Your frame width can vary across a pretty wide range. Generally speaking, the further apart the tubes are, the more stable the cataraft will be.
The diameter of the tubes enters into the picture indirectly. The bigger the tube diameter, the longer the oars you will need to reach the water at a manageable angle. For 24" tubes, your frame will be so high above the water that you'll need 10' oars. For 22" tubes, 9.5' oars. For 18-20" tubes, 9' oars.
The length of the oars determines the oarlock spacing. The oarlocks should be spaced apart by roughly 2/3 the length of the oars (this varies by personal preference--I like mine a bit closer together than 2/3). In theory, the oarlocks or pins for your 9' oars should be 72" apart. Use that dimension as a starting place for your frame width.
The other key dimension in determining the width of your frame is the length of your ice chest(s). My Yeti is 42" long. So I wouldn't want a cataraft frame that has any less than 42" between the tubes.
I'd like to mention the benefits of narrowness. too. Been rowing a Jack's Cutthroat that is 12" tube to tube ant it is a kick in the pants! fits in boulder gardens, pivots on a dime, crosses eddy lines without issues. A stable cat is one that is t'd up to shiat and in the right spot on the river. With 12' Legend tubes you have the potential to create a boat that can go places. I would hate for you to ruin that with too wide a frame. I just built up a new frame this winter that is 34" tube to tube and it still has a small feel, but I wouldn't go much wider.
P.S. - no cooler??!?!!? Are you in this sport for the right reasons?
when I need a cooler, I have my 16 avon and dry boxes. this rig will be whitewater day trips 1 person <5 day trips, SF Salmon, Illinois, that kind of thing. I don't haul a cooler on those trips. Which is certainly not to say I'm lacking in the various substances so often stored in the cooler ...
so for a legend, sounds like 28"-36" might be a basic range, narrower also being in play, with the sacrifice of a little side-to-side stability.
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