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!
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