The first answer was the closest. Yes there is a difference and yes in most
compasses including the one's we find on our sailboats and powerboats are,
in fact, different. Some compasses, such as the one on USNS Bob Hope, are
built to be self adjusting when crossing the line. On a previous ship one
of our main bridge compasses (magnetic not gyro) showed a marked tilt some
degrees south of the equator. It was recalibrated in Darwin, Australia.
Great seafood buffet place down there by the way. Smaller less expensive
compasses do not have this ability for self adjustment and this includes
those in the many hundreds of dollars category. No need since most boats
don't make it across the equator one way or another. The easiest way to
solve the issue is order up a compass from some company in Australia, NZ
etc. and have it airmailed. Get the exact same compass you already have.
When crossing the equatorial region at some point you will observe the tilt
or dip. Switch compasses and re-swing. Not hard to do even way out there.
In fact I think it was Ole Thom who first gave me an explanation on how to
determine exact bearings out of sight of land. This phenomenae does not
happen exactly at the equator. The reason it happens is the distance from
the mass of iron in N. Canada that is the Magnetic Northern Pole increases
while the mass of earth's iron making up the Magnetic Southern Pole
decreases in distance. At some point one becomes the major attraction to ye
olde compass instead of the other and it becomes off-balance. Here's the
humorous part. When the compass on our ship acted up none of the highly
trained and experienced deck officers realized what had happened. One of
the other AB's remarked to me, "Southern Dip?" "For sure", sez I. Finally
we were asked what we meant by that phrase. They didn't believe us and had
to look it up in Bowditch. Score one for the home team!

Three more days and we're outta here. OT is running 4 plus hours a day and
tomorrow for T-Day we will work about 12. You all enjoy the turkey . .. I'm
loving the paycheck!

Here's one for you. The new Chief Mate on board has the unlikely name of
Despot.She just finished being Captain of a smaller vessel and is now
qualifying in the big ones. Yes, I did say 'she'. Imagine "Captain Despot"
being your boss. Haggie, Katy, LP don't worry about ole Neale . . . .Your
gender is more than holding it's own in the commercial mariner's world. The
best part is it's no longer necessary out here for someone to say, "How do
you find working for a woman?" The question never comes up. But it's kinda
fun knowing I work for a 'Despot'.

Few more days and I'll talk to you all in two months or so . . . .

MST

Great and informative post. Thanks. It's nice to see some
people around here are more interested in providing in-
formation instead of only trying to belittle and build up
their own sagging egos (Oz).

Your observations pretty much confirm my suspicions
about compasses and nobody's mentioning dip in their
world cruises. It also confirms what I know about deck
officers like those who frequent this group - they know
little about what we sailors know instinctively because
we are so close to the water and the real world in
comparison.

Fair winds to ya. Enjoy your mission.

S.Simon


"Schoonertrash" wrote in message ...
The first answer was the closest. Yes there is a difference and yes in most
compasses including the one's we find on our sailboats and powerboats are,
in fact, different. Some compasses, such as the one on USNS Bob Hope, are
built to be self adjusting when crossing the line. On a previous ship one
of our main bridge compasses (magnetic not gyro) showed a marked tilt some
degrees south of the equator. It was recalibrated in Darwin, Australia.
Great seafood buffet place down there by the way. Smaller less expensive
compasses do not have this ability for self adjustment and this includes
those in the many hundreds of dollars category. No need since most boats
don't make it across the equator one way or another. The easiest way to
solve the issue is order up a compass from some company in Australia, NZ
etc. and have it airmailed. Get the exact same compass you already have.
When crossing the equatorial region at some point you will observe the tilt
or dip. Switch compasses and re-swing. Not hard to do even way out there.
In fact I think it was Ole Thom who first gave me an explanation on how to
determine exact bearings out of sight of land. This phenomenae does not
happen exactly at the equator. The reason it happens is the distance from
the mass of iron in N. Canada that is the Magnetic Northern Pole increases
while the mass of earth's iron making up the Magnetic Southern Pole
decreases in distance. At some point one becomes the major attraction to ye
olde compass instead of the other and it becomes off-balance. Here's the
humorous part. When the compass on our ship acted up none of the highly
trained and experienced deck officers realized what had happened. One of
the other AB's remarked to me, "Southern Dip?" "For sure", sez I. Finally
we were asked what we meant by that phrase. They didn't believe us and had
to look it up in Bowditch. Score one for the home team!

Three more days and we're outta here. OT is running 4 plus hours a day and
tomorrow for T-Day we will work about 12. You all enjoy the turkey . .. I'm
loving the paycheck!

Here's one for you. The new Chief Mate on board has the unlikely name of
Despot.She just finished being Captain of a smaller vessel and is now
qualifying in the big ones. Yes, I did say 'she'. Imagine "Captain Despot"
being your boss. Haggie, Katy, LP don't worry about ole Neale . . . .Your
gender is more than holding it's own in the commercial mariner's world. The
best part is it's no longer necessary out here for someone to say, "How do
you find working for a woman?" The question never comes up. But it's kinda
fun knowing I work for a 'Despot'.

Few more days and I'll talk to you all in two months or so . . . .

MST

Schoonertrash wrote:

The first answer was the closest. Yes there is a difference and yes in most
compasses including the one's we find on our sailboats and powerboats are,
in fact, different. Some compasses, such as the one on USNS Bob Hope, are
built to be self adjusting when crossing the line.

