Steel Boats and Compasses
I suggest that those who think that the compass solution on small steel
vessels is simple have not actually tried to implement the solution. The
solutions that work on larger vessels do not scale down to small ones
very well and in many cases not at all.
I have been researching this issue for about 3 years now and the
problems are almost insurmountable.
A true solution is very expensive.
Making the pilothouse out of non magnetic materials helps a lot, but can
be expensive.
Real binnacles with flinders bars and soft iron balls may work but the
reliable solution is big and expensive, and may not be effective if the
surrounding cabin is made out of steel.
Electronic solutions like a "GPS Compass" are very expensive and have
all the usual complications.
Moving the compass sensor high above the pilothouse to get away from the
effect of the steel makes the sensor gyrate in a seaway, which may
render it useless. This problem even affects flux gate compasses, which
I have used in a similar situation. This solution is expensive.
To be blunt, I challenge anyone to provide a solution that is
inexpensive, reliable and works inside of a small steel pilothouse. A
proposal is not a solution. Only a working solution fits my challenge.
By the way, the magnetic field is weakening worldwide and the effect is
more pronounced as one gets nearer the magnetic poles. The real
solutions that anyone presents needs to know how close to the poles it
will work and on what headings.
The problem is subtle and full of complications. I have not pointed them
all out here.
Regards,
Mike
Capt. Mike Maurice
Tigard, Oregon (Near Portland).
Mike Maurice mikem@yachtsdelivered.com writes:
To be blunt, I challenge anyone to provide a solution that is
inexpensive, reliable and works inside of a small steel pilothouse.
There are two (competing) problems with steel boats. First, if you install a
compass within a steel enclosure, the steel will interfere with the operation
of the compass. Second, if you install a compass in a location well away from
interference (e.g. the top of the mast) then the compass is exposed to a grea
t deal of yawing and pitching.
There is a solution to these problems. Here's a message I posted here about a
few years ago:
OK, first note that I am neither an electrical engineer nor a compass designe
r. But I am a mathematician, and I just installed a rate-stabilized ComNav au
topilot on Island Eagle, and during the course of debugging it I had occasion
to have a long and enlightening conversation with one of the engineers at Com
Nav. I will pass it on here:
First, recall how a regular magnetic compass works: it senses the variation i
n the earth's magnetic field, and in the case of a mechanical compass the flo
ating magnetic card will always point towards the magnetic north pole. Well,
that's the theoryB anyhow. The problem is that many things can affect this,
such as local magnetic interference (electric motors, the engine block), as w
ell as the mechanical action of the vessel pitching and yawing through the wa
ter. Still, a magnetic compass has a high degree of absolute accuracy -- that
is to say that if you stand still in a certain spot, N will indeed point repe
atable to the same place. (for the purpose of this discussion we'll ignore va
riation)
Now, gyros are sort of the opposite. Since they are inertial, not magnetic, t
hey are unaffected by interference, magnets, etc. This means that they are id
eal as the vessel pitches and rolls. However, the problem is that they have n
o absolute accuracy at all -- when you start up a gyro you have to manually t
ell it where North is. And over time a gyro will drift and loose "North".
So, here's the clever thing: Magnetic compasses are excellent at absolute acc
uracy, while gyros are good at relative accuracy. Therefor, suppose we were t
o mount a gyro and an mag compass in the same vessel, and then make an arbitr
ary course change of 30 degrees. (say from a heading of 90 to a heading of 12
0). We would expect that that both compasses would respond with the new headi
ng. However, that's not quite right. The magnetic would be affected by local
interference, and so might be out by a few degrees -- say it settles on 117.
The gyro, on the other hand, has no absolute knowledge at all and so will inc
rease by exactly 30 degrees from whatever it was at -- say from 63 to 93.
But hold on: since we know that the gyro is highly relatively accurate, we ca
n use the fact that there was a 30 degree course change indicated by the gyro
to correct the magnetic compass. In other words, the gyro is applied as inver
se feedback to the magnetic signal, to completely compensate for any locally-
induced errors in the magnetic compass. If we keep a table of every differenc
e between the magnetic and the gyro compass, we'll have an instantaneous corr
ection table.
Now the cool part: we are also correcting for errors caused by mechanical fac
tors (pitch, roll & yaw), since the magnetic compass is affected by that but
the gyro is not. This means that we will have a much more stable signal in he
avy weather. We can also be much more flexible with the location of the magne
tic compass. For example (and I know of a boat where this has been done very
successfully) on a steel boat we can put the magnetic compass at the top of t
he mast, put the rate stabilizer in the wheelhouse, and have a rock-solid com
pass signal free of interference from the hull.
So, as I said, don't take this as the absolute word on the subject, but it ma
kes sense to me. By the way on Island Eagle I mounted the both the rate stabi
lizer and the flux-gate compass in the pilothouse (which is about 10 feet abo
ve the waterline) and they seem to work perfectly.
So, if you have a steel boat my suggestion would be a flux gate compass combi
ned with a compass rate stabilizer (which is actually a solid-state gyroscope
). The combination of these two should give you a solid compass signal regard
less of the boat construction or the compass location.
