The conventional wisdom for owners of larger sailboats powered by an auxiliary diesel engine is to top off your fuel tank at the end of the season after adding additives to prevent water buildup, algae growth and fuel deterioration. The reason for keeping the tank full is to keep condensation from contaminating the fuel with water.
I have followed this procedure for the 35 years I've owned my O’Day 31. But I've learned the hard way that is not enough to prevent problems.
The problem is that many recreational sailors who don’t take long trips never go through a full tank of fuel in a season. My sloop, for example, has a 15-gallon tank. This past season I was unusually busy and used the boat only 15 days, including a three-day trip into New York City from Oyster Bay. I ended up using only about 5 gallons of fuel all season. Powerboat owners might be envious of how miserly the small diesels are with fuel but there is a downside.
The result of the diesel efficiency is that some of the fuel remains in the tank for years, in my case 10 years since I replaced a leaking tank and the fuel with it. There is always some water that settles out of fuel even from reputable marinas. I learned that lesson – expensively – a year after I replaced my fuel tank when my Universal diesel seized up over the winter. In the spring, it had to be pulled out of the boat and sent back to the factory to be rebuilt, which cost me $7,000 and several weeks of sailing time.
(The marina manager questioned whether I had mistakenly put water in the tank thinking I was filling my fresh water supply. I replied that while I can be absent-minded, the fuel filler fitting and the water tank filler fitting were on opposite sides and ends of the boat, which would make it hard to make that mistake.)
Last fall on my final sail of the season, I dutifully poured in additives and then topped off the fuel at a Huntington marina.
Still nervous about water buildup in my tank, I was intrigued when I saw an ad for a fuel and tank-cleaning company that would cycle the fuel in my tank through a filtering system and clean the tank at the same time. The service wasn’t cheap, but I decided it was worth the $375 cost to avoid another potential $7,000 rebuild.
I knew I had made the right decision after Tom Schultz, owner of Fuel Solutions, arrived at the boatyard. After he started filtering the fuel, he showed me a jar with a sample of the fuel coming out of my tank – with a shocking amount of water sitting at the bottom.
I ended up having him pump out and taking away all the fuel – including the five gallons I had just put in the week before – for an additional $3.50 per gallon. But it seemed to be a good investment in avoiding bigger problems down the road.
I learned I was not unique in having water build up in my fuel tank. “The major problem with boats is that people don’t use them enough, whether it’s gas or diesel,” said Schultz, who has operated the company for 13 years. Because a lot of sailboat owners don’t take long cruises or day trips under power, “the fuel just sits there. The quality of diesel nowadays, especially in a marine environment, is nothing like it used to be. It’s literally the bottom of the barrel. That’s where you get your sludge, your water.”
Schultz added that “diesel fuel has a long shelf life. In a home heating tank, it will last for years. In a marine environment with your hot weather, your cold weather,” not so much.
He said about 80 percent of his customers have gasoline engines and about 20 percent diesel, which includes a lot of sailboats. Schultz advises owners of the sailboats who don’t go through a full tank every season and use home heating oil or have another way to dispose of their unused diesel in their boat tanks to pump out the fuel at the end of the season and add it to their home heating oil tank or give it to someone else who uses home heating oil.
“If you only have a 15-gallon tank, you can stick a bilge pump down in there and drain it yourself into 5-gallon containers instead of paying someone a couple hundred dollars to come do it,” Schultz said.
He said mixing it with the home heating oil that is used more frequently and has less moisture in it to start with would not cause any problems for an oil burner.
Schultz advised doing this at least every other season.
“It’s best to get rid of it and put a fresh tank of fuel with stabilizer in there and run the engine before you put the boat away,” he said.
He said the usual advice to store the boat with a full tank still make sense. “You get less moisture keeping a full tank,” he said. “A dry tank will create moisture. With shrinkwrap or a cover, in the wintertime on a sunny day, it gets toasty in an engine compartment, and that’s where you get your condensation. You can’t get condensation with a full tank. Condensation forms on the sidewalls of an empty tank.”
To avoid problems, “run your engine,” Schultz advised. “Even if you’re just sitting at the dock. It’s good for the engine. Diesel engines are made to be run.