8 things every Northwest boater should know about fuel filters

From Three Sheets Northwest / Story developed in collaboration with Fisheries Supply.

Don’t leave the dock without a good fuel filter.

When Seattle-based commercial fishing boats head towards the rough and tumble waters off Alaska for the season, they need to make sure their engines can run without problems all day and all night for weeks on end. So what do you think the most important spare part they bring with them is?

Fuel filters. And a lot of ‘em.

On any boat, fuel problems can happen at almost any time (and usually, the worst time) thanks to the challenging conditions that exist in fuel tanks. And bad fuel can stop your boat’s engine dead.

That’s why every boat should have a high-quality fuel filtering system installed — and every boat owner should know how and when to change the filters.

I spent some time chatting with George Schillinger, an expert in fuel filters with the Racor division of Parker, and what I learned surprised me. Here are some of the common questions — and misconceptions — about onboard filters.

My engine comes with a fuel filter, so why do I need a secondary filtering system?

That primary filter might be enough for a car, but it isn’t designed to deal with the primary culprit that affects fuel aboard boats — water. You are asking for trouble if you rely only on the engine’s fuel filter. In fact, you might not even know you have a fuel problem until your engine starts sputtering to a stop. Just one trip to your local mechanic to fix engine injectors ruined by bad fuel will make the cost of a good secondary fuel filtration system seem like a bargain.

Why do boats have such a problem with water in the fuel tanks?

Because all boat tanks are vented to the outside. Depending on where the vent is located, water can splash directly into the tank when underway. That’s bad enough, but even if the boat isn’t moving, water can easily make its way into your tanks. The fact is, your boat’s tank “breathes” a lot of moisture-ladened air — all the time. It happens as temperatures change during the day and night and when fuel inside the tank sloshes about. Over time, condensation inside the tank can lead to water in the fuel. Water can also get into your tank if the deck fill plate leaks or even if the place you buy your fuel from doesn’t properly filter it before it goes into your tanks.

“Water happens when you use your boat or not,” Schillinger said. “You need to be prepared for it.”

So why is water such a bad thing?

Water in your tank allows algae to grow which eventually produces sludge. Water and sludge can cause big fuel problems. If either gets into your engine’s fuel system, it can lead to costly repairs. It is also a safety issue. Water and sludge settle to the bottom of the tank. So if your boat hits bad weather, the agitation in the tank will stir up those nasty components and could cause your boat’s engine to die just when you need it most.

How does a filter stop water anyway?

Two ways. Water is heavier than fuel, so a good filter housing will use gravity and centrifugal forces to separate the water and then let it fall to the bottom of the filter housing. Recent advances in filtering materials have improved the filter element’s ability to block water while allowing fuel to flow through. Think of it like a breathable rain jacket. Good filter housings will have a glass or plastic bowl at the bottom so you can see if water is collected. It should appear as a pretty clear line in the bowl, which will alert you to a problem.

Okay, so I need a secondary fuel filter. What size of a filter system should I get?

This will depend entirely on the horsepower of your engine and the amount of fuel that flows through it. The best approach is to check with your the manufacturer of your engine to see how many gallons per hour your filter should be able to handle.

The reason you want appropriate capacity is to give the fuel in the filter housing time to let the water settle out. “The larger the unit, the longer the filter will last and the more the water separation will have time to work,” Schillinger said.

Schillinger says that when it comes to diesel engines, the rule of thumb is that for 150 hp or less, a 60 gallon-per-hour (gph) filter will work. For engines between 150 and 300 hp, a 90 gph filter should work. And for an engine larger than 300 hp, step up to 180 gph or larger.

What about the filter element? How fine should it be?

Filter elements typically come in three ratings — 30, 10 and 2 microns. The lower the micron rating, the smaller the particles that the filter will catch. Schillinger recommends 2 micron filters for most recreational boaters because it will most likely be a finer filter than the engine’s primary filters.

“If the filter is applied correctly, than 2 microns is no problem,” Schillinger added. It’s a good idea to check with the engine maker to make sure the fuel flow with a 2 micron filter is adequate and doesn’t cause problems with the engine’s fuel pump.

How often should I change the filter?

That will depend entirely on the condition of the fuel and your tanks. The filter will grow darker if it is capturing sludge or other particles. You can add a pressure gauge to some filter housings that will indicate when the filter needs to be changed.

As for water, if you don’t see any in the housing, then you probably have a good fuel system and tank. But if you are draining water every week or month, then you have a problem in the tank that needs to be addressed.

Even if you don’t have a problem, it’s a good idea to change fuel filters out on a regular basis — perhaps every 100 hours.

Can I rely on my filters to deal with all of my fuel problems?

No. The best way to think about your fuel filter is as a canary in the coal mine. If you see sludge or water in the filter, it is an indication of problems in in your fuel tank. “It’s like having an alarm, visual rather than audible. If there is a little water in the filter housing, there is probably a lot of water in the tank,” Schillinger said.

If you find that you regularly have water in your filter, or if it gets clogged often, then consider cleaning the tanks out and having the existing fuel polished. Most importantly, get to the bottom of why you have so much water in the tanks to begin with. Water or a clogged filter is a symptom of a much bigger problem.

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About Marty McOmber

Marty McOmber is editor and co-founder of Three Sheets Northwest. He is an avid sailor and long-time professional journalist. You can find Marty aboard Meridian, a Passport 40.