Speed log and wind instruments
Navigation
A speed log measures your speed through the water, usually with a small paddlewheel under the hull. Wind instruments measure wind speed and angle at the masthead. With GPS and a compass, they let a sailor see true wind and how well the boat is sailing.
How it works
A paddlewheel spins as water flows past, and the instrument turns those spins into knots, so it has to be calibrated for each hull. Speed through the water differs from GPS speed over ground by the current. A masthead sensor measures apparent wind, the wind felt on the moving boat. True wind is calculated from apparent wind and boat speed; true wind direction also needs a heading source.
Using it
- Once the log is calibrated, the difference between log speed and GPS speed is mostly current.
- Calibrate the log from the display's calibration menu; one maker's system prompts you to run at a steady speed and warns if the wheel isn't turning.
- If the masthead sensor doesn't point exactly along the centerline, enter a wind angle offset in the display's settings.
- True wind is only as good as the boat speed and heading feeding it; a badly calibrated log gives wrong true wind.
- Wireless masthead sensors need a clear line of sight to the display; one maker lists masts, booms, rigging and sails as things that weaken the signal.
Looking after it
- Paddlewheels foul quickly. Many can be pulled from inside the boat and swapped for a blanking plug to clean them; practise it, because some water comes in (one maker says about 10 oz with practice).
- Keep the speed insert's O-rings intact and lubricated, and always refit the safety wire or cap afterwards, as the maker instructs.
- Antifoul the paddlewheel only with water-based paint made for transducers.
- Inspect wind sensor cables and connectors for damage or corrosion. On wireless sensors, check the battery compartment for corrosion and clean the solar panel.
Troubleshooting
Boat speed reads zero or too low
- Check the paddlewheel isn't jammed with growth; pull it and clean it.
- If the wheel turns freely, check the cable connections.
- Recalibrate once it's reading.
Wind reading missing or wrong
- Check the vane and cups move freely.
- Check the wind angle offset matches how the sensor is mounted.
- Check network cables and T-connectors, look for the sensor in the NMEA 2000 device list, and power-cycle the network.
- For a wireless sensor, check its battery, solar panel, and any new obstruction between it and the display.
True wind looks wrong while apparent wind looks right
- Check the speed log is reading and calibrated.
- Check a heading source is connected if you need true wind direction.
Buying a boat that has one
- Ask whether the blanking plug for the speed sensor is aboard; you need it to clean the paddlewheel afloat.
- Check wind sensor cables and connectors for corrosion or damage.
- Older instrument systems may use a maker's own network; check how they'll connect to newer displays before upgrading one piece.
Sources (6)
www8.garmin.com · www8.garmin.com · support.garmin.com · support.garmin.com · airmarweb.com · raymarine.com
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.