Hey, Michael - I have the outboard version of the Seafari, so I am not sure if his I/O tank has the same measurements, but here are my tank's diagram that was derived from Bushwacker's factory standard measurements for a 1972. The tank was sourced from Patriot Marine tanks, Forked River, NJ. Capacity is about 35 or so gallons.

Very cool Sir, had not seen that one before. The one removed from the Blank Canvas was a max/mismeasured custom rectangular tank requiring a Cob Job Installation. Wasn’t pretty. Have not done fuel calculation for gallon total. Hope it helps OP.

Michael

my fuel tank has the fittings on top of the tank

Two facts you need to know about Seafari tanks:
1. The I/O tanks are smaller because the engine takes up space that’s available in the OB models.
2. Potter changed the inner liner in about ‘75 or ‘76. He raised the deck a couple inches, moved the windshield forward so the tip of the V is only about 6” aft of the hatch vs. about 18” on the earlier models. He also made the step down smaller, just big enough to open the door, and that made room for a ~50 gallon tank. The tank in my ‘72 model was about 35 gallons, and I think the I/0’s were something like 29 gallons.

The sketch Sailorchlud shows is correct. The top taper at the back and bottom taper at the front are necessary for clearance when sliding the tank in through the hatch from the back.

One thing I had considered if I ever had to replace the tank was to have TWO smaller rectangular tanks made. They could have been made taller and no need for taper, so I’m sure you could stuff more fuel in the same space that way. Also, with the much lower fuel consumption of modern engines a big 50 gallon tank is often not needed, depending on how you use the boat. Plus it gives you the option to leave one tank empty and not have a bunch of old fuel in the boat. There is room to run the filler hose and vent line to the port side gunnel for the second tank. I would locate the vent port at the front of the second tank like the original but move it to the side opposite of the fuel filler port . . . Rationale is that if you’re filling tank at a dock and you’re on the port side, boat will heel to port and starboard side of tank will be higher, so that’s where you want the vent!

Great info. Thanks. I’ll take it into consideration when planning.

Took some measurements over the weekend of my fuel tank area. Opening is 22 inches wide x 37 inches long and about 9.5 inches deep. RDS has a stock tank that is 18 gallons that measures 36.5x16 3/8x 7. Would be cheaper than going custom. Looking at powering with a 140 Suzuki when the time comes. Should I go stock or custom.

I would go custom, and fill the space. The day may come where you want to run offshore, and having extra deck tanks is a pain.

Yep, have already gotten quotes on some. For a few hundred more than this stock one I get a bigger size with epoxy coating. Going to mock up a wooden frame to see what I can fit.