It's not just the diameter, it's also the lamination schedule. With the right schedule and proper reinforcements, 2 inches diameter is plenty for the main mast and 1" for the sprit is fine.

Here's a link to my webpage where I describe a 10-foot, 2-inch diameter mast for my 12-foot sailing dinghy.

The mast was made from 640Ksi uniaxial cf and 2 layers of 13 oz biaxial glass, weighed 4 lbs and lasted a bit over a year. Due to a measuring fault on my part the internal reinforcements were above the mast partner and the stress on on the unsupported CF broke the mast. It's actually amazing that it lasted as long as it did. So measure carefully if you're building your own.

Have fun,

Laszlo

My thoughts are something a little more simple. I want to buy carbon tubing to make into a mast. I plan on buying 1 7/8" tubing for the mast, and insert a smaller piece at the base for added support. Then wrap the base in leather for abrasion resistance.  Also wrap the area the spars come in contact with in leather.  I may make the mast and spar two peice for easier storage.  I am looking at this place to purchase the tubing from, http://www.carbonfibertubeshop.com/large%20tubing.html

Thanks

Have you considered sailboard masts, which often are almost 100% carbon and tapered to boot?

Unfortunatly, I already ordered the carbon fiber parts, but I really like the idea of using a sailboard mast. If the parts I ordered dont' work, or worse yet, it snaps, then I will do the sailboard mast. Thanks for the great input.