Yo guys

The snorkel idea sounds way dodgy to me. Somebody hit on the most likely cause of serious trouble: Dead space in the 'long' snorkel. If the snorkel is too long (and it doesn't need to be much more than a foot) you end up inhaling your expired breath which remains in the tube (just like it would as if you were breathing into and from a small paper bag held to your mouth). Your breaths aren't powerfull enough to expell the long snorkel worth of 'air' and tap into the limited kayak supply.

21% oxygen 79% nitrogen =air= breathed in. Metabolism means that we breathe out about 16% oxygen 5% Carbon dioxide and 79%nitrogen. Nitrogen is a pain in the butt and makes divers 'narcosed at depth' and makes divers bent if we stay too long at depth - far better (but more expensive) to reduce the nitrogen percentage with helium). 16% oxygen (at sea level) is the minimum we need for conciousness and that is if we are resting up.

Much less than 16% Oxygen leads to rapid unconciousness.

A sign (in rebreathers) of a build up of CO2 is rapid breathing, but that is moments before you pass out and drown.

Heck you can probably hold your breath for 30 seconds and that is a long time to practice a couple of rolls. Even if you get half a roll you should be able to sneak a quick breath and set up again.

This is exactly what I end up doing. My charming wife reckons I look like I am drowning when I am trying to roll. This takes the edge and glamour off the very few times I have been able to roll.

I had pondered about getting a little scuba tank rig and leaving it in the hull and sucking away while I butchered my rolls. However, you learn fairly quick even if you are teaching yourself, so your don't really need such paraphenalia.

Besides if you start in the shallows: you can cheat and don't have to swim so far when you fail a roll.

Stay safe

Pete