It occured to me that it might be a good idea to make snorkel tubes longer to allow people to go further underwater without the need for an air supply in a tank. The design would be something along the lines of having a float on the surface with a rigid plastic tube (about 6 inches long) sticking out of the top into fresh air, below the float would be a flexible rubber tube of some length that would be attached to a snorkel mask in the usual manner. My question is, how long can this tube be and still allow you to draw a breathable gas through the tube? I can see that the problem with having a very long tube would be that when breathe out the air may not leave the tube and you would end up breathing in the same air again, which brings problems concerning the level of carbon dioxide in the mixture. I suppose that if the diver breathed very heavily (particularly when exhaling) then the tube could be made longer but what is the limit? What if we made the tube wider so it held a greater volume of air, would it help?
It’s a question of pressure. Any more than around a metre underwater, the water pressure is too great for your chest muscles to overcome in order to pull your lungs large enough for atmospheric pressure to push air into them, thus requiring pressurised air in a tank to force its way in instead.
Also, the longer the tube, the harder you have to suck to get the air to move.
Ever tried to suck liquid through a very long straw?
Disregarding the really serious problem of pressure that the other’s mentioned, widening the tube might help, because air could actually circulate inside of it, but (again, disregarding the pressure thing), it would be easier just to run two tubes and use one for supply and the other for exhaust, with a valve arrangement, or just use one tube for supply and let the exhaust find its own way back to the surface as bubbles.
Dammit, I know full well there’s not meant to be an apostrophe in others - I have no idea how that got there.
So would we be able to breathe “comfortably” at a depth of about 3 feet? It seems to me that this is enough of an improvement to make it worthwhile as it shouldn’t be expensive to make the units and would allow greater freedom when snorkelling around reefs.
No, I think it would be quite uncomfortable to breathe through an unpressurised supply tube at even that depth; maybe a bit like trying to breath lying down with a couple of kids sitting on your chest. Not impossible, but enough of a pain to outweigh the improvement of snorkelling at a greater depth.
Also, because you would then be away from the surface, the top end of the tube could be inundated by a large wave or by your inadvertently stepping into a bit of a hole on the sea bottom; getting a six-inch snorkel tube full of water is unpleasant, doing the same with a three-foot tube could be dangerous, especially as you’re no longer in a position to gasp for air by simply lifting up your head.
Sucking air, as opposed to liquid, is no big problem as long as the tube is of a reasonable size (say, garden-hose as opposed to straw diameter).
And a double tube could easily deal with the problem of inhaling what you’ve exhaled.
But, as SentientMeat has noted, the pressure-difference issue is going to make this scheme unworkable.
Possibly. But if the tube is freely floating on the surface and the waves aren’t actually breaking, this should be quite rare. When I’m snorkling (with a normal snorkel) on the surface even in significant seas, it pretty much never happens.
Could you use an underwater accordion like device to pressurize the air yourself or would your arms get too tired? 
There is a scheme for doing what the OP is interested in: hookah systems. A low-pressure compressor floating on the surface sends air through hoses down to divers that may be well below the surface.
Wait. I have a brilliant idea!
Why not instead carry the pressurised air in some kind of container; probably some kind of metal tank; perhaps it could be carried on the back, like a rucksack.
This is BRILLIANT! Now, if we could just come up with a catchy name for this device!
Dunno, all I can come up with is Aquatic Respiratory Support Equipment, we need something snappy; an acronym perhaps
Right explanation, but you’ve got damned good chest muscles if you can go down as far as a meter – no matter how you dedfine that. I tried this as a kid, using a carefully-emptied garden hose and our swimming pool, which was four feet deep. I found that as soon as I got any depth at all I was unable to inflate my lungs against the water pressure. I was limited to not much more than the depth of your basic commercial snorkel, meaning I couldn’t even get fully submerged.
Nope – doesn’t work, in my experience.
I too tried this. It won’t work as described. In my experience, just being verticle (with your just under the water) is too deep. 3 feet? No way.
Yeah - pretty much any kid who has ever seen the the ‘breathing through a reed while hiding underwater’ thing in a book or film and then tried to recreate it in a pool/pond/river with a bit of hose or similar has either drowned or come to the conclusion that fiction =/= reality.
Water is really really heavy, and if you have much of it on top of your chest it’s not going to let you suck down any air, no matter what type of air delivery tube you use. Pressurisation is the only way to go.
Oh, man. It was a good thing I wasn’t taking a sip of coffee just then! Yes, I did laugh out loud!
‘How will we breathe underwater, daddy?’
‘You’ll breathe out of your ARSE, son.’
I’m sure you’re right - thinking about it, it would be like being hugged by a very strong grandmother. Is there a world record for this or something?
Besides the pressure thing which is pretty much a deal breaker there is the depth control thing.
I have done a fair amount of skin diving and SCUBA err ARSE diving.
Floating on the surface with a snorkel is easy. Trying to swim just under the surface (say at a steady 3’) is a real bitch. If you were to go just a little too deep, you would flood your snorkel. Now the problem becomes trying to get all that water out of the snorkel. It will be too long to be able to clear it with a forceable blow, so you will have to surface and drain it.