We’re boaters, so presumably we have at least some love for water. That’s all well and good, but when water gets into your fuel, either gas or diesel, it can really test that love. And the fact is, if you spend enough time in boats, you will have to deal with water in your fuel. Follow along as we walk you through the steps you need to take to make sure your days on the water end on a happy note—and not hooked to the line of a tow boat.

Sometimes a leak can be caused by a simple thing like a damaged O-ring on a deck fill cap.

Sometimes a leak can be caused by a simple thing like a damaged O-ring on a deck fill cap.


Where’s the water coming from?


Water in fuel can come from a variety of sources. Some of these might be obvious, like a broken O-ring on a deck fill cap. Other sources might be harder to detect, like a tank vent improperly placed by the boatbuilder. In my experience a vent that’s located amidships is vulnerable to bow wave exposure depending on boat speed and its height above the static waterline. Vent openings are also sometimes set facing forward, creating a strong possibility of picking up water coming off a bow wave at slow speed. Alternatively, a tank vent mounted on the topsides just forward of the transom with the opening facing downward is likely to stay dry in just about any sea condition.

Condensation can be another source of water in your tank, a situation made worse by ethanol added to marine fuel.

Use a fuel stabilizer recommended by your engine manufacturer (Mercury Marine, for example makes their own branded stabilizer) and don’t leave your tank for too long without a full load of fuel; try to keep it topped up. Doing this minimizes space inside the tank for condensate to build up and ultimately end up in your fuel system.

But be advised—your fuel tank is only one in a series of tanks that bring fuel to your engine. There are also large-scale distributors, delivery trucks, and marina fuel stations, all of which use tanks that are subject to condensation, and you may face a cumulative collection of water from all those links in the supply chain.

This Moeller Clear-Site water separator and filter allows you to see any water settled into the bottom of the clear plastic fuel bowl, and has a large opening valve to drain the water out. However, depending on the boat and fuel system arrangement, such filters may not be compliant with USCG or ABYC standards.

This Moeller Clear-Site water separator and filter allows you to see any water settled into the bottom of the clear plastic fuel bowl, and has a large opening valve to drain the water out. However, depending on the boat and fuel system arrangement, such filters may not be compliant with USCG or ABYC standards.


OK, water happens. How do I keep it from causing trouble?


Water-separating fuel filters are the key to success here. I’m a big fan of the clear-bowl kind that allow you to see and drain out water settled at the bottom. That said, some caution is advised. Both the U.S. Coast Guard (USCG) and American Boat and Yacht Council (ABYC) mandate that all components used in gasoline fuel systems pass a fire test and meet some other strict requirements. However, ABYC standards are much tougher to comply with than USCG minimum requirements. All of this can be confusing, but as a general rule of thumb, ABYC standards say that water separator/filters with clear plastic bowls for gasoline engines are acceptable for open-cockpit boats, but should not be installed belowdecks in boats with enclosed engine or tank spaces where a volatile fuel/air mixture can accumulate in the event of a fuel leak. In most cases, to meet all the requirements for both the USCG and the ABYC in those circumstances you will need to use all-metal water separator/filters and do without the convenient see-through plastic bowls.

How often should I change my filters?


At the minimum, follow your engine manufacturer’s recommendations. I typically put between 50 and 150 hours per year on my boat, and I change my filter every spring before starting my engine for the first time. And I always keep a spare filter on board in case the worse happens and I need to change out offshore.

Remember that most outboards today will have a small secondary filter mounted on the powerhead. It will have a clear bowl so you can see if any water has settled to the bottom. Be extra vigilant and check this filter for water a few times during the season. In fact, some manufacturers have a sensor integrated into this setup that will alert you if water reaches a level too high in the bowl. Change this filter element at least every two years, or according to your engine-maker’s recommendations.

An engine-side canister with a filter and a water sump is the last line of defense before water works its way into the engine.

An engine-side canister with a filter and a water sump is the last line of defense before water works its way into the engine.


What about diesels?


As far as diesels go, the same basic rules apply but one additional detail should be mentioned. Filters are rated to stop particles of a certain size, typically measured in microns. One micron is equivalent to .00004”. If the diesel engine manufacturer says to use a 10-micron filter stick to that specification. Some people have thought that if 10 is good, a 5-micron element might be better. No! It is actually possible to create a situation where the filter is too fine and will restrict fuel flow, causing a fuel starvation condition with your engine.

The best strategy for minimizing the risk of water in your fuel is to purchase your fuel from a place that sells a lot of fuel. This helps to ensure that the fuel is fresh, and that it hasn’t been sitting around in a storage tank for an extended period, collecting condensed water. Follow the recommendations relative to fuel filters, and you can enjoy the water on the outside of your boat—not suffer from water inside your fuel tank.

Editor’s Note: Promotional consideration for this article was provided by Sea Tow.

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