[Rhodes22-list] Magnetic or True? By a famous nautical author... Other than Bill Effros

[Tootle]

Bill:

Is this what you want to talk about:

"Excerpts from The Practical Encyclopedia of Boating 

by John Vigor

If you wish to steer accurate courses by compass, you must know the
compass’s deviation, or the amount by which it deviates from the magnetic
course. Deviation is complicated — it changes as you swing the boat around
and varies for every course, so it’s convenient to make up a graph or a
table that can be displayed on a deviation card for quick reference.

The process for calculating deviation is know as “swinging the ship.” All it
involves is measuring the difference between a known magnetic bearing and
what your steering compass says the bearing is. These differences are
measured every 15 to 45 degrees while the boat is swung around the clock in
a tight circle.

The results are displayed on a deviation card, and tell you at a glance what
compass course to steer for any given magnetic course, which you can either
read straight off a magnetic compass rose on the chart or calculate for
yourself from a true course.

It’s not difficult to make your own deviation card, and it’s a good skill to
learn because deviation usually changes with time. All you need is an
inexpensive pelorus from a marine hardware store and a modicum of patience
while you swing the ship. There is no great mathematical skill involved,
just simple addition and subtraction. 

There are several other ways to check for deviation and many good books that
describe the methods, but Chapman’s Piloting, Seamanship & Small Boat
Handling is the old standby, providing as much detail as you’re ever likely
to need.

Some small boat compasses will introduce a surprising degree of error into
courses steered if they are not checked for deviation — as much as 20
degrees is quite common. They must, therefore, be compensated, or adjusted
so that deviation is reduced as much as possible — that is, within about 5
or 6 degrees.

There are professional compass adjusters who can do this for you, but you
can also do it yourself by means of the adjustable external magnets found on
most marine compasses. It’s not black magic, and you can hardly do any
permanent damage to the compass during your experimentation. If you are
planning to cross an ocean, however, it might be wise to call an expert,
just for peace of mind. Once your compass is corrected, keep it free of
nearby magnetic influences such as anchors, binoculars, and steel beer cans.

John Vigor’s book, Practical Encyclopedia of Boating, is available from the
Good Old Boat Bookshelf at $29.95; 352 pages (hardcover). "

Ed K
Greenville, SC, USA



Bill Effros wrote:
> 
> 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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>>   
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> 

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