Johnny,

Here is what I know about props and what size to use with a certain size engine: Nothing, zip, zero!
I just started buying props and trying them until one of them seemed to work the best. The one on my boat is an 18 x 10 (which is the biggest/highest pitch glass prop I could find) . and the engine has no problem turning it over.

My best guess for prop size for a 30 cc engine would be a HMMMMM.................16 x 7 ???just a guess though.

What I have noticed is that a chainsaw engine has LOTS more power than a weed wacker engine, I dont know if it has something to do with the carb or if they just have larger displacement and longer stroke. But the have more power.. and more power is GOOD

As for hull material.....i use, Acrylonitrile Butadiene Styrene, (ABS) plastic, 1/8 " thick.

See the forum titled "prop walk with my airboat" (PAGE 2)[link]http://www.rcuniverse.com/forum/m_3524121/mpage_2/key_/tm.htm[/link] that should give you some Idea if you really want to build with plastic.

I seem to be the odd duck whereas building material is concerned, these other guys have made some real nice looking boats from wood, look around the forums, LOTS of high quality workmanship out there. If you decide to build with wood, these are the guys to ask.