Why breathe a long hose?

The idea of a long hose came out of penetration diving - caves and wrecks. The basic problem was that, in a cave or wreck, you frequently have very limited room to manoeuvre. It’s not uncommon to be in an area so narrow, you have to go in single-file, one behind the other.
So, imagine you’re in a cave or wreck, and there’s a narrow passageway between you and the outside world. Suddenly, you have a problem, and you’re out of air. You have to leave using your buddy’s air.
How are you going to do that with a conventional octopus?
Answer: You’re going to hold your breath and pray you can get to the other end of the passage in time, ‘cos you sure as hell aren’t going to be able to use your buddy’s octopus while you’re swimming behind him.

Now, imagine that the octopus was on a hose longer than your body. Suddenly, it’s always possible for both divers to breathe off one supply, even when you’re going one behind the other. You have to go slowly and carefully to make sure you don’t get a fin in the face, but that’s fine, you have plenty of air, you can go slow as you need.

And once you’ve found a way of carrying that long-hose with you conveniently, you start to remember all those air-donating drills you practised when you were learning to dive. And you remember how difficult they were, with the two of you inches apart and the reg breathing wet and feeling like it’s about to be yanked out of your mouth by the too-short hose. And you think "Hey, a long hose would make it a lot more comfortable to share air even in open water."

So the only problem is to work out how to carry a two meter length of hose.

Now, let’s back up, and think about your regulator setup.
Way back when, divers only used a single demand valve (DV). If one of them went out-of-air, they used buddy breathing to get back to the surface, with both divers sharing one DV.
Then some bright spark said "Hey, if we carry two DV’s instead, it’s really easy to share air." And people saw the sense of it, and they bought themselves a new DV, and they attached it to their existing setup.
And there was the problem. Instead of re-thinking their configuration to revolve around the new setup, they just bolted a bit on. Everything was kept the same, except for an additional DV.
Was this the best way?

Consider this hypothetical example. Units are arbitrary.
You have two DVs: A high-performance DV that delivers up to 100 units of air per minute for 5 units of effort; and a low-performance DV that delivers up to 50 units of air for 10 units of effort. You usually breathe only 20 units of air per minute.

Now, consider these questions.

1. You are about to go diving. Which DV do you want to breathe during the dive?

During a dive, you want to put as little effort into your breathing as possible. The less effort it is to breathe, the less tired you get, and so the less gas you use up. Therefore, you want to use the high-performance DV.

2. Your buddy is out of air (OOA). He is holding his breath, swimming madly towards you, stressed and panicking. Which DV would he benefit from most?

If you hand your buddy the low-performance DV, you are handing him a DV that offers a limited amount of air, for a lot of effort. If your buddy needs to breathe more than 50units of air per minute, the DV will not supply him with enough air. He will therefore forcibly take the high-performance DV from you, in a desperate attempt to get enough air. Your buddy, therefore, needs, and will end up with, your highest-performing DV, regardless of which you hand him - it is the only one that can give him enough air. You, on the other hand, have not had to hold your breath, are not panicking, and are only mildly stressed. Even if your breathing rate doubles, this can be easily supplied by the lower-performing DV.

You may think, therefore, that since both of you seem to need a high-performance DV, the best thing to do is use two high-performance DVs, for an octopus as well as your main reg.
However, at this point, the ease of breathing itself can cause problems. The easier a DV is to breathe from, the more likely it is to freeflow. This is unavoidable, in the same way that a light door, while being easier to push open than a heavy door, is more likely to be blown open by the wind. The DV you are not breathing from should be low-performance, to avoid problems with freeflows.

So, you need a high and a low performing DV. During a dive, you should be on the highest performance DV. In an OOA situation, your buddy should be on the highest performing DV. Therefore, the conventional setup of short-hosed main DV and long-hosed octopus goes out of the window.
Instead, you have a short-hosed, low-performance DV that only you will breathe from, and a long-hosed, high-performance DV, that you will breathe from normally, but donate to your buddy in an OOA situation.

However, with a conventional octopus hose, this will result in a hose sticking out a long way.

This has potential to snag on things, therefore an alternative routing for the hose must be found. But since an octopus hose has very little length in it to allow for creative routing, the hose must be made even longer, to allow for flexibility.

At this point, your conventional-but-more-thought-out rig requirements meet the less conventional cave-diver’s. Throw your slightly-less-short hose out of the window, and get a truly long hose. This is routed down your back to the right hip, up and across your chest, across the back of your neck, and round into your mouth - This is the Hogarthian method.

Some say "It’s madness!"

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