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    Re: Should all electronic navigation devices on a yacht be set to True or Magnetic?
    From: Lu Abel
    Date: 2009 Nov 21, 21:46 -0800
    By that reasoning, you would trust the oscilloscope blip-matching inherent in Loran-A with the automatic calculations of Loran-C or GPS?

    I don't mean to insult you, but the way electronic compasses do their compensation is to measure magnetic direction as the boat circles through 720 degrees.   Making just one reasonable assumption -- that the boat's angular velocity is constant -- a deviation table can be created.  It's not a whole lot different from the way one creates a deviation table or a Napier diagram the old fashioned way.

    Having said all that, I firmly believe that one should at least once do a manual deviation table.   Why?   Because the one thing that this method can't detect is whether the lubber line (whether on a traditional compass or its equivalent in an electronic compass) is aligned properly with the boat's keel.  On my otherwise high-quality boat, it wasn't until I created a deviation table that I discovered that my lubber line was out of alignment by a couple of degrees!   (with a steering-pedestal compass it's hard to see if the lubber line is aligned with the tip of the bow -- especially since there is this vital propulsion component called the "mast" in the way).

    For those unaware, if one creates a plot with 0-360 on the horizontal axis and deviation on the vertical, deviation should form a sine wave symmetric around the X-axis.   Vertical displacement means the lubber line isn't aligned with the keel.

    Greg Rudzinski wrote:
    The corrections aren't displayed or quantified in any way. I prefer a
    deviation table.
    On Nov 21, 8:21 pm, Lu Abel <lu...@abelhome.net> wrote:
    Greg Rudzinski wrote:
    New units today are suppose to
    automatically correct for deviation after circling a few times. A
    feature which I would find hard to trust.

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