NavList:

Hi everyone;

I thought that with so much discussion on this topic I aught to add my two
pence.

No one has said much about the compass instrument so far.  You need to buy a
compass with some care.



It is easy to be sold a compass intended for a power boat and then fit it in
a sailboat, or vice-versa . The result is that the damping effect of the
liquid fill is not correct for the motion of the boat, it will be more
sluggish or more lively than it should be. Also it some sea states the
resonant swing of the card may be 'tuned' to the motion and this would cause
a build up of movement with every wave making the card to swing wildly about
. Fortunately the effect usually only lasts for a few minutes.



It is most unwise to rely on the boat yard to set your compass. If they are
anything like the ones over here in the UK They have no knowledge about it
and will tell you anything to impress you. My last experience of this
resulted in a compass fitted with 5 deg of  misfit and a Loudspeaker on the
other side of the bulkhead. All courses were North!



Dip, A compass made for the southern ocean would be compensated for dip
which is in the completely wrong direction, i.e. away from the South
magnetic pole instead of towards the  North mag pole. Not a common fault but
it can happen if you buy secondhand. Elaborate correctors are necessary for
boats travelling both oceans.



The method of using a land mark with a bearing check using the GPS requires
that you have a steering compass designed for taking bearings. It needs to
be fitted either with a prism,as in a hand bearing compass, or have a Gnomen
or shadow stick in the center of the compass card. The compass also needs to
be mounted so that an all round (or as much as possible) view of the horizon
is available from across the compass. Standing on the opposite side of the
compass and looking across at the mark lets you see the Gnomon against the
scale on the card and the bearing noted. The Gnomen also allows you to use
the bearing of the Sun for checking .  Many power boats have only the
steering compass in the cabin and this cannot be used in such a way.



A single bearing observation can only gives a single reading of deviation
error and the error needs to be observed on N,NW,W,SW,S,SE,E,and NE courses
at least to plot a good deviation card. Once you have such a card then
single bearing checks are always a good idea. Using the deviation measured
also needs care as the deviation error is related to direction the boat is
pointing NOT the bearing direction measured.



Compass adjusting is an interesting science and I am surprised it doesn't
come up in this list more often.



Clive Sutherland.

----- Original Message -----
From: "Red" <hellosailor@verizon.net>
To: <NavList@fer3.com>
Sent: Tuesday, July 25, 2006 4:55 AM
Subject: [NavList 911] Re: Deviation Card with GPS


>
> Robert, how would human error make a compass SPIN ? (Without some active
> and
> creative helmsmanship.)
>
>
> >
>



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