Sounds like a good but probably expensive feature to have.

.... The easiest way to
solve the issue is order up a compass from some company in Australia, NZ
etc. and have it airmailed. Get the exact same compass you already have.

Actually, you don't need to order from Australia (not intending to hurt the
Aussie economy, sorry). All good compass makers will have the same model compass
made in several dip ranges. The dip angle changes by latitude, not just when you
cross the equator.


When crossing the equatorial region at some point you will observe the tilt
or dip. Switch compasses and re-swing. ..... The reason it happens is the
distance from
the mass of iron in N. Canada that is the Magnetic Northern Pole increases
while the mass of earth's iron making up the Magnetic Southern Pole
decreases in distance. At some point one becomes the major attraction to ye
olde compass instead of the other and it becomes off-balance.

Total malarkey. Compasses are polar, north poles & south poles of magnets both
attract metal but only opposite poles of other magnets. Try it yourself with a
kids toy bar magnet.

The reason for the dip angle is that the Earth is round (well, it's an oblate
sphere) and the lines of magentic force between the North and South Poles are
more apple-shaped. So, the Earth's magnetic field pulls either the north or
south pole of the compass card down more as it approaches the poles.

Here's the
humorous part. When the compass on our ship acted up none of the highly
trained and experienced deck officers realized what had happened. One of
the other AB's remarked to me, "Southern Dip?" "For sure", sez I. Finally
we were asked what we meant by that phrase. They didn't believe us and had
to look it up in Bowditch. Score one for the home team!

I guess if you don't think about a compass very often, you forget stuff like
that. I can't believe a qualified sea officer would have never learned about
compass dip. For that matter I am surprised you can't explain it more clearly &
accurately yourself, ST!

Fresh Breezes- Doug King

Well . . it was clear to me?????

Subject: Compass dip - questions for the southern Hemisphere people.
From: "Schoonertrash"
Date: 11/28/2003 18:52 Pacific Standard Time
Message-id:

Well . . it was clear to me?????




I'd be curious as to what they had to do in Australia to fix the "dip" problem,
plus, curious how they "balance" small yacht compasses to make them good, world
wide.

As Doug mentioned the cost of doing that is prohibitive. Much easier to get
two compasses. When you see the need, and it will be very apparent. Switch
to the other one. Then you only have to swing the compass for correction
which is not all that difficult. Remember you now have things like GPS to
help you out. Or use a star sight for N. and S. Figure amplitude on the
sun's rising and setting to find E. and W. Sounds much more intimidating
that it really is.

MST

One day left.

Subject: Compass dip - questions for the southern Hemisphere people.
From: "Schoonertrash"
Date: 11/29/2003 10:31 Pacific Standard Time
Message-id:

As Doug mentioned the cost of doing that is prohibitive. Much easier to get
two compasses. When you see the need, and it will be very apparent. Switch
to the other one. Then you only have to swing the compass for correction
which is not all that difficult. Remember you now have things like GPS to
help you out. Or use a star sight for N. and S. Figure amplitude on the
sun's rising and setting to find E. and W. Sounds much more intimidating
that it really is.

MST

One day left.


If that's directed at my response, I'd have to disagree.
Since balanced compasses are available, I'd consider it a better investment to
go for the single, potentially better compass, rather than two cheaper models
(which may end up costing about the same).
Naturally, there are always advantages to having two.....and even though it may
be unbalanced, doesn't mean it won't work.
I'd still be curious as to what they had to do to that ship compass to correct
it, as I've never heard of any problems with dip on one of these (might have
something to do with hanging the magnets well below the compass card and pivot)
Taking azimuths/amplitudes/polaris, should be a normal occurence for anyone
doing serious offshore work..... BG devising methods to get accurate ones on
many small boats, is another story

We're moving soon and will be within cell phone range one more time before
heading 'way' out. I'll make a point of asking some tech questions and
report back.

MST

Schoonertrash wrote:

Well . . it was clear to me?????

Massive iron deposis in Canada have nothing at all
to do with compass dip. It's only that the earths
surface is not parallel to the lines of the Earths
magnetic force. Closer to the poles, the magnetism
of the Earth pulls either the north or south pole
of the compass down more. Close to the equator,
the force is almost parallel.

You may be thinking about variation, which is
definitely affected by local magnetic anomalies
like massive iron deposits.

BTW in a couple of "around the world" sailing
books, the skipper/author talks about either
correcting for compass dip or switching compasses.
One of them is 'My Old Man & The Sea' which is a
good read all in itself; but they planned to take
three compasses with them (on a 25' boat no less)
and in the last minute rush, forgot. So they
disassembled their compass and remounted the
magnets to correct for dip. Interesting book, I
gave my father a copy.....

Fresh Breezes- Doug King

DSK wrote:

Massive iron deposis in Canada have nothing at all
to do with compass dip. It's only that the earths
surface is not parallel to the lines of the Earths
magnetic force.

100% correct.

Closer to the poles, the magnetism
of the Earth pulls either the north or south pole
of the compass down more. Close to the equator,
the force is almost parallel.

Not really, the region of zero dip wanders about 12 degrees north and
south of the equator. Check out Chart 30 for this info?


You may be thinking about variation, which is
definitely affected by local magnetic anomalies
like massive iron deposits.

Dip is also affect by anolmalies.


Cheers MC