compasses are the easiest, cheapest and most reliable instruments used
for boat navigation.
are in the middle of an ocean, out of sight of land, the magnitised
needle or card will still point to the magnetic north.
it will do this unerringly anywhere on the surface of the globe,
without the need for electricity and without any reliance on the
outside such as GPS satellites or other high tech navigational aids.
A magnetic pointer, as a navigational instrument,
was first used
in China as far back as the 11th century.
liquid allows the card to swing with the minimum of friction and damps
any excessive swing or wobble, making it easier to read.
Friction needs to be kept to minimum as the Earth's magnetic
fairly weak at the surface.
However, the basic principle remains the same.
today we think of the needle or card as pointing towards magnetic
north, the original Chinese navigators thought of their needles as
pointing towards the south.
All magnetic compasses rely on a magnet which is allowed to
align itself with the Earth's magnetic field.
This field however is not aligned perfectly with the
So, the needle points to the magnetic north or south rather
geographic north or south poles.
reason this happens is not entirely clear, one theory is that it has
something to do with molten iron swilling around in the earth's core.
is known and important for the navigator to remember, is that the
magnetic north and south poles are not at the geographical north and
south and that they move.
difference between the direction of true north and magnetic north is
known as the Variation or Declination and can vary by as much as 25
miles in a year.
And the angle of
this Variation or declination varies across the surface of the globe.
nautical charts should indicate the Variation for that particular area
at the time of publication, as well as indicating by how much it will
change over time.
Something that isn't accounted for on your charts is any
which is specific to your boat.
the magnet in your instrument has to be so sensitive it will also be
affected by any nearby ferrous materials and any nearby electromagnetic
needle or card can end up showing an error due to nearby lumps of
ferrous metal such as the boats engine as well as phones, speakers or
even a hand held compass left nearby.
To avoid the latter causing problems you will just have to be
where you or your crew leave such things.
As for the engine or other fixtures, these will have to be
can be balanced out using small correction magnets placed in a binnacle
but this is a specialized job and only used on ships.
The usual practice for the small boat owner is to create a
card by doing a compass swing.
this would be done by taking bearings on fixed land features, nowadays
however GPS which is not subject to magnetic deviation can be used to
The deviation card can then be used by the navigator when he
converting between true and magnetic headings.
There are a few, fortunately rare, locations in the earth's
field where magnetic anomalies occur.
Fortunately these sources of magnetic anomaly will be
indicated on your
There are, as per usual with sailors, a couple of easy to
for remembering how corrections should be allowed for.
The CADET rule for variation;
When converting from magnetic: ADd East to get True.
It follows that converting from true to magnetic is the
For deviation you can use the saying;
Deviation east, magnetic least, in other words subtract from
Deviation west magnetic best, in other words add the correction.
hand-bearing-compass is an important piece of equipment to have on
Its main use is for
taking bearings of shore features in order to get a fix.
it is also useful when monitoring the relative bearings of other
vessels where there is a risk of being on a collision course.
is possible to do this using the steering-compass but they are rarely
mounted in such a way as to make this either easy or accurate.
accuracy is important, which is why the better compact variety are
designed for ease of handling and fitted with a sighting device.
Many come with either a
battery powered backlight or a photo-luminescent dial to allow for use
And a prism for sighting
through that eliminates any parallax error.
with the steering-compass the bearing will be a magnetic bearing which
will need to be converted to true bearing before plotting on a chart.
fluxgate-compasses are becoming increasingly common, often included as
a component part of other solid state electronic devices.
While they do require a
stable supply of electrical power, they don't have any moving parts
use magnetic field sensors which provide data tor a microprocessor
which then sends the information to either a digital or analog display
They are often built in
to auto pilots, GPS receivers, mobile phones and such like.
Because of their size
they are becoming increasingly popular on small boats.
However, a magnetic
compass which is not dependent on any sort of electrical power is still
the navigator,s best friend.