MacSailing.net

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.

I use FileMaker as my log system and get it to query MacENC on demand. I have FileMaker display true wind speed and direction calculated from the data available.

I can let you have the trig calculations as FileMaker formulae if you want.

regards, Tony

I also got this working, using an Arduino to calculate true wind speed and direction and speed upwind. The program uses RMC from a GPS unit to provide SOG, COG, and Magnetic Variation, MWV from a wind instrument to provide apparent wind speed and direction relative to the boat heading, and compass input (from an electronic tilt-compensated compass) to provide magnetic heading. The Arduino then computes the true wind speed and direction (relative to earth, not the water) in a second MWV sentence, and true and apparent heading and speed upwind in a VHW sentence. All this is displayed in MacENC and iNavX automatically.
Happy to provide the details for anyone interested.
Eric

Hi Eric Frank

This sounds very interesting. Do you have the Arduino code for this?
I have all the nmea inputs needed and multiplexed to wifi. True vind unfortunatly isn´t computed anywhere in my system.
But in my plotter I can choose to output RMC+MWV+HDG seperatly - feed into Arduino and back to the multiplexer-
If I have a code :-)
Hope to hear from you
Cheers

Ebbe

Shipmodul (http://www.shipmodul.com/en/index.html) converts NMEA sentences commonly found in chart plotters to TRUE with their software.