Of course this will not help you if you are in very high latitudes, for that
you will have to go to a true gyrocompass or a GPS-based compass. The latter
will probably come down in price as flux-gate compasses are now being include
d on GPS chipsets, in fact the rumor is that the new iPhone will include a co
mpass.
Scott Welch
Product Manager, Open Text Collaboration and Social Media Group
www.opentext.com
905 762 6101
"Things turn out best for people who make the best of the way things turn out
." - John Wooden
Scott posted excellent advice and summary of the new generation of
fluxgate compass stabilized by rate gyros.
KVH was among the first to offer such a compass to the recreational market.
Now ther are several other brands available.
The Azimuth 1000 can be run on a small portable battery and can
initialize from a cold start in much less than a minute. Certainly less
than the time it takes for a GPS to acquire signals and display a position.
For a steel vessel in lightning prone areas, my suggestion would be to
have an independent back up power supply to such a compass. If you feel it
necessary to disconnect all electronic equipment, switch to the alternate
power supply so that you still have a compass to steer by.
A GPS compass is not as good a choice since it has an external antenna. It
is usually the antenna that collects inductive voltage and current from a
lightning strike. But a fluxgate compass can remain safely inside the
faraday cage of the steel hull.
And with reference to D.N. s description of having someone with a
flashlight stand near the compass to read the heading . The Azimuth 1000
compass has internal illumination. Total current draw is around 120 ma.
Even a small battery can keep it going for many hours.
I have tested this compass inside a steel car a steel boat and in
contrived tests I placed the compass les than 3 feet from a diesel engine
block. A friend also tested ( inadvertently) his by placing it 18 inches
away from a microwave oven. The galley was below and the compass was
mounted above at the helm station.
Arild
----- Original Message -----
From: "Scott H.E. Welch" swelch@opentext.com
So, if you have a steel boat my suggestion would be a flux gate compass
combi
ned with a compass rate stabilizer (which is actually a solid-state
gyroscope
). The combination of these two should give you a solid compass signal
regard
less of the boat construction or the compass location.
Of course this will not help you if you are in very high latitudes, for
that
you will have to go to a true gyrocompass or a GPS-based compass. The
latter
will probably come down in price as flux-gate compasses are now being
include
d on GPS chipsets, in fact the rumor is that the new iPhone will include a
co
mpass.
Scott Welch
----- Original Message -----
From: "Scott H.E. Welch"
if you have a steel boat my suggestion would be a flux gate compass
combined
with a compass rate stabilizer (which is actually a solid-state gyroscope
The combination of these two should give you a solid compass signal regard
less of the boat construction or the compass location.
Of course this will not help you if you are in very high latitudes, for
that
you will have to go to a true gyrocompass or a GPS-based compass.
REPLY
Whether you have a steel or plastic boat, you should have a deviation table
for any magnetic compass on board..
In fact Scott Welch who is boating in Canadian waters in a Cdn flaged
vessel must comply with the Transport Canada requirement to have a valid
deviation tabel on board. regardless of what electronic and electrical
navigation equipment he also has.
Although this requirement has always been on the books, it was not enforced
for recreational boats.
CCG has determined that more people are now getting into trouble by
venturing further from land in boats equipped only with GPS. This in turn
means more demand on SAR facilities.
There is a misconception by many recreational boaters that any GPS
automatically serves as a compass.
CCG issues a Boating Handbook which is a summary of legally required safety
equipment. Bailers paddles anchores flare fire extinguishers etc.
In the 2007 edition I noticed a new paragraph stating all boats over 8
meters (approx 24 feet) must have a valid deviation card and of course a
magnetic compass installed.
This rule makes sense. It provides for a direction indicating instrument
that is functional despite total lack of electrical power. But without a
deviation card it cannot serve its intended purpose.
Cheers
arild
----- Original Message -----
From: "Mike Maurice"
By the way, the magnetic field is weakening worldwide and the effect is
more pronounced as one gets nearer the magnetic poles. The real solutions
that anyone presents needs to know how close to the poles it will work and
on what headings.
REPLY
Florida boaters may not notice this weakening as much, but up in PNW and
Alaska the deviation is 22 degrees or worse. The dip angle is far greater
( around 70 - 80 degrees) and with reduced field strenght overall it does
mean a floating card compass is less likely to track properly.
One of the more subtle failings of a floating card compass is the pivot
point wear.
Even when the boat is in a marina, the card rests on the pivot point. Any
motion from waves or wakes will cause movement. Over time this dulls the
point and increases bearing friction. If your compass is subjected to
constant motion check to see how well it reacts. Check it at least
annually.
With boat stable and fixed in direction, position a steel object such as a
screw driver close enough to cause a five degree deflection of the card.
Now remove the magnetic object.
If the card does not return EXACTLY to the same heading within half a
degree the pivot is likey worn.
Do not deflect the card more than 5 degrees. This provides too much
leverage and masks the bearing friction you are testing for.
Arild