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breezetrees

decimal places of speed through the water?

By breezetrees on Mar 9, 2009 at 4:51 PM 4 Vote 1 Vote

Hi,

Can anyone tell me how many decimal places the DST 800 reports speed in knots?

For driving a sailboat to target speeds hundreths of a knot can be useful for telling if you are speeding up or slowing down, even if you could never calibrate that accurately.

I was told once that the NMEA 0183 DST 800 only outputs speed to tenths of a knot, can anyone confirm this? Do the NMEA 2000 messages have more decimal places (I don't know if the native units are even knots)? Are all the nmea 2000 speed transducers the same? Spec sheets always tell how many decimal places the temperature is measured to and never the speed.

Thanks,
-Mike

4 Replies

  • Ben E
    Ben E | March 9, 2009 6:45 PM

    I'm pretty sure that the speed data coming from the Maretron DST 100 (which is very similar to the Airmar N2K DST 800) at least presents itself with several decimal points of precision. But I don't know if that "precision" is meaningful, even in terms of trend, and I don't think many, if any, displays are willing to express it in more than one decimal point. I'll check further on my test gear one day soon.

  • Russ
    Russ | March 19, 2009 9:18 AM

    I have a DST800 and my Furuno instruments will display hundredths of a knot, but I have no idea if the data is any good.

  • Mistaken
    Mistaken | May 5, 2009 3:51 PM

    I use the Maretron DST 100 which feeds data into my B&G Hercules system - the hundredths of a knot works very well especially from a relative speed perspective. Within the B&G system I can damp it down using various averaging parameters...

    Highly recommended for racing sailboats...

  • via e-mail
    via e-mail | May 6, 2009 6:06 PM

    From Kees (who is a good man, but whose comments somehow mess up the forums ;-)

    Hi Breezetree,

    The NMEA 2000 PGN 128259 “Speed” has a 16 bit value containing the boat speed in m/s in 1 cm resolution.

    I happen to have received my N2K Airmar DST-800 this week, and am happy to report that it indeed does send out values at 1 cm per second. At very low speed there is a bit of irregularity (quantization); the values I saw on the bus were (determined by slowly spinning the sensor manually!)

    0.00 m/s
    0.07 m/s
    0.15 m/s
    0.22 m/s
    0.25 m/s
    0.28 m/s
    0.30 m/s
    0.33 m/s
    0.34 m/s
    after that I saw pretty much any value. It reports speed once per second, log and trip once per second and temperature twice per second (with each of the previous).

    Note that 1 m/s = 3.6 km/h = 1.94 knots so the minimum resolution is about 0.02 knots.

    Hope this helps!