Chartplotter heading vectors

Discussion in 'Electronics Q&A' started by Painkiller, Sep 9, 2019 at 10:47 PM.

  1. Painkiller

    Painkiller Member GOLD Sponsor

    May 18, 2015
    Grand Haven, MI
    2005 Sundancer 420
    Twin Cummins 6CTA 8.3
    I have a Raymarine C120 chartplotter, GPS, and radar system in my 2005 420. As you can see the picture showing the heading vector (red) and COG vector (green) are different. The boat in the chartplotter is pointed along the heading vector. However, the boat is actually heading in the COG vector direction. Is this deviation between the two vectors typical or is something wrong? I believe something is wrong for a number of reasons such as autocorrect on my autopilot takes me off actual course. Does anyone have an idea on what I need to do to correct such as recalibrate the system, call a professional, etc? Thanks.

  2. Jaybeaux

    Jaybeaux Well-Known Member SILVER Sponsor

    Jan 3, 2016
    Upper Potomac River
    420 Sundancer 2004
    Naught On Call
    Cummins 6CTA-8.3's with V-Drives
    9KW Onan Genset
    Variations in COG and Heading are "normal" and can be attributed to external forces such as wind or current. From your picture, I would assume that the wind that day was coming from your Starboard side.

    Have you ever seen a video of an airplane landing in a strong cross-wind? Pilots call it crabbing...

    Steve S and FootballFan like this.
  3. Woody

    Woody Well-Known Member PLATINUM Sponsor

    Nov 20, 2007
    N. Wisconsin/Lk Superior
    2005 420DA
    Cummins 6CTA8.3
    It's not always the same is it? I mean, some times the boat symbol/heading vector could be pointed to port, or anywhere between.

    The projected COG comes from your GPS(satellites). It knows this because it knows where you are now and where you were a bit before. It knows that's the direction your moving at that point in time but I does not know which way your boat is pointing, only which way it's moving.

    Your fluxgate compass properly orientated/installed knows which way you boat bow is pointed. It's the fluxgate that feeds the autopilot so correct steering inputs are made to keep you on course, like when you go to a waypoint.

    Sometimes the COG and heading should be the same or pretty close, like no wind, waves, swells, tides.

    It is possible though that your fluxgate needs to be calibrated/orientated, there's probably a procedure for that in the manual. I think it involve driving slowly around in big circles.
  4. Creekwood

    Creekwood Well-Known Member GOLD Sponsor

    Apr 26, 2009
    Oakville and Georgian Bay, Ontario
    '97 330 Sundancer, Raymarine C80 suite with radar, Mercury 310 Hypalon w/8hp Yammie 2stk
    2X 454 carbs w/ vDrives
    You can also align the compass to the COG. Firat recalibrate it and then align it to COG. Look it up in the manual. I can’t recall the steps.
  5. juggernaut1

    juggernaut1 Active Member

    Apr 19, 2015
    Perth, Western Australia
    2006 38 Sundancer
    8.1S / ZF 63V Drives
  6. Painkiller

    Painkiller Member GOLD Sponsor

    May 18, 2015
    Grand Haven, MI
    2005 Sundancer 420
    Twin Cummins 6CTA 8.3
    Thanks everyone. I will try to calibrate this weekend if weather permits. It just seems like the heading vector is always 20 degrees off no matter what the conditions are.

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