Air Bag

I wouldn't bother with an inflating one, Crewsaver make a 40L float you can hoist with the main and doesn't cost you £20 every time you capsize. Probably considerably cheaper too. Another alternative is some old 5L bottles tied to a bit of rope and again hoisted with the main. In my experience 40L of buoyancy at the masthead will prevent inversion even with someone stood on the mast. One question though: is that a good thing? In any breeze the boat will go downwind far faster than you can swim. The speed they move on their side is suprising. If you decide to go down the route of adding masthead buoyancy I hope there's good rescue cover wherever you're sailing cos if you're no longer with the boat you won't reach it without the aid of a rib.