January 29, 2014 09:54
How does MacENC compute true wind info from the apparent wind info it gets from my Navman 3150 wind indicator? That unit sends out NMEA ($IIVWR,001,L,6.79,N,3.49,M,12.58,K and $IIMWV,000,R,6.79,N,A)
which is the apparent version, with reference to the boat heading (0 degrees). The instrument itself also displays "true" wind info if it receives the boat speed (I give it the RMC sentence that contains SOG). But doesn't this assume that the boat is actually going in the same direction as it is pointed (i.e. its "heading")? That is a good approximation, but does not account for cross currents or leeway. Shouldn't we use the GPS COG as the reference, which is the actual direction the boat is moving relative to the earth?
I'm not sure about this, because apparent wind direction relative to the boat is obviously useful for sail trim. But to estimate one's speed upwind (for example) it would seem to be important to relate wind info to the actual movement of the boat relative to earth. That should be easy by using COG from the GPS. Does anyone do that?
BTW, there is a good description of the trig needed to convert apparent to true wind info on the US Sailing website (http://www.sailingcourse.com/keelboat/tr...ulator.htm) if anyone needs a refresher (I did).
I got into this playing with an Arduino computer connected to a GPS source and a tilt-compensated magnetometer to measure the ship's heading, but then realized I didn't understand whether to use COG or not.
Comments are very welcome.
which is the apparent version, with reference to the boat heading (0 degrees). The instrument itself also displays "true" wind info if it receives the boat speed (I give it the RMC sentence that contains SOG). But doesn't this assume that the boat is actually going in the same direction as it is pointed (i.e. its "heading")? That is a good approximation, but does not account for cross currents or leeway. Shouldn't we use the GPS COG as the reference, which is the actual direction the boat is moving relative to the earth?
I'm not sure about this, because apparent wind direction relative to the boat is obviously useful for sail trim. But to estimate one's speed upwind (for example) it would seem to be important to relate wind info to the actual movement of the boat relative to earth. That should be easy by using COG from the GPS. Does anyone do that?
BTW, there is a good description of the trig needed to convert apparent to true wind info on the US Sailing website (http://www.sailingcourse.com/keelboat/tr...ulator.htm) if anyone needs a refresher (I did).
I got into this playing with an Arduino computer connected to a GPS source and a tilt-compensated magnetometer to measure the ship's heading, but then realized I didn't understand whether to use COG or not.
Comments are very welcome.