[Rhodes22-list] Magnetic or True?

Tue Apr 10 08:58:15 EDT 2007

Also valid.. I forget sometimes that sailboats are not power boats since
(dare I say it :) I frequently use both.
        But it all kind of relates to where you are standing. In general
terms ( a phrase that will always get you in trouble)..
Either by sail or power, You are trying to make a point. The difference with
sail is that you can sometimes head right for a point (accounting for drift
etc,etc etc ) and other times not. But you still have a conceptual point in
mind that the vessel needs to make. We could go on and on about the
difference between making a point and getting to a destination.. But I leave
that to the philosophers among us.


Bill Effros wrote:
> 
> 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
>>>
>>>
>>>
>>> ************************************** See what's free at
>>> http://www.aol.com.
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