Sailor Cruiser Tech logo

Solved problems · Electronics · Crew answer

Why does my boat's speed log read zero or too low?

A paddlewheel that reads zero or too low is almost always fouled with growth or debris. Pulling the sensor briefly to clean it, using its blanking plug, usually fixes it.

Symptom

Speed through the water reads zero, much lower than GPS speed over ground in slack water, or jumps around. True wind data may also be wrong.

Likely cause

Paddlewheel sensors spin in the water flow under the hull. Barnacles, weed or debris stop or slow the wheel, especially after the boat sits. Worn bearings or a missing shaft, a loose connector, a damaged cable, or a calibration that was never set correctly also cause wrong readings.

Stop now if

Water comes in faster than you can stop it when the sensor is removed, the housing is cracked, or the blanking plug doesn't seat. Push the plug or sensor home, secure the cap nut and safety wire, and pump. Get help if it keeps leaking.

The fix, in short

  1. Compare with GPS in slack water. In calm water with no current, compare log speed with GPS speed. A large difference suggests fouling or calibration.

  2. Clean the paddlewheel. With the blanking plug ready, pull the sensor and insert the plug immediately. Clean the paddlewheel with mild detergent, check that it spins freely, then swap back quickly. Expect some water to come in.

  3. Secure it properly. Airmar warns that the valve alone isn't watertight. The insert or blanking plug must be fully seated, its O-rings intact and lubricated, and the safety wire attached. Check for leaks afterward.

  4. Check the connection. Airmar's test is to spin the paddlewheel with the sensor connected and see whether the display responds. No reading points to wiring or the sensor.

  5. Calibrate. Once it reads, calibrate it using the instrument's procedure, usually against GPS on runs in opposite directions.

  6. Use the blanking plug when idle. Leaving the blanking plug in while the boat sits keeps growth off the paddlewheel.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Instruments: Paddlewheel speed and temperature sensor, through-hull housing with blanking plug, instrument display

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

Members get the full step-by-step for their boat — join

This page is the short version. Members ask about their own boat and gear, get a guided fix matched to their equipment, and keep every case on file.

See membership plans More solved problems