Compasses
Power or Sail
Sailboats heel and powerboats vibrate and pound - your compass must compensate
for these conditions. Most sailboat compasses have an internal gimbal
so the card will stay level when the boat is heeled. Powerboat compasses
compensate in one of four ways. First, by using heavy dampening fluid.
Second, by installing a weight under the card to slow its motion. The
third, designed by Aqua Meter, relies on an air chamber to pivot the card,
which floats in the compass fluid at almost neutral buoyancy. Fourth,
Aqua Meter's Navagrid System uses lubber and bearing lines floating above
the compass card to provide a cushion of dampened fluid over the card.
Size of Card
Recreational marine compass cards range in size from about 2 1/2-7" dia.
The larger the card, the more accurate your course is likely to be because
a larger card is more stable and easier to read. Smaller cards are sufficient
for short runs or occasional use, and they're popular because small compasses
are easy to stow below. You should select the largest card your space
and budget can handle. On some compasses, the card will appear larger
due to the magnification of the dome. This is referred to as apparent
diameter.
Type of Card
There are three basic types of compass cards: the dish or back-reading
card; the dome, or front-reading card; and the Ritchie CombiDial, a combination
dish/front-reading card. The dish card and the Ritchie CombiDial are most
often used on larger boats while the dome card is suited for sailboats
with tillers so the helmsman is able to see the bulkhead-mounted compass
at or about eye level.
Fluxgate Compasses
Fluxgate compasses feature a digital or electronically reproduced "analog"
display rather than a card or dish. Instead of a rotating compass card,
the fluxgate compass uses a stationary electronic sensor mounted below
deck. This sensor detects minute changes in electrical voltage caused
by a deflection of its north/south orientation. In other words, it "senses"
its position relative to the earth's magnetic poles and reports it, via
hundreds of readings per second, to a micro-computer. This microcomputer
averages the readings to arrive at a single accurate and steady electronic
display.
Compass Deviation
The accuracy of your compass, once installed, will be affected by its
proximity to iron and other metals on your boat. You should verify its
accuracy on points north, south, east, and west by aiming your boat at
structures on land, several miles distant, with known bearings from a
navigational aid. If your compass readings are more than a few degrees
off in any one direction, you should have a professional adjuster correct
it.
Balancing Compass Dials
For The Southern Hemisphere
The dials in magnetic compasses should be balanced to compensate for dip
caused by the earth's magnetic field. All Ritchie compasses come balanced
for Zone 1 (essentially all of the Northern Hemisphere). If you need a
compass calibrated for the Southern Hemisphere, contact our Special Order
Center at 800-568-0319.
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