Need a lot more information. Size of the vessel, gallons of the tank, etc. Even so, there is really no way of knowing unless someone here has the exact same model boat you do. Tanks come in so many shapes and sizes. A good rule of thumb is if the tank is in the center of the boat, it usually runs between the bulkheads and the stringers. Example of this is my tank is between the ski locker and the engine compartment bulkheads. Photo shows this example.
My 1975 bayliner 20 food cuddy, i could slide a tape measure up on the top of the tank until it hooked the end of the tank. Good luck. Mine is 61 inches, i believe it is 40 gallon.
