Support ?

What are your guys thoughts on the span between bracing for 1" square tubing (1/8" wall). The braces on the bottom of my boat are set at various spacing (12' to 18") so I thought I would frame those in with 11/4 angle and then put the 1" tubing on top, but I'm not sure about the spans. Thanks in advance.
Scott

12-14 inch on center that will let you use 1/2 plywood if you go to 14-16 inch on center 5/8 plywood, and 16-18 inch on center 3/4 plywood (this is not code but a a quick rule of thumb for floor weight distribution and stress distribution. The best thing to do for your layout is get some painters blue tape and layout the frame in your head down in the boat and see how it looks. Tape is cheap and in the long run it will save you a lot of trouble, time and money.