About rotating masts, angle detection of the mast, and deploying LCJ Capteurs wind sensors
Article MT24046, January 29, 2016
If a sail boat's mast rotates, a wind sensor must know the angle of rotation in order to report the correct wind angle. Hardware for judging the mast angle is available.
Background information about rotating masts
While tacking to changing wind conditions, rotating the mast into the wind clarifies the leading edge of the airfoil and minimizes turbulence. This can improve sail performance 10% or more compared to the fixed mast configuration seen in convential sailing boats.
However, a rotating mast introduces a complication for measuring wind angle: a wind sensor measures angle with respect to the mast center-line so any rotation of the mast by X degrees shifts the sensor's reading by X degrees. To correct for this, the mast angle can be sensed, quantified, and then that rotation angle used in a final calculation of wind angle. As a practical matter, precision measurement of mast rotation and associated re-calculation of wind angle may be argued to be "over-kill". After all, a mast rotation is typically slight (perhaps just 3 or 4 degrees) and the resulting degree shift in s wind angle calculation may be within a range of error similar to other systematic errors in data from a flux gate compass reading, instantaneous speed, tilt and trim angle, etc. Given the several data error-bars varying from moment to moment, some sailors simply do mental math to guess the mast rotation's effect on the wind angle reading coming from the sensor.
But assuming there is a need for as much certainty as possible and to reduce guesswork, there are hardware methods of measuring mast rotation and then feeding that factoring that data into a wind angle calculation.
Different hardware to measure mast angle
- Potentiometer -- Inexpensive, simple to wire. Analog output unless converted to NMEA data. Potentiometers are usually limited to much less than a full 360 degree rotation.
- Rotary Encoder -- Digital output, highly accurate, no limit on rotations. Larger, initial setup more complicated.
Typical equipment and experience
Most commonly, the sensors used for rotating masts are the same as the ones used for rudder feedback. All marine electronic brands have these in their catalog as part of their auto-pilot kits. These sensors are generally potentiometers and the input is designed for rotating masts (rather than generalized for any category of hardware). The sensor's calibration is done in the same way as it is for a rudder: port-end, starboard-end and middle. Fugawi Marine 5 often receives data from a LCJ Capteur CV7-C-RM ultrasonic wind sensor and such a unit receives mast angle data from a potentiometer at the base of the mast.