How to Calibrate Boat Gauges

Why it's not always smart to trust your gauges.

March 24, 2009

Forget about politicians, used car salesmen, and stockbrokers–if your boat is bigger than a canoe and smaller than the QEII, chances are it’s lying to you right now. Gauges on boats are notorious for being inaccurate. They often give bogus readings or fail completely, and even expensive models can’t always be counted on to provide reliable information for very long. Sick and tired of getting stuck with counterfeit claims? Check out these instruments, and learn how to keep them honest.

Fuel Gauges
Often misinform boaters. In many cases this is true from the day a boat leaves the factory. Why? Because marine fuel tanks often have V-shaped bottoms so they can fit into a boat’s hull. Because of that shape, when the sender’s float is halfway down, there’s actually only a third or less of a tank of fuel remaining. Also, consider the wear and tear a marine fuel sender endures, as opposed to one installed in an automobile. Constant pounding, rocking, and rolling means that the float and arm never sit still, but instead are always being worked up and down, and back and forth. If you want an accurate reading on your fuel tank, replace your gauge and sender with solid state instruments such as Racor’s FS240 series ($153.55, Your other option? Install a sight gauge-it’s the ultimate in idiot-proof.

Can also be way off the mark. The root of this problem lies in the sender, more than the gauge itself. Many speedos rely on paddlewheel transducers to provide a reading, and those paddlewheels-which usually aren’t accurate below 10 mph-are easily thrown off kilter by marine growth or seaweed bits that lodge in the assembly. Others use a pitot tube to measure speed. These are usually more accurate than paddlewheels, at least until the tube gets clogged. Also, a gauge that uses a pitot can be fouled if the tube running to it gets a kink or sharp bend in it. There are only two sure ways to know your exact speed on a boat: GPS or Raymarine’s Ultrasonic Speed Sensor. If your dash doesn’t allow for a chartplotter at the helm, you may have room for a dedicated GPS speedo such as the ones made by Livorsi ($230 to $400; or a multifunction digital gauge such as those made by marine electronics manufacturers including Garmin, Furuno, and Raymarine. Or just bring aboard your handheld.


Provide bogus readings to boaters each and every day. Most used in stern drive or inboard applications are made for use with four-, six-, or eight-cylinder engines, and a tiny selector switch on the back of the gauge must be set for the proper number of cylinders. You’d be surprised at how many boats leave the factory without this simple adjustment being made-or at how often that switch gets bumped out of place by accident. Even when the switch is set properly, grime collects on the terminals over time, which can eventually cause faulty readings. If your gauge starts acting weird, try clicking the switch back and forth a few times to clean off the contacts. Often, this is enough to get it working properly again.

Counterfeit Readings
Can be caused in any type of gauge by three problems. Check the ground wire first. Then consider moisture, which leads to sticky needles and corroded internal parts. If you see beads of moisture inside a gauge, it probably won’t last much longer, but you may get by for a while by using the time-honored technique of tapping the glass. The final culprit is a power surge. This can be caused by anything from bad wiring to a lightning strike, prompting your gauges to work erratically or not at all. In either case they need to be replaced. Once they’re fried by a surge, they’ll be compulsive liars forever.


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