Martinini...any diesel tank that has been standing around for a while will bet water in it from condensation. That's why we have fuel/water separators on the fuel lines. So you may not have a leak type issue at all. Suggest you hire a fuel polishing service to pump and clean the existing diesel (or dispose of it if really old) and put some new stuff in. Keep tank FILLED to prevent condensation. Use a biocide to kill the alge that grows in the fuel/water barrier.
As to the tank..Practical Sailor reports:
Freshwater capacity is variously reported at 80 and 90 gallons in a stainless steel tank under the main cabin sole. Fuel is reported as 40 gallons, first in a black iron tank, later in one of Corten steel, which is preferable.
They do not disclose the year the switch was made but Corten is not much of an improvement over black iron (surface looks like rust which is how to tell the difference. Problem is at welds and in a marine environment Corten is not designed for salt water.)...and a boat 30 years old or so with original tanks may need them sooner than later...but test the easy stuff first.
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