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If I was to opt out on fabricating a new tank to fit my whaler, what's your opinion on how it would affect the boat's balance? Figure the 40 gallon tank weighs roughly, 300lbs when full, but the weight is centered. If I were to leave the fuel cavity in the hull empty, seal it to prevent water from getting in, do you think it would throw the boat off? Would the boat be more likely to tip? The fuel tank option would be plastic on the deck. This is a work boat.
My 21 never had the belly tank. I installed a PVC tube into the rear sump so that the water empties into the sump and is pumped out. My console has a built in front seat. Under that I have a 26 gal tank. You could put a tank like that in front of your console and build a box of some sort to sit on. That should give you about a 75 mile range. Don't worry about stability, I doubt you could flip that hull if you tried. I have had all 10 of my lobster traps on one side and it didn't list very much.
I did the same thing as Tom after my fuel tank failed; connected the tank cavity to the sump with a PVC tube. My deck cover was waterlogged and in poor condition, so I bridged the tank coffin with 5/4 mahogany and used it for wet storage.
You will have no issues using your boat in this manner.
The early 21's did not have the tank in a cavity below deck. You should be fine doing what your propose. Maybe even consider making a fish or storage locker in that area since its a work boat. My 21 has the fuel tank under the driver seat - it came from the factory with two saddle mount tanks. (one under each gunnel)
