OK, I have put on my flack jacket for this one!!

I have attached a rather basic drawing of a catamaran I am attempting to build with my kids.

Not being an engineer, I could do with some advice on the size of box section aluminium tubing I could get away with as I am keen to keep weight down to a sensible minimum.

The cross beams will be about 5'6" across, the overall boat length is 12' and I plan to use approximately a 10square metre windsurf rig. This is really meant to be a single hander, or one adult plus one child.

Looking at the dimensions I was thinking I could use 3' box section with a 3mm wall thickness. The smaller sections at the end of each cross beam would be 2.5' box section (these are the pieces that fit into the beams running along the length of the hull.)

I am wondering however if I could get away with 2.5' box section for the main beams?

Any suggestions or advice on any other aspect of my project would be appreciated. (yes, very funny, I meant constructive advice )

Thanks
Andy