Speed Sensor ..... - The J/109 Class Association Forum
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Most speedos can be calibrated. Just be sure to check several sources (GPS, other boats, measured mile, etc) to determine what's really correct before making any changes.
Depending on how "gummed" up the speedo is, I use a stiff bristled toothbrush to clean my speedo. Sometimes baby barnicles get rooted near the paddlewheel, so I take my fingetnail to work it lose. After that, and a recalibration, we're are generally OK. Not too glamorous is it!
Another approach is to remove the paddlewheel sensor after your in the slip and put it back in before you go sailing. Its a minor pain, and lets a small amount of water in, but beats having to clean the paddlewheel with your fingernail/toothbrush, and quickly becomes habit.