I have had a 16 ft poly square back canoe for several years. I found it impossible to sit on the factory bench seat and be able to start and tiller a motor that was directly behind me. I mounted a side motor mount that worked but the center of gravity was way off when I put my Merc 4 on the side .I actually carried a barbell weight to compensate. I was always somewhat afraid of the very limited flotation the air pockets in the seats provided. Especially with a 50 lb motor and other gear. So I modified things. I installed a 6 inch motor board on the extreme left side of the transom on the outside . It sticks up 2 inches above the transom so the motor clamps can grab it. The canoe molded corner braces only allowed the motor to be mounted in the center. With this setup the motor is mounted on the rear and all the way to the left. No cavitation problems with the 2 inch lift . Next I removed the center and rear seats. I bought a 54 inch round towable $59 overtons and installed it in the center of the canoe and it is held in place with treated 1x6 and a few pickets. WaLa 850 lbs of flotation. Next I installed a rear 1 x 6 crosspiece to carry a tempress seat w swivel disconnect. That seat I installed as far right as it would go and still swivel to access the motor . The motor being left and seat right balances things nicely while providing a normal seat / tiller orientation . I fashioned the same tempress seat to the one remaining front factory bench. Have to keep the 1 st mate happy. Mods are fantastic and boat is comfortable and safe. See pix . Ps I am 68 live in So Jersey and boat in the tidal rivers along our coast.
