We have both the diesel fuel tank and fresh water tanks in the keel on Illusion.
The diesel tank is clearly visible, and sits on top of the encased lead ballast. It is a conventional aluminum tank that is painted with Gluvit and sits on two G10 fore-aft runners so that water can pass underneath it from forward in the boat into the keel well.
We fill the fuel tank directly via the visible port. We had no easy way to run a fill hose from the keel well to the deck. It also turns out that filling a fuel tank directly into a large opening is easy because you can see the fuel level, it is trivial to avoid overfilling or spills, and there is never a problem with a vent spurting. The key is to have a large opening.
Because the vent doesn’t need to have large capacity to let air escape during filling we use a tiny 1/8 inch vent line that is visible. The vent line is only there to let air in to replace the used fuel, which is very slow, 2/3 of a gallon per hour maximum in our case. The vent line runs to a clear PVC pipe that is inserted in the cabin vertical support pipe, and is full of silica gel. The vent is open to the inside of the boat through a pin hole at the top of the PVC pipe full of silica gell. The idea is that the silica gel keeps the fuel dry and keeps moisture out of any air that enters the tank via the vent, and there is no possibility of water entering the vent line because the vent line does not go above deck.
It seems to work. The bottom of our tank is clearly visible through the fill port and we’ve never seen any moisture. My suspicion is that most water in fuel tanks gets there through leaks in the deck fill, and directly via the deck vent when there is green water on deck or during wash downs. Many race boats use the same approach as we use on Illusion, and fill the tanks directly down below, and only have a tiny vent line that runs high up in the accommodation but does not go above deck, avoiding any possibility for water entering through the fill or vent.
Directly aft of the lead ballast we have a G10 partition that keeps a 6 inch area open to the bottom of the keel for bilge strum boxes and bilge pump hoses, and it allows us to visually see to the bottom of the keel well. Aft of that partition is an off-the-shelf fresh water bladder tank.
The diesel tank holds about 50 gallons and the freshwater tank holds about 35 gallons.
We put the freshwater in the bilge because it was a good use for the unused volume, and it seemed dumb to have a keel full of air.
Also visible in the photos is our engine raw water intake. The corrugated hose runs to the cockpit operated diaphragm bilge pump. The white PVC hose and pipe run to the diaphragm pump in the head, via a Y-valve, so that we can use the diaphragm pump that is associated with our Lavac head as a backup bilge pump.
Our electric bilge pump is a portable, 3800 GPH, centrifugal pump that has a coil of collapsible hose and a long wire with battery clips. We carry it in a bag as an emergency pump.
Below is a close up of the cabin post that supports the deck. You can just see the blue silica gel that is inside the clear PVC pipe, that is inside the post. We have to re-dry the silica gel about every three years since we installed it.