: I'm interested in building my next boat using a central aluminum rectangular
: tube. Any ideas for a source, and recommended size?

hi tom;
i work with metal as my trade and have a couple of suggestions for you.
if you are married to the idea of useing aluminum as your box-beam don't use anything less than 6061-t6, 2"x4"x1/4" wall thickness would be my suggestion for the dimensions. get square edge if you can, it'll be easier to square lines around than round edge would be.
6061-t6 is an alloy/heat treatment designation. it's used in aircraft and while weldable is desinated "no bend". i have worked with it and found it to be less flexable than the "lower" alloys.
as for availability i can't recomend a supplier because i don't know where you are. look in you yellow pages under "metals non-ferrous".
hope this helps.
best regards;
daren......