[Rhodes22-list] Magnetic or True?

Bill Effros bill at effros.com
Mon Apr 9 19:44:49 EDT 2007


Thank you Michael.

Dave, Rory,

This is really my point.  You can plot either magnetic or true if you 
know what you are doing.  What a new sailor can't do is derive courses 
from a GPS plotting true and then try to sail the course steering by a 
magnetic compass.  Many people do just this.  They zig zag through the 
water, and grumble about why their boat keeps getting blown off course.

Our boats cruise at 3-4 kts. on a good day.  Steering off by just a few 
degrees will substantially lengthen the amount of time it takes to 
arrive at a given destination.

In my area, variation is 13.4 degrees.  (The compass rose of every chart 
will tell you the variation.)  That means that if I'm in Oyster Bay on 
the other side of Long Island Sound -- around 4nm -- sailing back to my 
mooring in Greenwich and my GPS is set to true, and I follow the course 
on my magnetic steering compass -- I'm going to wind up in Stamford.  A 
1 hour trip is going to take 2 hours.  I can't take advantage of the 
best wind and sail conditions, because my GPS seems to be telling me 
that I'm continually getting blown off course, when actually I'm aways 
pointing 13.4 degrees in the wrong direction and then compensating for 
my navigational error.

3-4 kts. is way too fast when you are learning navigation.  There isn't 
enough time to take all the readings you need to take.  You continually 
create your own chicken tracks with just a chart, a ruler, a pencil, and 
a decent hand bearing compass.  The difference between magnetic and true 
soon becomes obvious.  But as you do it more, you find you can sail and 
navigate at the same time.  The time flies.  You always know where you are.

Just read a story in March USBoat Magazine about some guys who read the 
wrong numbers out of their GPS and sent the coast guard to the wrong 
place.  And another about a guy with a GPS who plotted a course right 
over a 27 mile long Island that didn't show up on his outdated charts.  
Grounded the boat beyond repair.

I think we do new sailors a disservice by telling them to get a GPS and 
sail by the numbers before they understand what the numbers mean.

Bill Effros



michael meltzer wrote:
> You always plot true because the deviations card for your compass is
> dependent on the direction you are moving, plus variance does change
> generally updated yearly.
>
> -mjm
>
> -----Original Message-----
> From: rhodes22-list-bounces at rhodes22.org
> [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Bill Effros
> Sent: Saturday, April 07, 2007 3:07 PM
> To: The Rhodes 22 mail list
> Subject: [Rhodes22-list] Magnetic or True?
>
> Rory,
>
> Do you chart magnetic or true?
>
> Bill Effros
>
>
>
> Rory Orkin wrote:
>   
>> I think a charting GPS is the greatest navigational tool ever invented..
>>     
> It
>   
>> gives you an accurate fix instantly and reports your speed as well. If you
>> get one that interfaces with the radio it will broadcast your position in
>> the event of a Mayday.
>> Not likely but when you want it,  the best piece of information you can
>>     
> have
>   
>> is your exact position. The GPS is the best way to come up with that
>>     
> quickly
>   
>> and precisely
>> That being said I always use my paper chart book because it gives a much
>> larger perspective on where you are and where you want to go..not to
>>     
> mention
>   
>> giving a better overview on hazards..
>> The compass is also essential for an easy way to maintain a heading or to
>> tell someone else how to point the boat.
>>
>>
>> Frazier Scott wrote:
>>   
>>     
>>> I plan to have my new rhodes 22 this spring.  I will be sailing on Long
>>> Island Sound.  I will add a VHF radio and a compass but wonder what else
>>> by way of navigation equipment I should have.  A GPS seems to be
>>> advisable, but is it necessary?  What type of compass is best and where
>>> should it be mounted?
>>>   Any advice is helpful.  (Rummy, I expect to stock Mount Gay, so I have
>>> that covered).
>>>   Thanks,
>>>    
>>>   Frazier Scott
>>> __________________________________________________
>>> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>>
>>>
>>>     
>>>       
>>   
>>     
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