I thought I could set up my Annapolis Wherry Tandem with fixed seat and short 6" outriggers made of 3/8" flat aluminum stock.  I set the seat up about where the center of the boat was and maybe a foot aft.  Foot braces were a bit of a problem since the most comfortable position was with my legs over the last frame, and digging into the back of my calves.  Had to reposition the seat as a result. 

It really didn't matter.  Even 3/8" flat aluminum bolted to the rail, and reinforced with back plates was no match for the torque of the oars...or maybe it was just me being too strong a rower. (Blackburn Challenge winner in home built wherry 1999 & 2000)

Either way, within 5 or 6 strokes, the aluminum bent and twisted so that no rowing could be done. Thank goodness I brought a paddle 'just in case'.

Think stainless if you do a fixed system.  And try to keep your butt in the center of the boat. Too far forward (as was my position after I installed the Row Wing) caused bobbing, which slowed me down considerably.

Hope that helps.

john Zeigler