Understanding Boat Fueling

Fueling a boat is very different than fueling a car. Why?

• Boats tend to have larger fuel tanks;
• Gas dock nozzles tend to pump fuel faster than those at a service station; and
• Boats have an overboard vent.

All of these factors can lead to an unintentional spill at the fuel dock.

By learning more about boat fueling systems we can better understand why boats and their fuels can be a challenge to handle.

Boats Are Not Pressurized Like Cars!

Cars have pressurized systems to deliver fuel directly to engine injectors. This is done under high pressure by a pump in the fuel tank to assure peak performance and fuel economy. This type of closed system prevents air or fuel from escaping the tank during operation.

The Boat Safety Act of 1971 prohibited the use of pressurized fuel systems in boats. If a pressurized fuel system on a boat failed, fuel would collect in the bilge, and a very dangerous situation could develop. Therefore, a fuel delivery system that has an air vent is used on boats.

On this boat, the fuel vent is directly below the deck fill

An air vent allows for the movement of air in and out of the tank during the fueling process and regular boat operation. A vent also accommodates for the expansion of fuel when the temperature warms up or when the boat gets jostled about as a result of normal use. When pressure builds, it must be released – on boats it is released through the tank’s vent. Unfortunately as many boaters know, sometimes fuel, as well as air, escapes through the vent.

Why do Boats Burp Fuel?

When refueling, the fuel goes into the tank and displaces air. This air escapes through the boat’s fuel tank vent. When the tank is nearly full, especially when refueling with a high speed pump, fuel can gurgle up and spill out the vent. This can happen because the fuel is entering the tank faster than the air can escape. The expansion of fuel may also cause leaks from the fuel vent. This may occur when cool fuel from an underground storage tank is pumped into a boat on a hot day. Resist topping off your fuel tank and leave 10% of tank capacity empty to allow for expansion.

Another potential problem is “backsplash” or “blowback” from the deckfill. This can happen quickly, especially when refueling faster than your fuel fill and air vent can handle. This problem is made worse if a blockage exists in the vent and air from the tank cannot escape. Be sure to check for blockages caused by spiders or other insects that have taken up residence in your vent. An absorbent fueling donut around the nozzle can also help to absorb backsplash and minimize a spill.

To avoid air locks, blockages and sudden spills, always be certain the boat is level when refueling. Send all passengers ashore so the boat will sit on its lines or level your trailer if refueling at a gas station. Always refuel slowly and listen for a change in pitch that indicates the tank is nearing capacity.

Why Won’t My Fuel Gauge Work?

To better understand fuel gauge accuracy we spoke with many manufacturers. Their response was “don’t blame the fuel gauges!” Instead, the most common culprit cited was the tank sending unit - that’s why tapping on the gauge glass rarely works!

Most fuel gauges use a floating sending unit that is often manufactured by a different company than the gauge. The float rises and falls with the fuel level and sends an electronic signal corresponding to a unit of measurement, usually in ¼ tank increments, to your fuel gauge. If the sending unit and gauge are two different brands, compatibility could be the problem.

Problems can also arise when new boats are shipped on trucks when the fuel tanks are empty, causing the sending unit to bounce and become misaligned. Another potential reason for inaccuracy is that tanks are often designed to fit the bottom contour of a boat and sometimes are shaped like a triangle. This means that the sender and gauge must be compatible with unique tank shapes. A stock sending unit is not likely to read accurately.

In addition to issues with the float arm and tank shape, there may be calibration, voltage fluctuation or grounding issues at play. The bounce, heel, trim and general abuse that a boat takes when operated affects proper readout as well.

Here are some trouble-shooting tips:

• Manually inspect the sending unit in the fuel tank. Be certain there aren’t any signs of internal rubbing or obstructions.
• Check for proper voltage, wiring and ground connections and determine if the sender and gauge are compatible.
• Visually calibrate your tank by refilling it in known increments and make a few hash marks with permanent marker on your in-dash gauge at these intervals.

Have you ever been frustrated with a nozzle that didn’t click off in time? As fuel flows through a nozzle, it also passes through a small tube within the nozzle called a venturi which starts suction and creates a vacuum. Look inside a spout sometime and you’ll see this tube and a “sensing” hole at the tip. These two components are important in signaling the nozzle to “click off” before an overflow can occur. Courtesy of Husky Corporation As fuel flows through the venturi, air gets sucked in through this sensing hole at the tip of the nozzle and up through the tube to the top of the venturi in a circular fashion. So, as fuel is flowing out of the nozzle, air is flowing into the sensing hole, getting mixed into the fuel again at the venturi and moving back out with the fuel flow. Essentially, the air within the nozzle is traveling in a continuous fashion, and everything is in balance. As the fuel tank becomes full, the level of fuel rises in the fill pipe and eventually reaches the sensing hole in the nozzle. When this occurs, the circular pattern of air begins to suck in fuel through the sensing hole creating an imbalance in airflow and causing the automatic shut-off feature on the nozzle to trip. In theory and at gas stations this works great. Unfortunately at fuel docks, due to the increased volume of the pump’s flow, the nozzle and attendant don’t have enough time to react and fuel can spill from the deckfill. It is important to attend the nozzle at all times when refueling. Do not rely on the hands-free or automatic shut-off features to alert you when the tank is full. Fuel slowly and listen carefully for changes in pitch indicating that you are nearing capacity. If you suspect a spilling problem is a result of a faulty nozzle, report it to the dock attendant.

For more information on Boat fueling systems see the Boater’s Toolbox.