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

Hank hnw555 at gmail.com
Tue Apr 10 08:02:18 EDT 2007


Ed,

The deviation this article refers to is the difference between the actual
magnetic heading and the magnetic heading displayed on the boat's compass.
Compasses are affected by nearby metal, electrical currents or other such
things.  When swinging the boat to determine the deviation, you should have
all normal devices turned on so that the electrical interference most
closely duplicates what is found during normal sailing.

I started off my army career in avionics and this was something we had to do
with all of the aircraft on a periodic basis.

I think Bill was talking about the magnetic declination, which is the
difference between true north and magnetic north, since the true north pole
and the magnetic north pole are not in the exact same location, for most of
the world there will be a diffeence between the two.  So, if you plot a true
north heading and then sail a magnetic north heading without correcting for
the deviation, you will end up in the wrong place.

See http://en.wikipedia.org/wiki/North_pole for everything you wanted to
know but were afraid to ask.

Hank


On 4/9/07, Tootle <ekroposki at charter.net> wrote:
>
>
> 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
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>>
> >> __________________________________________________
> >> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >>
> >> __________________________________________________
> >> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >>
> >>
> > __________________________________________________
> > Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >
> >
>
> --
> View this message in context:
> http://www.nabble.com/New-Rodes-Owner-tf3523399.html#a9912156
> Sent from the Rhodes 22 mailing list archive at Nabble.com.
>
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list


More information about the Rhodes22-list mailing list