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One of the most common causes of boat engine malfunction is also one of the easiest to prevent. Here's how.
Photo, Patricia Longman
While electricity and alternative fuels may propel the boats of the future, for now most of us still depend on good ol’ gasoline and diesel to keep our boats moving. Modern and well-maintained late-model internal combustion engines are extremely dependable when fed clean, pure fuels. But things can go south quickly when an engine’s worst adversary enters the room – water.
A creeping trespasser that catches most boaters by surprise, water in the fuel is one of the top causes of engine malfunctions – and one of the most preventable. With the frustrating ability to make its way into a fuel system undetected, the same water that provides us so much joy can also become the bane of our existence when it mixes with the fuel that propels us.
Commonly identified by a rough or nonrunning engine (many engines now feature water-in-fuel alarms), water in the fuel is easy to diagnose. Any boater can perform the age-old “jar test” by simply removing the fuel line from the boat side of the fuel system and pumping a pint or so of fuel into a clean glass jar. Cloudy fuel is an immediate giveaway that something is awry, but let the fuel settle for a few minutes, and if water exists, it will stratify and sink to the bottom of the jar, clearly visible to the naked eye.
Most boaters will at some point face this inconvenient dilemma. The good news is that if caught early, the remedy is often as simple as flushing the system and replacing it with fresh, clean fuel. However, left unchecked, damage to pumps, injectors, and other mechanical engine parts can lead to ongoing headaches before the offending three atoms are removed from play. Either way, the ultimate solution remains the same: Find the water’s entry point and stop it in its tracks.
As sneaky as it may be, water only has a finite number of opportunities to mix with your prized fuel, and most are easily preventable with some periodic maintenance and inspection, so keep the water under your boat and out of your engines by safeguarding these common entry points.
Left: The fill cap’s O-ring forms a crucial seal that prevents water from entering the tank but must be in good condition to do its job. Below left: Installing a spacer between the boat and the fuel fill prevents water from accumulating around the cap. Below: Sending units are another common entry point for water. Periodically inspect them to ensure the gasket is in good shape and water is not collecting on them. Photos, Mike Longman
1. Deck fill cap. While many styles of fuel fills exist, almost all are sealed with a simple rubber O-ring to keep contaminants out when the lid is closed. This design works remarkably well, but like most materials living in the harsh marine environment, the O-rings can dry out, degrade, and crack over time, allowing water through the fill and into your tank. Fortunately, this situation is easy and inexpensive to prevent: Periodically inspect the O-ring and replace it when necessary.
Another common issue is water pooling around the fill cap, usually due to poor design or placement of the fill assembly. The fuel fill should never be recessed in a way that allows water to accumulate around it in heavy rain, seas, or while washing. I’ve seen it more than a few times, even on some newer production boats. Because moving the fuel fill is usually impractical, some manufacturers offer spacers, which are essentially very thick washers that sit in between the fill’s flange and the deck, allowing water to run around the fill and not under the cap.
Portable fuel tanks found on small boats and tenders typically have fuel fill caps with an integrated vent that can be opened or closed. Most are designed to prevent water from pooling and making its way in, but sometimes it does anyway. Removing portable tanks from the boat when not in use and storing them in a safe, dry, and well-ventilated space is a surefire way to prevent rainwater from entering. In situations where that isn’t practical, having a cover for the tank that keeps rain out – but lets fresh air in – can be helpful.
2. Sending unit. The sending unit is the device that measures the fuel level in the tank and sends the signal to your fuel gauge. It relies on a synthetic rubber gasket to seal the tank and is also a common place for water to enter the system. These gaskets usually last a long time, but it’s prudent to inspect them and check the tightness of the screws periodically.
Sending units become troublesome when water accumulates on top of the tank, as it has the uncanny ability to find any minuscule entry point to infiltrate the system. Pay close attention to deck plates and access hatches above or near the sending unit, ensuring seals are in good shape and all deck hardware is also well-sealed to avoid water falling on the top of the tank.
If your boat has a poly (plastic) tank, look for warping on the top. Over time, these tanks can develop a concave top surface that allows water to pool and prevents the sending unit from making a perfect seal. In severe cases, this situation can only be remedied by replacing the tank.
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3. Venting system. EPA-compliant fuel systems on new boats use carbon filters and alternative venting methods, but traditional fuel systems rely on a simple vent line running from the tank to a location on the outside of the boat that allows the tank to “breathe.” While a necessary component of the system, the vent line is also a potential entry point for water. If you suspect your vent line may be the culprit, ensure there is a loop in the line to catch any water, and examine the location of the vent itself to see if water has the opportunity to enter. Heavy seas can sometimes force water into hullside vents, so moving them to a dryer location or covering them with an aft-facing clamshell can help keep water out.
4. Corroded fuel tank. Perhaps the most dreaded (and dangerous) cause of water intrusion is a compromised fuel tank. Aluminum, the most commonly used material for internal tanks, can be plagued by corrosion, especially on boats used in saltwater. A common boatbuilding practice involves pouring flotation foam around the tank, which can allow moisture to become trapped between the foam and the tank walls. Over time, this moisture inevitably leads to corrosion that can eat right through the tank if left unchecked.
The obvious first signs of a leaking tank are an abnormal fuel smell or, more obviously, fuel in the bilge. To confirm, a technician can perform a pressure test, which involves draining the tank, slightly pressurizing it with air, and measuring for any pressure drop. Aluminum tanks are not often repairable, so if a leak caused by corrosion is found, the entire tank usually needs to be replaced.
5. Purchasing contaminated fuel. This is the least common cause, but remote destinations; fuel stations with old, buried tanks; and other questionable fuel sources can have fuel contaminated with water or other debris. When possible, purchase your fuel from a reputable marina or gas station. If you are in a new area or have doubts about fuel quality, water-separating fuel filter funnels are a great tool to have aboard, as they allow you to catch any water before it hits your tank.