Tan PS wrote:
You've got to do a compass swing to determine the effects of the ships
magnetic interference.

Quite a few steps and you need a means to determine the magnetic heading of
the ship usign an external reference, preferable but is probably beyond the
means of most of us.

Not really, most ports have a compass swinging place set aside have a
look at the charts or ask. It usually has a set of posts you can tie to
& hold the boat on the various headings & "usually" they're sited so
there are at the very least a set of transits (2 or more landbased
objects that are on the chart & when in line mean your boat is pointing
to a known true heading as close to nth as reasonable given the
available transits)

Alternative is a GPS set to magnetic reading or if you
have local magnetic variations, then you can use the true North and apply
the variation.

Notwithstanding even some cheap compass manufacturers suggest this;
it's not a goer & indeed is just more of the dumbing down of things in
general.

A GPS has no clue which way the bow of the boat is pointing & this is
what you're trying to determine. All a gps can do is tell you your
history track across the seabed & clearly you need to be moving for it
to even be able to tell you that.. You might be moving sideways,
backwards or whatever the GPS will not know so can't tell you.

If you use this method it's OK for runabouts or gin palaces but yours
sounds like a proper boat & few boats travel through the water in the
direction the bow is pointed, even a slight cross wind will slightly
heel the boat so changing it's shape enough that it actually crabs
along, then if there's any sort of cross tide/current the game is off
anyway, so if you're serious DON"T use a GPS to swing a compass. The
object is to make you confident you know what your compass will read
when the bow is pointed in a certain direction, experienced boaters of
course know only too well that the boat won't be going there, but with
experience you can then make your course to steer adjustments from that
known starting point

I also note point 15 below??? DON'T ever do this, the compass must be
mounted so it's aligned with the boat's fore/aft centre line. It doesn't
need to be "on" or even near the centre line but must be completely
parallel with it. Otherwise every time you turn 180 degs the error will
just reverse & multiply. It's tempting for play boats but again not a
proper boat where you'll rely upon the compass as more than a talking
point at the club bar:-)


K


In the following steps, try to be as close to the cardinal points as
possible.

1. Point ship North, take compass reading. Take reference reading. Note
error. If compass underreads, error is negative.
2. Point East, note reading. Take reference. Note error again.
3. Point South, note reading. Take reference. Note error.
4. Now, calculate North-South compensation. Sum both error and divide by
2.
5. Point North or South (easier to keep the South you are in)
6. Ajust the N-S screw to change reading by the results.
Example,
North your compass reads 003 deg, your external ref is 359deg ,
error is +4deg,
South compass reading 185deg, external ref is 180deg. error is 5deg
Step 4 calculation gives (4+5)/2=4.5 degrees.
Point South, first reading 185deg (assuming you managed to get the
same heading), adjust N-S screw until you get 180.5deg (185deg - 4.5deg)
7. Note new South reading and external reference reading. Calculate new
error.
8. Point West, repeat readings.
9. Calculate correction like in step 4.
10. Point West. Adjust E-W screw like in step 6 and example.
11. Note new West reading and external reference reading. Calculate new
error.
12. Point North, take compass and external ref reading.
13. Point East, take readings
14. Calculate index error, sum all 4 errors and divide by 4.
15. Physically rotate compass mounting to corrent for the error calculate
in step 14.
16. Now go through all the 4 cardinal headings, preferably 8 (the 45 deg
positions) and note compass readings and tabluate a Steer-by card.
17. You should get reading that are slightly out by 1 or 2 deg when you
tabulate the steer-by card and set you heading to those readings and you
will get the actuals.
Example: If the Steer-by card says 46deg for NE, it means you are
actually pointing 45deg when the compass is reading 46deg

Hope this helps.

Tan PS


"Jürgen Spelter" wrote in message
...

Hi,

I bought a dutch steal steel shipband I want to install a new compass.
Has anybody a tip how to adjust compass deviation?

regards

Juergen