NavList:

Thanks Lu

My compass does not have corrector magnets. It is one of those (new) old
fashioned flat top steering compasses. Dirigo, made in Seattle.

Nav aids are few and far between in my part of the world but I imagine I
could find a spot where there are some identifiable landmarks that I could
use.  I will have to get my chart out tonight and start plotting some
positions.

Your idea sounds a lot slicker than the way I have been doing it.
Regardless, I am getting deviation errors as much as 25 degrees. How this
can happen on a fibreglass boat is a mystery to me.  Perhaps the diesel
engine in the back is throwing out some fearsome magnetic fields. I have
also been told that GPS heading can sometime be off.

Robert

----- Original Message -----
From: "Lu Abel" <lunav@abelhome.net>
To: <NavList@fer3.com>
Sent: Sunday, July 23, 2006 1:08 PM
Subject: [NavList 876] Re: Deviation Card with GPS


>
> First and foremost, unless you're on a steel-hulled vessel, you should
> be able to reduce deviation to nearly zero with your compass's
> compensating magnets.  That does, of course, still require swinging your
> boat.
>
> The way to do it with GPS is to find a good spot on the water with
> navigation aids or other visible landmarks nicely around a 360 degree
> circle.  Using a chart, enter them as waypoints in your GPS, entering
> their latitude and longitude as accurately as possible (for floating
> aids, sailing past them and using the "Mark" button on your GPS is a
> great way to get exact coordinates).
>
> Now go to approximately your chosen spot (with GPS you don't have to be
> exactly there).  Bring up each of your landmarks on your GPS (there are
> a variety of ways to do this and they differ somewhat on each GPS).  Get
> the bearing from your current location to the landmark.  Point your boat
> at the landmark and read its compass.  Voila!  Deviation....   Repeat
> for other landmarks.
>
> This is a heck of a lot easier than the traditional methods of either
> anchoring at a precise spot and rigging a yoke so you can point your
> boat at various landmarks, or running courses between landmarks (where
> you have to worry about getting set sideways).
>
> One hint:   PLOT your deviation curve.  X-axis is 0-360 degrees heading,
> Y-axis is deviation.   It should look like a sine wave (again, except
> for steel boats).  If the axis of the sine wave is not at zero
> deviation, then your compass's lubber line is not aligned with your
> keel!   Many compass alignment books (or alignment chapters in more
> comprehensive navigation texts) for some reason do not mention this.
> When I followed this advice I discovered that my otherwise
> well-constructed 36' sloop came from the factory with the compass
> misaligned by two degrees.  Once I correctly aligned the lubber line I
> was able to get my deviation below 1 degree all the way around using my
> compass's compensating magnets.
>
> In addition to the usual chapters on compass adjusting in all the
> standard navigation textbooks, it is my understanding the the
> International Marine division of McGraw Hill will be publishing a
> "Captain's Quick Guide to Compass Adjusting" soon,  but I don't have a
> date.  It will be a waterproof fold-out guide, similar to their current
> Captain's Quick Guides" on other topics (
> http://books.mcgraw-hill.com/getdiscount.php?q=quickguides&template=im )
>
> Lu Abel
>
> Robert Eno wrote:
>> Anyone out there ever use their GPS to swing a steering compass.
>>
>> I'd like to hear about your experiences.
>>
>> Robert
>>
>> >
>
> >


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