Chapman Compass
A New Direction in
Compass Design
A compass mariners can look up to.
Viewing: This compass can be viewed from below with the same view as from above.
This provides the user many mounting options; flush, bracket, bulkhead or
binnacle housed with either a top view or a bottom view.
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Compasses shouldn’t be looked down upon, they are directional instruments and should be looked across, either to observe, at a glance, the direction of the vessel’s heading or the direction of an object. |
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With an oblique view of the card, optimum optical benefit is achieved from the spherical bowl. The clarity of the graduation markings is further enhanced by being perpendicular to the plane of the card and the observer’s line of vision. |
Lubber line: The
compass card rotates outside the lubber line marking which is
from a shipshape in the middle of the compass.
This arrangement mimics the
egocentric view of the vessel and her surrounding directions allowing for
intuitive ease of reading and comprehending.
This is significantly different from
traditional compasses where the compass card rotates inside the lubber line
marking. Confusion can arise resulting in the helmsman trying to steer the
card, a common error called “chasing the compass”.
Furthermore, on compasses read from
an aft lubber line (“direct reading dial”, “front reading”) the display is
counter-intuitive. The card’s graduations show starboard directions to port of
the lubber line and when turning, the part of the card being read will move in
the opposite direction of the external environment.
The style of the lubber line
utilizing a bowsprit from a ship shape minimises parallax error when reading
the heading from a position not directly behind the compass. This is further
enhanced by the marking behind the card.
Compass bearings can be taken using
the sighting peg on the card (when deck mounted) and the sighting peg from the
ship shape (when deck head mounted).
Compass card:
The card achieves greater stability by the moment of inertia derived
from the circumference ring. The directive magnets are neodymium to provide;
¨ a powerful torque
¨ small sweep
¨ The closer positions of the
directive magnets’ poles generate a weaker field outside the compass bowl.
(Having the least power required to give the card directive force minimises
deviations arising from soft iron induced by the compass card’s directive
magnets.)
The gimbal assembly,
pivot, lubber line and directive magnets all lie in the same plane. This
maintains the relative position of the directive magnets to surrounding
structures and importantly, any magnets or soft iron engaged to correct for
deviation.
The card is kept level
by engaging buoyancy. By placing the buoyant chamber at the top of the compass
card’s sighting peg, a maximum righting force is developed. This righting force
will keep the card near level in all navigable latitudes. The buoyant chamber
also provides a platform on which to display a logo.
To achieve similar roll
periods of the gimbaled mounting base (supporting the ship shape) and the
compass card mounted on it, a weight is placed on the bottom of the lower
sighting peg. The weight having of a mass and position to harmonise the roll
period of the components. This weight also provides a platform on which to
display a logo.
Materials:
The use of clear plastics with a refractive index similar to the compass
fluid gives a clear uncluttered view of only the;
¨ shipshape and lubber line
¨ card’s graduated rim,
¨ pivot and directional magnet assembly.
Lighting: The clear spherical bowl allows for many lighting options including
illumination from within the binnacle or the other side of the observer with
flush mountings. As the lighting arrangements are not on the side the card is
viewed from, more elaborate lighting arrangements can be accommodated such as
“heading only” for steering and “all around” if bearing are being taken.
The bowl: The spherical bowl is kept full
of fluid by having an adjacent chamber that can accommodate the expansion and
contraction of the fluid by the elasticity of air. By this method, there is no
diaphragm. The failure of diaphragms is a common cause of compass break down.
¨ Top viewing mounting; the air is
trapped in the chamber below the bowl by the joining orifice being at the bottom
of the chamber (via a tube). This also allows for easy removal of bubbles
(should they occur) by temporarily inverting the compass.
¨ Bulkhead mounting; as for top
viewing but the tube is bent to have the opening at the bottom of the chamber
when the compass is mounted
¨ Bottom viewing mounting; The air
naturally will be at the top of the chamber.
This
innovative design delivers a versatile reliable compass with many features
attractive to boat owners, operators and skippers.
Deckhead mounting: There are many benefits in
mounting a compass from the deckhead (ceiling).
¨ The area in front of the helm can be
clear for,
o
instruments
and equipment
o
chart
space
o
general
useful space for coffee cup etc
¨ Being mounted on the deckhead, the
compass is well clear from instruments and equipment, such as tachometers,
steering mechanisms, flexible lamps and radio microphones that may have a
variable effect on the compass.
¨ Radio sets and microphones (or other
equipment that require manual attention) can be placed in front of the helm,
removing the need to reach up, which can be dangerous in a seaway.
¨ There is little chance of objects
being placed near the compass, inadvertently affecting the deviation. Tools
(especially screwdrivers) and small portable appliances have a history of
causing unexpected deviations.
¨ The compass will not be exposed to
direct sunlight and the subsequent deteriorating effect of UV rays and
excessive heat.
¨ It is unlikely that a bubble will
form between the observer and the card.
¨ The simplicity and reliability of
the expansion chamber being above the compass bowl is fully utilized.
¨ The compass heading is easily read
at a glance up.
This new compass design has patents pending.
Paul Chapman
May 2010