[Rhodes22-list] TVMDC

[Bill Effros]

Slim,

Good Start.  Now, as Bob suggests, it's time to put the compass on the 
boat where it will be when you are using it.

The first thing you must understand is that your compass doesn't tell 
you the direction of your street.  You learn that from your charts, and 
increasingly from maps as well.  You use a steering compass to determine 
which way to point your car or boat on the street or in the water if you 
want to go to Lake Superior.

Your steering compass cannot be kept "true".  Instead, you must know how 
to compensate for the errors that are particular to your compass and 
location.

Timid Virgins Make Dull Companions is the way I learned the mnemonic 
device for remembering the formula for adjusting the output of a compass 
to the grid of a chart.  (Some people remember "TV Makes Dopey Children")

Here's what the letters stand for:

T=True
V=Variation
M=Magnetic
D=Deviation
C=Course

You add and subtract variables inserted in TVMDC to determine unknowns.

So far, all you've got is M--Magnetic.  You look at your magnetic 
compass, and it kicks out a number.  You already know that number isn't 
what you expect when you walk down your street.

The next thing you've got to do is determine D--Deviation.  I like to 
mount my steering compass in the grooves of the companionway because it 
is far away from magnetic metal and electromagnets, even though it is a 
pain to step over.  It is also quite true to the direction the boat is 
pointing--thank you Stan.

In any event, you must pick the place you will mount your compass, and 
put your compass in that place.

Then you are ready for the next lesson -- constructing a Deviation 
Table, so you can fill in variable "D".

Bill Effros


Bob Weber wrote:

> Slim, now do the same on the boat and you got it SWUNG.  Bob
>
>> From: Steve Alm <salm at mn.rr.com>
>> Reply-To: The Rhodes 22 mail list <rhodes22-list at rhodes22.org>
>> To: Rhodes <rhodes22-list at rhodes22.org>
>> Subject: Re: [Rhodes22-list] Compasses
>> Date: Fri, 08 Apr 2005 15:44:10 -0500
>>
>> Step one checks out.  I took a full sheet of newspaper and creased it in
>> half both ways and diagonally and drew lines with a straight edge.  
>> Since my
>> compass is still mounted on the splash board, it's an easy and sure 
>> way to
>> line the board up on the lines on the paper.  I laid the newspaper 
>> out on my
>> wooden dining room table and went to work on the adjustments. After 
>> about 35
>> minutes of fiddling, I got it to be consistent on all lines (8 
>> positions) to
>> within a half of a degree.  Then I repeated the test in other rooms 
>> of the
>> house and basement and out on the front walk.  Results were 
>> consistent, so
>> my compass is working.  However, I always thought my house and the 
>> street in
>> front ran exactly north and south but the compass is saying it's a few
>> degrees off.  And maybe the street isn't exactly north and south but 
>> I don't
>> have a second compass to cross reference.  I have a couple small 
>> hand-helds
>> on the boat but not here at home.
>>
>> Slim
>>
>> On 4/8/05 11:32 AM, "Bill Effros" <bill at effros.com> wrote:
>>
>> > Slim,
>> >
>> > From comments sent previously, you can infer the problems manifest in
>> > all compasses: they will all kick out numbers, but there is no fixed
>> > relationship between any two numbers until you discover what that
>> > relationship is.
>> >
>> > Let's start with your compass.  First rotate it 360°, slowly.  Is it
>> > capable of stopping at any number?  If the magnet has been badly
>> > damaged, it will jump over some numbers, no matter which way you point
>> > it.  If that's the case, you will not be able to use this compass
>> > because you will never be able to find another compass that makes the
>> > same mistakes in exactly the same way.  Consistency is the critical
>> > function of any compass.
>> >
>> > Assuming that your compass is capable of pointing in 72 different
>> > directions (5° intervals) you must next determine if it is consistent.
>> > Point the compass so it reads 0°.  Make a line in the sand.  Turn the
>> > compass 180°, and write down what number the compass points to.  
>> Turn it
>> > back to 0° using your line in the sand.  Does it say 0°?
>> >
>> > Please note, if the compass is working properly, the compass card 
>> should
>> > not move at all -- only the housing and the board you may have mounted
>> > it on should move.  It may not be pointing in a predetermined 
>> direction,
>> > but it should be pointing in the same direction all the time.  That's
>> > the way properly functioning compasses work.
>> >
>> > Using a protractor, draw a line in the sand at a 90° angle 
>> intersecting
>> > your first line.  Put your compass on this new line, first one way, 
>> then
>> > the other.  It should read 90° and 270°.  If it doesn't, you've got a
>> > problem.  Make more lines at 45° angles.  The compass card should not
>> > move.  Your computed angles should equal what you see on your compass.
>> >
>> > If they do not, someone may have "corrected" your compass.  The
>> > adjusting magnets, built into the housing of your compass, must be
>> > pulling your compass card in different directions as you rotate the
>> > housing around your compass magnet. Try to twiddle with them, so that
>> > they don't change the orientation of your compass card when you rotate
>> > them around.  If you can't make this happen, it's time for a new 
>> compass.
>> >
>> > If your compass is consistent, it's time to mount it on your boat, and
>> > figure out what direction it's actually pointing.
>> >
>> > Timid Virgins Make Dull Companions.  Lesson II.
>> >
>> > Bill Effros
>> >
>> >
>> >
>> > __________________________________________________
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>>
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