The "day tank" is a fuel tank that supplies the engine for a while (not really a day in this case), and is refueled from other tanks in the boat. The day tank simplifies switching between fuel tanks, and can be used to help separate out water and dirt in the fuel (being heavier, they sink to the bottom and can be removed if there is a drain at the bottom of the tank).

The old day tank was a cylindrical plastic affair, with fittings Sikaflexed (Sikaflex is a brand of glues and sealants) into it. It had been leaking, and the Sikaflex used to repair it was getting into the fuel and starting to clog the fuel filters.

I wanted a bigger day tank (the old one was only good for about three hours of motoring) that didn't leak and would help separate out the water and dirt from the fuel. This is the new day tank, made of stainless steel, much bigger, minus the fittings, which were installed after the tank was installed. The new tank is built to fit into an existing cutout in the cockpit floor.