I have two converted Ryobi engines and have always wanted to build a hovercraft. Anyone have an idea as to how big, and heavy it could be? I would like to use both engines, one for lift and the other for propusion. Anybody seen any plans any where?

MANY years ago you could buy plans for a hovercraft from Herb Bartlett in Arizona...
Might be worth a search....
I don't think a Ryobi powered hovercraft would be able to get off the ground..

It can get off the ground.

You need to make an estimate of the final weight of the craft, say, 3~4 times 1 engine weight for a rough estimate.
The you will have to calculate the area of the hull that gives you a small pressure (weight per square area unit) - better ask this on the hovercraft forum.
For the lifter engine, use a 4 blade prop. If you use for example, a 14x6 2-bladed prop for normal running, use a 12x7~12x8 (or 11x8 for more rpm). Multi-bladed props are great for making pressure - that's what you need for the lift, a compressor-like engine.

Good luck!

stgrlee,
You may want to run them on gas/glow to save weight. This would save a pound for each engine. The typical mag ignition Ryobi weighs about four pounds.

Dave