[Rhodes22-list] Magnetic or True?

[Bill Effros]

Nope.

Not on a sailboat.

The GPS is measuring the direction the boat is actually traveling.  The 
steering compass measures the direction the boat is pointing.  All 
sailboats slip sideways to some extent due to current and wind. 

You can't simply take the number off the GPS and apply it to the 
steering compass.  You must create some sort of deviation table.  You 
must understand that the table will be different when sailing in 
different directions.  Thus, when sailing due north you may have to add 
5 degrees to the number shown on the GPS, and when sailing due south you 
may have to subtract 5 degrees.  This table will differ on every boat, 
and the corrections may not create any smooth curve.

It is possible to build the deviation table into the GPS, so that you 
can transfer the number directly from the GPS to the steering 
compass--but only if you understand navigation well enough to understand 
how to do that.

And that still does not truly solve your problem.  Because one device is 
constantly computing the direction to your destination without showing 
you how it derives that information, and the other device is telling you 
only which way you are pointing your boat, you are not traveling either 
the shortest or the fastest route.  Yes, you will eventually arrive at 
your destination, but you will probably have to turn your sailboat into 
a motorboat to do so.  Your method of navigation will force your boat, 
over time, more and more into the wind.

Racers understand that you can't just aim your boat at the next mark.  
This is even more true when the legs of your trip are substantially 
longer than racing around buoys.

If you are navigating home from any substantial distance you rarely 
expect to be able to point your sail boat in that direction and sail 
right to it, even if there are no obstructions in the way.  The fastest 
way home will be to point the boat in some other direction in such a way 
that your boat actually travels the shortest route possible.  Again, it 
is possible to adjust the GPS so it gives you some of this information, 
but not if you don't already know what I'm talking about.

If you're flying a jet, or piloting a motorboat you can overcome the 
forces that throw you off the shortest route home.  But if you're on a 
sailboat, and you are sailing, you must learn to calculate how to 
overcome these forces, or you will not sail the shortest/fastest route.

To me, that's what sailing is really about. 

Bill Effros





Brad Haslett wrote:
> Dave,
>
> Yup, that's about right.  It doesn't matter what electronic device you are
> using for guidance, if you are following an electronically generated lubber
> line you only need to steer relative to the track.  It doesn't matter what
> the variation is at your location or the deviation for your craft, or how
> the compass is marked.  It could be a Russian compass and marked in
> 'potatoes' - if it takes 10 potatoes to the left to hold course it takes 10
> potatoes to the left to hold course.  Now when the electronics go kaput, all
> bets are off.
>
> Brad
>
> On 4/10/07, DCLewis1 at aol.com <DCLewis1 at aol.com> wrote:
>   
>> Bill and Ed,
>>
>> I don't think it matters much if I were to try to sail a true heading as
>> magnetic, my chart plotting GPS  would straighten me out  quickly.  With a
>> chart
>> plotting GPS I enter  my destination/way points on the electronic charts
>> in
>> the GPS unit and  then  I  watch my track evolve in real time as I
>> sail.  Of
>> course, I always try to start on the right magnetic bearing, since I steer
>> via
>> compass, but it's not critical at all.  If I'm not headed where I want
>> to  be
>> headed relative to my desired way point, the plot on my chart plotting GPS
>> will tell me immediately and quantitatively.   I can look at the  track
>> display
>> that's automatically displayed (no transferring plot points)  and
>> instantly
>> know if my course is too high or low and with that crude info  I can make
>> a
>> steering adjustment, note the new reading on the compass,  and then hold
>> that new
>> compass reading.  I  never make the  calculation to go from degT to degM,
>> instead I just note I'm off  track and adapt.
>>
>> As to deviation and variance, I'm not sure it matters when   a compass is
>> used with a chart plotting GPS.  The compass becomes a  relative
>> instrument that
>> is used to hold a heading appropriate to keep on,  or near,  the desired
>> trip
>> track, that's it.  The numbers on the dial  are just a way to help you
>> keep a
>> heading - whatever that heading  really is, and that heading is ultimately
>> determined by your chart plotting  GPS.  Making progress to the next way
>> point is
>> what's important, and you  can see that evolve (or not evolve) on the GPS
>> display.  Steering  accurately by compass, as per John Vigor, is now
>> besides the
>> point - I use  the compass to hold a bearing that will keep me on my
>> desired
>> track (or get me  back to my desired track) and that track is monitored
>> accurately in real time by  my chart plotting GPS.
>>
>> And all of the above is accomplished without keeping my head
>> down  navigating
>> via GPS because the chart plotting GPS unit does all of the  calculations
>> and
>> plotting.  All I have to do is glance at the display  to see that I'm on
>> track or off track and if I'm off track what I have to do to  get back on
>> track.
>> If I want to know quantitatively how far off-track I  am, speed, or
>> whatever,
>> I have to change the display page - but that's pretty  quick & simple.
>>
>> And with GPS I always know where I am (Lat/Lon) much better than the
>> paper-plotters.  I don't think there's any question about that.
>>
>> Of course we should always know how to fall back to paper and pencil,  but
>> I
>> think that fall back is a long ways back.  I think a chart plotting  GPS
>> can
>> really be a very useful asset - and it can be an especially useful  asset
>> at
>> night or in inclement weather (low vs).
>>
>> Dave
>>
>>
>>
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>> http://www.aol.com.
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