The old 60 gallon vertical steel tank on the starboard side of the boat was leaking fuel into the floorboards.  Out it came, to be replaced by a brand new custom made 100 gallon aluminum fuel tank.  The tank gets its capacity by being fitted into the entire space to the side of the passage way, only raising the top by a few inches.   It is designed to accommodate a Wema tank level sensor, access plate, vent, and a top draw pickup with anti-return valve.  A small (6" dia.) sump allows sediment and water to accumulate where it can be sucked out with a simple outboard motor squeeze bulb.

I think aluminum is by far the best material for diesel fuel - cheap, easy to manufacture, and compatible with the fuel. The new starboard tank is heavier gauge than off-the-shelf. I can't remember the exact thickness but it as at least 1/8".  It is also easy to have the tank  fit the curve of the hull.

Cross-section view of 100 gallon tank.  

Once the old tank is out, make a plywood mockup out of doorskin, plywood, and other bits. This way you can check the fit, and more importantly, if it is installable. Then you can just take the mockup to the fabricator and say: "make this". They should know how to install the sump and sensor, feed, return, and fill fittings. Allow for an inspection port. I used a "Wema system" inspection plate which incorporates a fuel level sensor and tank vent fitting. The Wema stuff is fairly inexpensive. I love having real fuel gauges.  I never could stand those expensive and difficult "Tank Tender" pressurized systems.

For the sump, a 2" peice cut from a 6" aluminum tube and welded to the bottom works well.  Also weld on a tab for attaching a ground wire if you want to ground the tank.  If desired, weld a couple of scrap pieces of aluminum tube to the top of the tank through which you can run a rope for lifting it into place. These can be cut off later with a Sawzall if you need the room.  Pressure test to 3 PSI before installation.  This tank should cost $700 to $900.

When installing the tank it should not rest on any hard points.  Closed cell foam was used to fill gaps and provide insulation.  

Jeff Stander - s/v Beatrix  (www.svbeatrix.com)