I'm deisgning/building new rowing frames for my raft and new cat. I want to get the oarlocks farther out that what's on my 60" NRS frame, so I was thinking double rail would give me an extra few inches each way. I'm using aluminum and planning to use diamond plate for walk-ways along the top or the tubes(raft frame only, I think). I think 4.5 inches wide would be enough and 6" would seem like a sidewalk...I think.
So my questions are;  
How much space is typically between the two tubes of a twin rail frame?
and
Is there any reason NOT to have the oar towers extend out beyond the sides of the tubes? Obviously not sticking out a lot!