I Bought New Tank
Pete, I had the same failure in my C36 -- the flat-bottomed tank was parked on a slight bulge in the hull and after many years of stress, it finally cracked. I was told that because the tank is made of polyethlene, no permanent repair is possible. Kind of like Teflon, nothing "sticks" permanently and well to polyethlene. Catalina put me in touch with the tank vendor, Ronco Plastics, who confirmed this, saying that even the hose connections are "spun" in at such a high speed that the poly gets hot, melts and fuses to them. Glueing is not possible. The new tank cost me $175 and they made it and delivered it to my door in three days time. I suppose you could try a cheap repair and with winter coming on, there's time to try it, and you're not out much if it doesn't work.