Condensation Conundrum

Clyde Collins of Seneca Falls, New York, asks:

“I’ve moved up from a Catalina 22 to a Bayfield 29 with a 20-gallon diesel fuel tank. I sail on the Finger Lakes of New York and only use about 5 gallons of fuel a summer. My understanding has always been that I should store the boat over winter with a full tank to minimize condensation. But that means I have 15 gallons that won’t get used next summer, which means keeping old fuel in my tank year after year.

Although I do put in additives, I’m concerned about the quality of my leftover fuel. What do you think about putting a small amount of a water-retention additive into the fuel that remains in the fall and then pumping it all out in the spring before I add five gallons of clean fresh fuel?”

Nigel Calder replies:

Although I’ve always advocated topping off a fuel tank to avoid condensation, recently I’ve seen comments from a yard in New England that deals with this issue on a regular basis and they suggest that condensation in a half-empty tank is negligible—at least in New England. So if you leave the tank half empty, things probably will be all right. However, if you can insert a line down to the bottom of the tank, I would suggest pumping out a fuel sample at the start of next season to remove any water that may have collected and to suck out any accumulated sediment. If you can’t do this, keep a close eye on the transparent bowl on your primary fuel meter when you first crank up the engine next year.

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