One of the profound mysteries of operating sailboats is maintaining the fuel system. There are several theories regarding your fuel, tank and filters. Should the tank be topped off and kept full? This minimizes the space for condensation, yet, if you are like us, we rarely motor, so a tank of diesel fuel lasts a year or more. Some claim, with the various additives, diesel fuel has a shelf-life and should be consumed within 6 months or so. I don't claim to know the proper balance between topping off the tank vs. running it down and refilling.
I do know that microbiological organisms tend to grow in the tank which settle to the bottom. If you've ever looked at a typical diesel fuel tank, the pickup tube stops about 1" from the bottom of the tank. In theory, this prevents the the various sediments on the bottom of the tank to not be drawn into the fuel system.
Theory is fine, but what happens when your tank level is low and you are motoring into a choppy sea? The sediment becomes stirred and any growth may dislodge into the tank. This will be picked up through the fuel line and hopefully filtered by your primary fuel filter. Unfortunately, this also tends to block the filter, effectively shutting down the fuel supply to your engine. In this case, your engine will stop.
This happened to us one, years ago on a friends boat. We were motoring off the west end of Santa Cruz Island, through a region fondly called the "potato patch". We hadn't planned on sailing through this, but this seemed like the best course to our planned anchorage for the night. Of course, our fuel tank level was low. Within a mile of the anchorage, the motor just stopped. We tried starting the engine, it ran for a few seconds and stopped again. The cause - blocked fuel system. Believe me, it is not fun to change the fuel filters and bleed the fuel system while underway in choppy seas.
In preparation for our trip to Mexico, I am trying to anticipate problems before we head south. Since this trip will entail a fair amount of motor sailing, I was curious about the state of our fuel tanks. In preparation for this inspection, I deliberately ran the fuel level less than 1/4 tank.
Our 2000 Tartan 3500 has a 25 gallon fuel tank, located under the starboard locker with two 5" inspection ports. Clearing out all the junk in the locker (spare jerry can, paddles, dinghy wheels, lifejackets, siphons, fishing supplies...), I ventured into the locker to open the port. It was very easy to unscrew 10 or so screws and to my surprise, the inspection cover came right off (I was expecting it to be sealed with gasket material - instead, just a crude rubber gasket).
I pumped out about 1/2 the remaining fuel (2 1/2 gallons) to inspect. Peering into this mystery void, I found black/brown goo lightly sticking to the bottom of the tank. Diesel fuel is red in color. What you see here is sediment on the bottom of the tank. The red squiggly lines are where the bottom of the pump hose scraped against the bottom of the tank, dislodging the debris. I was very pleased to see that the rest of the tank is pretty clean (lower left of the photo).
Looking at the 2 1/2 gallons of fuel that I pumped out, it was cloudy and dark red in color. It was pretty ugly. Obviously, this sediment was stirred up and became suspended in the fuel. This dirty fuel is what would be pumped into your engine. I'm waiting to see if this will settle down and I can filter/siphon the top clear good fuel.
Thinking about what I found, taking advice from friends, and considering our trip, I decided to clean the tank.
Cleaning the tank is actually two processes. The first is to physically clean the tank. Commercial systems either steam-clean the tank or pressure wash the inside with cleaned fuel. The second process is polish the fuel, by filtering the suspended debris from the fuel. The Alamitos Bay Fuel Dock provides a fuel-polishing service; however, the service is pretty expensive. For our boat, there's a flat fee of $170 plus $85/hour. I could see this costing $250 or more and I'd have to move the boat to and from Alamitos Bay. I called a few mobile fuel polishing services, and I'm still waiting to hear back from them.
Using my iPhone, I took several photos of the tank interior. I didn't see any nasty bits of growth on the tank. In fact, the tank looked remarkably clean.
Rather than put the cover back on this and hope that the tank was clean-enough, I decided to pump out the remaining fuel and clean clean the tank. Armed with a couple rolls of paper towels, micro-fiber cloth, heavy-duty gloves, I jumped into cleaning the tank. Most of the debris was along the bottom of the tank, with more buildup by the pickup-tube and baffles.
Using the paper towels, I mopped up most of the debris. An interesting side note - diesel fuel makes a remarkable good cleaner. Dipping my paper-towels in the remaining diesel fuel, made it easy to clean the tank. Reaching around through the inspection port, I was able to reach all of the sides, top, bottom and tricky corners.
Finishing up with the micro-fiber cloths, I took a dozen photos, to verify that the tank was clean!!! I was very surprised how clean the tank was and how quick this process actually went. After deciding to clean the tank, on the way to the boat I purchased 5 gallons of diesel to refill. I refilled the tank with new, clean fuel. Now my next dilemma - will the boat start?
Part of my concern is the fuel pickup tube is 1" diameter tube running from the top of the tank to near the bottom. By emptying the tank, I was convinced that the fuel in this tube would drip out, leaving a large air-pocket in the fuel system. Considering this, I bleed the fuel system at the secondary fuel filter, and a little bit of air came out of the system. The engine started and I ran it for about 20 minutes at the dock.
I still wasn't convinced. The fuel that I bleed, was the old fuel, pink in color. Recall, that I filled my jerry can with auto diesel, which is blue in color. I was hoping to see blue fuel at the bleed screw. I didn't.
Yesterday was a gorgeous day for sailing. We filled the tank with 22.8 gallons of new, clean fuel, with additive, and sailed nicely in 20+ knots of wind. Not trusting the fuel system, we ran the motor hard at 3000 rpm for 30 minutes, without incident. A quick mental calculation, .66 gallons per hour, 30 minutes is 1/3 gallon which is more than a quart. There's not a quart of fuel in the fuel-pickup tube and 4' of 1/4" fuel hose. I'm pretty certain there's no evil air pocket waiting to kill our engine at the most inopportune time! I still don't understand why this wasn't a problem, but I'm happy it wasn't.
My only remaining issue is what to do with the 5 gallons of old fuel? At $4.80/gallon, this is $24 worth of fuel. It is not worth a lot of time or money to polish - it might be better to simply dispose of it. Out of curiosity, I'm going to watch the fuel for a few days and see if the it the sediment settles to the bottom. If so, I can siphon the clean fuel, filter, and possibly reuse. If not, I'll check with auto supply stores if they'll recycle used diesel fuel. As a last resort, I can dispose of the fuel at the Alamitos Bay Fuel Dock, for $2.00/gallon. Perhaps I can use 1/2 of what I have and dispose the rest. I've got time to sort this out.
I'm pleased how easy this entire process was and I'm now comfortable that large nasty bits won't be clogging our fuel system. Once your tank level gets less than 1/2, I would strongly recommend opening inspection port and checking out the state of your tank. This will give you piece of mind that your fuel system is clean and reliable!