I think I know what you talking about but, lets be sure.

You are thinking about using straight 10" al tubing with some kind of end caps for the amma's (outriggers) and maybe the main hull. This can be done but it will be harder than you think.

I looked at this as part of a fun contest, build a catamaran out of a 10"x 20' section of PVC pipe. It can be done but it turned out to be almost as much effort as building one right. The main problem is, a straight circular section hull is fast and too stable, it is very hard to tack.

You could probably build a tri out of a 14' al canoe and 2 10' sections of pipe. You will need to taper the tubes so that they are pointy on one end and taller and fatter in the middle than on the ends. It would help if you put the dagger boards in the ammas

Post a reply if you are still interested and I will tell you what else I know.