In this section you will learn about:

• Function of the Anemometer
• Definition of Apparent wind and how it's measured
• Definition of True Water wind and how it's measured
• Definition of True Ground wind and how it's measured
• Video lesson supporting and comparing the above
• Ultra Sonic Anemometers
• Barometers
• Instancing Considerations

Anemometers.

Anemometers measure wind speed and direction and convert this information into an analogue signal that is then further converted into a recognised standard such as NMEA 0183 and NMEA 2000.

Wind information can be displayed in a number of ways:

Apparent wind

Apparent wind is what you feel on deck and what your sails are subjected to, it is the raw wind un corrected as measured at the anemometer. As you go faster the apparent windspeed increases and the direction moves forward. As you slow down the apparent wind decreases and the direction moves aft. This wind comprised of the combined forces of the true wind and the boat's movement.

True wind (Water)

True water wind is calculated from the apparent wind and boat speed through the water. It corrects for the water speed of the vessel by comparing the apparent wind to the water speed from the Paddle wheel speed transducer. A wind gauge set to display True wind shows a wind speed and direction you would experience if you were drifting with the tide, bot not making any progress through the water.

True wind (ground)

True Ground wind compares the apparent wind to the course over ground and speed over ground data from the GNSS. It shows the wind speed and direction you would experience if you were anchored, or in any way attached to land.

Ultra Sonic Anemometers

There are a small number of solid state anemometers that can measure wind speed and direction, but also a number of other parameters making the device into a sort of on board weather station. This device from Airmar can, in conjunction with standard wind speed and direction, measure barometric pressure, humidity and temperature.

Barometric Pressure

If you already have wind information and you would like to have air pressure, there are a number of standalone NMEA 2000 barometers that can be useful as they can log the pressure over a period of time. This is a really handy feature as you can rewind back through this log and see what's been happening prior to setting off on a voyage. With additional hardware you can monitor this information remotely too. The device shown below is from Yacht Devices and simply connects into the NMEA 2000 Data Bus, with no additional sensors or wiring required.

Instancing considerations

It is unusual to have more than one Anemometer on board, but not unheard of, often this is more to do with redundancy than anything else. Of course, if there are more than one - device instancing must be considered.

The usual instancing protocols apply with forward sensors numbered from zero increasing further aft, and port side devices numbered from zero increasing further to port. The usual considerations apply when it comes to displaying multiple sources of the same data, NMEA 0183 does not support any form of instancing, neither does the old Seatalk 1 system. For multiple versions of the same type NMEA 2000 is the only system, and even then not all MFD's will support the displaying of multiple wind sources.

If the purpose of multiple sensors is purely for redundancy, viewing both simultaneously is not a requirement, so simply having the spare anemometer connected and 'instanced' correctly means it can be easily selected from a list of available devices should the primary device fail.