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    Re: digital compass, etc
    From: Richard B. Emerson
    Date: 2000 Sep 12, 6:35 AM

    FWIW, I can confirm that calibrating the fluxgate in an Autohelm (now
    Raytheon) autopilot is a two step process.  Specifically, I have a
    Raytheon Type 300 course computer and fluxgate kit connected to a Type
    2S linear drive.  As part of the setup, the computer is put into setup
    mode (surprise!) and the display is advanced through a number of
    options including drive type, etc.  In the "Swing Compass" page, the
    boat is taken through two turns at very low speed and, if completed
    properly, the computer displays a value called deviation.  In my case,
    that just happenes to be "2" although it's unclear if that's two
    degrees, two oersteds, or two bananas; from the documentation it's
    best to just view this number as a figure of merit.  If it's less than
    15, Raytheon says the value is acceptable, if it's above that level,
    it's time find out what's wrong.  Having established the deviation
    value, the user *can* (not "must") adjust the display to match the
    ship's compass.  Frankly, given the state of some compasses, this is a
    bit leap of faith.  Anyway, the point is, the adjustment isn't listed
    a requirement.
    
    The system also uses latitude and hemisphere (that is, enter a value
    for latitude and whether the boat is in the northern or southern
    hemisphere - these are two separate display pages in the process) to
    assess magnetic dip.
    
    Going back to the figure of merit, it took three tries to find the
    right place for the fluxgate.  The first location, located on a
    bulkhead near the galley gave a value of 45... until we removed the
    hand vacuum cleaner from the locker (oops!).  We couldn't get out of
    the low to mid 20's so we tried another location a but further forward
    with better but still not acceptable results.  It wasn't until the
    sensor was placed on a bulkhead outboard of the mast that we got the
    results given above.  The explanation is, I think, instructive.
    
    One With The Wind is a Baba 35 (you knew that, right) with a modified
    full keel.  I'd been lead to think the keel was weighted with lead but
    it's not.  The ballast is steel punchings, etc.  The initial locations
    were close to the bulk of the keel's mass and, therefore, magnetic
    influence from the steel.  Moving forward, towards the cutaway
    forefoot, meant the bulk of the steel was far enough away to reduce
    its influnce on the fluxgate (or to at least allow the computer to
    adjust the readings accordingly).
    
    Rick
    S/V One With The Wind, Baba 35
    

       
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