NavList:

Lars Bergman wrote-

Some findings about Robert's deviation (having received his data off
list):

Compass deviation could (approximately) be described by the equation

dev = A+B*sin(cc)+C*cos(cc)+D*sin(2*cc)+E*cos(2*cc),

where cc is the compass heading. Making a least square fit of Robert's
data (defining easterly deviation as positive) into this equation I
found the coefficients to be

A=-1.9d
B=-18.5d
C=12.3d
D=1.4d
E=-1.5d

Coefficient A represents a fixed offset, this could be due to e.g.
alignment error of lubber line or an errouneous variation value used.
Coefficients B and C represent permanent magnetic field components,
fore-and-aft and athwartship respectively. B and C are the main
contributors to the deviation in this case.

The greatest deviation is found on a SE compass heading, amounting to
25d W. With such large values there are a few reasons to compensate
the
compass by external magnets, or finding a new location for the
compass,
or finding and removing the magnetic source creating the deviation.

One reason is safety: If it for some reason is necessary to steer a
reversed course in a hurry, the easiest way is to make a 180d turn of
compass heading. With Robert's compass, worst case, the resulting
actual
change of ship's heading will be 43d off. This happens at cc=135 where
the deviation is 25W, thus magnetic=110. Turning to cc=315 the
deviation
is 18E, thus magnetic=333 instead of the desired 290d. I have never
seen
this phenomena described anywhere, and I don't think it is very well
known.

Another reason is that the compass on certain headings will be "slow"
or
"fast", i.e. a certain change of actual ship's heading do not
correspond
to the change of compass heading. With Robert's compass we can look at
cc=205 with dev=5W making mag=200. If we make a ten degree on compass
turn to starboard we find the deviation of cc=215 to be 1W, thus
mag=214. Although the ship's head had turned 14 degs, the compass
shows
only ten. The compass is "slow". When changing course from cc=055 to
cc=065 we will find that the ship's actual change of heading is only 7
degrees; the compass is too "fast".

=============end of Lars Bergman quote.

Comment from George-

As usual from him, there's good insight in Lars Bergman's analysis,
about Robert Eno's compass errors.

The major problem that surfaces is that of the two major
contributions, B and C, which correspond to permanent magnetism,
perhaps originating from the engine.

And it's the crosswise B component that's particularly worrying.
Presuming that the engine is mounted fore and aft, it implies a lot of
magnetisation, transverse to the engine block. To me, that's something
of a surprise. It prompts a number of questions for Robert to ask
himself.

Is the compass (or perhaps even the engine) mounted a long way off the
line of symmetry of the boat?

Does the compass share a panel with other instruments or components
close by that could be generating their own magnetic field? I'm
thinking particularly of a windsceen wiper motor, an inverter or
anything else with a transformer in it, loudhailer, analogue meters.
Is it mounted in a bulkhead, in which case, what's on the other side
of the bulkhead?

Is the compass nearer to the engine than it really needs to be? For
example, can it be mounted close under the roof of the steering cabin,
just as high up as it can possibly go?

Is there any vertical mast or other component close by, that's made of
steel?

Robert has to be aware that he is especially vulnerable to the effects
of magnetised metal on board, because of his high magnetic latitude
and the resulting weakness of the horizontal component. And
particularly vulnerable to any nearby piece of vertical(ish) metal,
that might come to an end near to the level of the compass

Lars puts an interesting slant in his analysis, that I haven't come
across before, in pointing out zones where the compass was "fast"
(turning faster than the boat does) and conversely slow at others. I
take it that Robert's course, in Frobisher Bay, when his wildly
varying compass showed up, was roughly Northwest. I might have
expected such instability to result from being in a particuarly fast
zone, but my understanding of Lars' analysis indicates a slow zone
around that direction.

I agree completely with Lars, that this compass installation indicates
a particularly dangerous state of affairs. It needs some sort of
urgent remedy. I would think hard about installing some sort of
remote-reading compass up a stick, if it was my boat.

I don't know how much change of latitude comes into Robert's voyaging,
but he should be aware that any deviation table or magnetic correction
will be local to his high latitude, and may need redoing if he
journeys far South (or North).

George

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.









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