I heat my boat with a diesel furnace system and use a gallon or two a day heating her during the winter. Recently, I topped her off; there was a wicked nor’easter heading our way, adding to the difficulties of an already weeks-long cold snap.

Before the storm struck, I pumped in about 10 gallons into the 33-gallon, portside, aft tank. It was a nippy evening, in the mid-teens to low twenties, but I knew doing so would leave me with plenty of fuel for the days ahead, forecast to be in the teens and single digits.

The following day, someone noticed that diesel had accumulated in an area of my boat where the fuel tanks vents. This does not typically happen when I fill up. So what could cause that? How could a tank be full in the evening and subsequently overflow?

I blame the cold.

Those 10 gallons were stored in jerry cans on the docks and so the diesel’s temperature was about 15 degrees F (-9.5 degrees C.) The fuel already in the tank was about 55 degrees F (13 degrees C.)

And as those 10 gallons (37.9 liters) gradually warmed to the temperature of the rest the fuel, it also expanded. According to an online calculator, it grew to roughly 10.3 gallons (39 liters.) ( I used a constant expansion coefficient of 0.0007.)

By comparison, when filling up during the summer, the temperature differential inside and outside the boat would be much smaller. For example, when going from 65 degrees outside to 85 inside, pretty extreme and unlikely for the summer,  10 gallons (37.9 liters) would increase in volume to less than 10.1 (38.2 liters.) A more likely summer scenario would have the temperature outside the boat warmer than the inside, meaning the diesel’s volume would shrink.

So here is my best guess what happened: The gradual increase of 0.3 gallons was enough to slightly overfill the tank and force a small amount of fuel to expand up the vent line.