How big??
And why? I sailed all summer and probably used less than one-fifth of the tank. Additional space is at a premium unless some things are moved. And the additional quantity would be a major consideration. Are you thinking of backup? Then a six-gallon outboard tank in a rear locker would take you a long way(ten hours?). Gene Greunder found lots of storage space on his H37C but didn't use any of it for extra diesel. See his projects at: www.geocities.com/TheTropics/Cabana/7414/upgrades.html He claims the tank is only 38 gallons, still seems like a lot.
