How to calibrate fuel gauges

How reliable are fuel gauges on boats? Is there anything I can do to get my gauges calibrated because on our last outing, we ran dry when the fuel gauge showed there was enough to get us home.

From: Peter Thornett – Lady Godiver

Angus Answers: My experience across a range of boats suggests fuel gauges on boats should not be relied upon unless they have been calibrated. I was once, on a 50’ motor cruiser when we ran out of fuel in the starboard tank and just crept back to the fuel berth on fumes from the port tank, when the gauges were showing a healthy ¼ of a tank! This problem is often down to the manufacturer not calibrating what is a fairly simple and fool-proof system.

At Seaward, we use two styles of fuel gauge and detection system.

Standard Gauges

The first and most frequent on our smaller boats is the ‘car’ style fuel gauge:


A float within the tank triggers the needle . At Seaward these are calibrated during installation so that when the float is near the bottom of the tank the gauge shows empty, and full when the float is at the top. This means that the gauge is properly calibrated and should continue to show a pretty accurate picture of the tanks contents. Most Seaward tanks are rectangular in construction and when the float is halfway down this represents half a tank of fuel. Care needs to be taken on some boats where tanks are shaped or follow the line of hull. This can lead to less fuel being available than shown on the gauge.

Tank Tender

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Some owners, particularly with larger boats opt for the multi-tank, ‘Tank Tender system’. This utilises a vacuum tube in each tank which, when the pump is operated, draws up fuel into the tube to match the level in the tank. When calibrated, this is a very accurate system. The calibration measure guide is made during manufacture and supplied as a laminated sheet. (it’s worth taking a copy and keeping it safe in case the original is lost). This system is excellent for shaped tanks since the calibration guide shows how much liquid is in the tank.

The Tank Tender can be attached to multiple fuel, water, grey waste and black waste tanks.

The tank tender is easy to operate. Press and hold down the button for the tank to be tested, then pull and push the pump lever twice. The gauge then reads the level of fuel in that tank. This can be repeat with other tanks.

If there is a grey or black waste tank, the Tank Tender will also have a ‘read/purge’ switch. This blows down the tube prior to taking a reading to clear any blockages that may be caused by solid debris in the tank.

Simply flick the switch to ‘purge’ then prime the pump a few times before switching to ‘read’ and carry out the same process for reading fuel levels.

Gauge Calibration – for all gauge types

However accurate these systems may appear, calibrating fuel gauges will give you peace of mind.

For best results, start the process with near empty tanks, Dip the tank with a stick to find out the level of fuel in the tank and note the gauge reading (take off 10% to ensure there is some in reserve when the indicator shows empty) Then add 10% of the tank capacity at a time and re-measure with the stick and record until the tan. This will give a reasonably accurate guide and take account of any shape issues. Once you have established the figures, make up a calibration guide and laminate it to keep on the boat.

Don’t Get Too Low

Now that you have an accurate idea of what your gauges are telling you, the temptation to use more fuel within the tanks before filling up. Don’t! Instead, keep tanks full whenever possible, because this prevents water build up and the dreaded fuel bug. Alsways allow 1/3rd safety margin when planning a passage and try not to let our tanks drop below 1/4 full. Remember that water and muck will always be at the bottom of our tanks and, ands when mixed with a small amount of fuel, will become an issue for the filters to deal with.

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