive conditions with it. One question, to begin with, if I'm backmounting a single and sidemounting another single, would my 38 lb wing be sufficient for such a setup? Per my understanding, I'd need more lead to keep the AL 80 from getting floaty near the end of the dive. I also have a pair of LP 72's, if those would be preferred for such a setup.

@SouthernSharktoothDiver
As I've said before: I will be happy to walk you through different configurations.
On my scuba skills camps, I show people how to sling an AL80
It is not hard and the tank sits behind your shoulder like I have my AL80
https://scubaboard.com/community/attachments/deco1-png.819668/
Don't take my word for it: I'm just an idiot on the internet

 

[halocline]

@grantmac I'm scratching my head why I would cut off a tank in the event of gas failures in independent doubles. Sure, it might be nice to keep water from getting into the tank, but in the unlikely event of gas failure behind the reg, either my valve, tank neck, or regulator first stage is busted, and all three of those necessitate a trip to the dive shop for repairs in any case.

The reason you close the tank valve with a gas hemorrhage is to prevent all the gas in that tank from leaking out. The majority of leaks are due to regulator failure, not tank valve. And the reason that divers use doubles 99% of the time is because they are doing a dive that requires the extra gas and/or redundancy.

In BM doubles with a manifold, when a regulator starts leaking badly, you simply close the valve and all the gas in that tank is available through the other regulator. In side mount independent doubles, you have easy access to the valves and can feather them to save the gas in the event of a regulator leak. With BM independent, you don’t have either of those advantages. That’s one reason why tech divers in this century basically never use BM independent doubles. This is really basic stuff that you get taught at the beginning of any tech diving training.

BTW, someone posted something about a BM single tank and a side mount extra tank. That is also basically unheard of as a set up, for lots of other reasons. What that person probably meant was to use a single BM tank and sling an extra tank. That’s quite a bit different than a side mount setup, which requires a specialized harness. Slinging a tank is basically like carrying a stage; typically it’s attached to your chest D-ring and hip D ring with leashes, and the tank rides on one side (most often the left) of your chest area. You might start trying that with a small bailout bottle like a 19, 30, or even 40 cft tank. You’ll find it to be pretty manageable.

 

[inquis]

What that person probably meant was to use a single BM tank and sling an extra tank. That’s quite a bit different than a side mount setup, which requires a specialized harness.

Actually, it just requires a length of bungee from one of the holes in the edge of a normal backplate. I do that with my backmounted CCR BO for better streamlining, less chest clutter, and zero movement when kicking.

The tank has the standard stage rigging for getting to the water, but it's trivial to unclip from the chest & bungee it once in the water.