jeffies, one at a time

Magnetic compasses can
certainly be accurate to 2 degrees,

we were discussing recreational sailboats here, so let's stay with the magnetic
compasses one would find on such.

but in the
old
days it was pretty common to have it professional done, and to check it with
known ranges at every opportunity.

and read by an amateur in a moving boat under at sea conditions. and how many
compasses on the boats under discussion could *you* read to 2* or even 5*?

All of them right, because you read it in a book.

until this very minute you didn't know that a compass rotates one way as th
boat goes up a wave and rotates the other way going down a wave. Same thing as
the boat rolls one way and then the other.


Holding a course to 2 degrees may be difficult for a long period, but for the
short time needed for a bearing is not too hard.

ever helmed a boat, jeffies?


And errors multiply??? That's one of the stupidest things you ever come out
with jaxie! Are you claiming that a 5 degree compass error and a 5 degree
course error yield a net 25 degree error??? I think that one's another
keeper!

jeffies, you stupid cluck. YOU claim to have a degree in physics (okay, an
arts degree and from Potato State), and if you were even qualified to take
freshman courses in the subject you wouldn't make such a dumb statement.

So, let's start by admiting you lied about what you know and what you should
know.

Then, consider this, jeffies. why is it you think a 2* error here and a 5*
error that makes for a 3* to 7* total? It does not.

Now follow this. I'll use % of error rather than * because it makes it easier
for a rank beginner math person like you.

a 2% error means the actual might be anywhere from 98% to 102%. Understand?

Now bring on the next error, of 5%. you do NOT take the 5% error against the
original 100% but rather against the 98% to 102%.

THIS MEANS you take 95% to 105% against the errored 98% to 102%. Giving you a
potential error of 0.95 times 0.98, or just over 93% on the lower side and 1.02
times 1.05 or just over 7% on the upper side.

The incremental error is small in this case because the original errors were
small AND there were just two errors to cascade. EACH succeding error is
MULTIPLIED by the total of the previous errors. A string of four or five
errors, each small, can make for a highly uncertain result.

In the case of the radio compass, yo have the error of reading the mag compass
by the helmsman, the error of maybe forgeting variation, the error in the
compass, the error of maybe local deviation, the error of the compassman in
placing the compass, the error of the helmsman in holding course, the error in
the compassman's listening to and hearding the nullness of the signal, the
error of alignment of the compass dial with the antenna, and the error of
reading the dial after the reading was taken.

Now, you HAVE AT LEAST TWO READINGS to take, *****each**** with potential
errors above.

In addition, you have the error of the boat movement, which can only be
estimated (no gps onboard, remember?)

In addition, you have more -- and potentially large -- errors if the two read
stations at not at right angles to the boat.

And why does RDF calibration depend on the ship's compass? There are others
ways to check for deviation that don't depend on the compass.

the discussion was a recreational sailboat.


"JAXAshby" wrote in message
...
shen, think about it for a minute. *Just ONE* of the issues is that a
radio
compass is aligned using a magnetic compass.

think about it? how many mag compasses have seen on boats that are
accurate
to
2*?

Think about it more. How many helmsmen on sailboats have you seen that
could
hold a course to 2* over any period of time?

Think about it. the error of the mag compass reading TIMES the error of
the
helmsman gives the potential error UP TO THAT POINT. Already, you are
waaaaaaaaaay beyond 2*. waaaaaaaaaaaaaaay beyond.

Think about the additional errors associated with just the radio compass
itself
and its usage.

think about it, shen.

Think.

....And you haven't come up with one fact to say it can't be better than 2
degrees, other than some dumb comparison to taking a bearing on sounds (a
known
no-no), and have shown you don't know the basics of piloting, when using a
compass.

Shen