Peanut butter cup?

Spring 2023

By Christopher Birch

Diesel means, well, diesel. Photo by Christopher Birch

Fuel tanks are not like Reese’s Peanut Butter Cups. The fortuitous merger of chocolate and peanut butter depicted in the classic Reese’s ad works. But an accidental mix of flavors inside your fuel tank won’t be a winning delicacy. Your engine won’t like the taste, and you will find the aftertaste bitter, indeed.

Chocolate happens, though, and people make mistakes. I used to think that pouring the wrong fluid into one of your boat’s tanks was the most careless error imaginable – a mistake only the most wretched sailor could make. Today, I don’t just think that to be true; I know it to be true. Last fall, I accidentally pumped diesel into my boat’s water tank, a shameful tale recounted in Points East Winter 2023 issue.

Others have made, and will continue to make, the inverse mistake; inadvertently loading dock hose water into their boat’s fuel tank – an equally unfortunate error, but one that requires an entirely different remedy. The deck fills are clearly labeled, but people get distracted and make a mess of their boat tanks with surprising regularity.

It’s worth noting that a missing or cracked O-ring on the fuel deck-fill cap will also allow rain, wash, and spray water into a fuel tank, producing the same problem and requiring the same fix – ditto for a lost or stolen cap.

If it was a cracked O-ring that got you here, and you’re feeling sorry for yourself, just remember that a failed O-ring did far worse to the crew of the space shuttle. Replacing older deck fill cap O-rings is good preventive maintenance, the nautical equivalent of flossing.


The Remedy

So, what do you do with a fuel tank that’s all peanut-butter-cupped-up with a fuel/water mix? You painstakingly pump all the fuel and water out of the tank into jerry cans and lug them to a waste oil facility for disposal. Here’s a time when it’s good to have a small fuel tank. Water is heavier than fuel, and it dives to the bottom of the tank upon arrival. It then lurks under the fuel, like vinegar beneath oil in a salad dressing jar. It’s tempting to try to suck the water out and leave the fuel in, but it’s hard to get the last bit of water out with all that fuel sitting on top of it. The tank’s pickup pipe extends down to a point just above the bottom of the tank. The space below the pipe will remain full of water until someone extracts it.

Inspection ports on the top of the tank allow access for a fuel transfer pump hose to be inserted. Those ports will also allow you to reach in with an oil-absorbent pad and paper towel and dry the tank entirely after it’s pumped out. If the tank lacks inspection ports, you will need to pump most of the fuel/water out through the tank’s own pickup pipe and then pull the entire fuel tank out from its cavity in the boat. The last few cups of water can be drained out through the tank fill hole with adequate reorientation of the tank. (Warning: Gasoline is extremely volatile, and precautions must be taken when handling that fuel to prevent an explosion.)

The project can really snowball from here. Now that you have your boat torn apart with a filthy fuel tank sitting awkwardly somewhere in the cabin, why not clean out the space where the fuel tank usually sits? And what an excellent opportunity to replace all those old fuel tank hoses! And if the gauge was kinda wonky, why not install a new one now? And if, upon close inspection, you happen to notice an alarming amount of corrosion on the bottom corner of your aluminum tank, why not take it to the good people at Luther’s Tank in Bristol, Rhode Island, and have them build you a brand-new replica before this old one starts to leak? These things don’t last forever, after all. And while the new tank is being designed, why not have them include inspection ports? Because, at this point, you really wish your fuel tank had inspection ports.

An empty tank with open inspection ports provides an excellent opportunity for cleaning. The average person would be surprised by the amount of scum that accumulates on the inside walls of their fuel tank and by the size of the slimy mound of sludge in the bottom corner of that tank. Even if you don’t accidentally pour water into your fuel tank, H2O tends to accumulate anyway, thanks to condensation.

This is the filth that clogs fuel filters. Exacerbating the problem is the fact that petroleum products contain microbes, and the addition of water to a fuel tank will stimulate algae growth. When the boat rolls in a seaway, the gunk in the bottom of the tank will slosh around to where the pickup pipe can find it, and the next thing you know, the engine dies right when you need it most. If your engine quit in the heavy chop in the Hog Island Channel at the west end of the Cape Cod Canal, this is probably why.

It’s wise to keep your fuel tanks full to prevent condensation in the airspace at the top of the tank. Especially in winter when large temperature swings will bring many daily cycles of heavy condensation producing large quantities of water that will eventually grow algae and clog fuel filters. When boaters are confronted with engine trouble due to clogged fuel filters, they often mistakenly assume they took on bad fuel at a suspect fuel dock somewhere. The more likely cause is that they didn’t take on fuel often enough, and condensation in the partially empty tank is what brought the engine to its knees. No air space in the tank = no condensation in the tank = clean tank = clean filters = happy engine = happy sailor.


Another potential pitfall

Sadly, there is a third way you can go wrong at the fuel dock. The careless captain can pump the wrong type of fuel into their boat’s fuel tank. Just because you ask for diesel doesn’t mean the fuel dock attendant will hand you the diesel hose. These people are human too and, as such, are capable of mistakes. Color coding of the different hose nozzles is helpful, but it’s not totally consistent throughout New England or the world. Many years ago, a salty old-timer taught me a simple, foolproof trick: Always smell the fuel hose nozzle before pumping fuel. If it smells like diesel, it’s diesel. If it smells like gasoline, it’s gasoline.


Three strikes rule

Be warned. The boating world has an unwritten but strictly enforced three-strikes-and-you’re-out rule: If within your lifetime, you accidentally pump fuel into your water tank and accidentally add water to your fuel tank and accidentally pour the wrong type of fuel into your fuel tank, you must immediately sell your boat and take up golf.

I understand this tough stance on boat abuse is necessary, but I already have one strike myself. Like a baseball player who is behind in the count, I get an uneasy feeling in my stomach whenever I see a fuel dock these days. Deck fills scare the hell out of me. And they should scare the hell out of you too.

Christopher Birch is the founder of Birch Marine Inc. on Long Wharf in Boston. He is now out cruising full-time with his wife, Alex, aboard their 36-foot Morris Justine. Follow their voyage at