How to Troubleshoot a Boat Speedometer
Boat speedometers work by means of a tube called a "pitot" that sticks into the water, usually from the stern of the boat. As the boat speeds up, water is forced into the tube, which in turn puts pressure on the air in the tube. The speedometer gauge in the boat is calibrated to convert that air pressure into miles per hour. If your marine speedometer is displaying a speed that you think is too slow, if it seems stuck at the same speed or shows no speed at all, the problem is usually with the pitot.
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Instructions
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Check that the pitot is in the down position. Many pitots are designed with a spring-loaded release that will cause it to flip up if it hits an obstruction in the water. If the pitot is flipped up, push it back into the down position.
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Check that the pitot line is not kinked or broken anywhere between the stern and the instrument panel. Also check that there are no heavy objects sitting on or against the pitot line.
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Clear debris from the pitot. Detach the pitot line from the speedometer gauge and blow forcefully into it. This should remove any obstruction that may be blocking the pitot. Never blow into the pitot from the port, which is the end that sticks into the water.
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Clean the pitot port with a length of stiff wire if the pitot seems blocked but blowing on the gauge-end of the tube won't clear it.
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Remove the gauge itself for service. If the pitot is clear and properly installed, the gauge is probably defective and will need to be professionally serviced or replaced.
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Tips & Warnings
Most marine speedometers don't really measure low speeds accurately; to adequately test your instrument, you need to be traveling at planing speeds.
Newer marine speedometers use GPS coordinates to calculate speed and so are immune to pitot problems.
Never blow into the port end of the pitot; this will damage the gauge.
References
Resources
- Photo Credit speedometer image by AGITA LEIMANE from Fotolia.com
