polyethylene
Buddy: I too have a 1980 vintage Oday. It is a 28 footer but I suspect that both boats had the same tank installed by Oday at the time they were built. Mine was Polyethylene or polypropylene (I'm not sure which). Whatever the material was it was impossible to seal up. It split at the top seam and was located under the starboard settee. There was no access to remove it so it was installed when the boat was built. So, what did I do? I got in thru the access panel with a roto zip tool with the plastic metal cutting tool and cut the thing to pieces small enough to remove. It took about an hour of determined cutting (being very careful to avoid the hull) but I got it out. I then replaced it with a collapsable tank of a similar size and capacity. They come in many sizes and different geometric shapes so you'll be able to find one which fits the space your current "hard" tanks occupy. However, If you can pull your tank out you may be able to "weld" the split by heat or chemical or replace it with another tank of like size. I am very pleased with the collapsable tank as it is ventless so no air can get at the water and it stays fresh tasting much longer. Also I pull it in the fall, take it home and store it in the basement. No freezing worries that way. I bought the tank through West Marine, it is a 20 gallon tank and cost about $110. Good Luck Mike
