For those who are willing to pay the price, you can now get fluxgate compasses. These use an electronic sensing device – the fluxgate – to detect the earth's magnetic field. This can be placed anywhere on the boat to minimize magnetic influences, even on top of a mast. You can then place one or more readout, often digital repeaters that read directly in degrees, wherever they are needed.
BUYING A COMPASS
If your boat did not come already equipped with a compass, you need to buy one. Not surprisingly, they come in a wide range of sizes, styles, and prices. The first thing you must consider is where the compass will be mounted. Will it be close to the helm, say in a binnacle right at the wheel? Or will it be farther away and dash-mounted? This will give you an idea of the size you will be looking for. Within limits, bigger is usually better because it will be easier to read.
Most compasses available today have a domed top. The effect of the fluid-filled dome is to magnify the card, enhancing visibility. Compass cards are marked in degrees, with 0 (or 360) being North and proceeding clockwise. Thus East is 90 degrees, South 180, and West 270. Depending on the size of the unit, the card may be marked every five degrees, every two degrees, or even every degree on large cards. Some large compasses may also include markings for the traditional 32 points, which were used before mariners adopted the system of degrees to steer.
Take some measurements on your boat, then head to the store. When comparing compasses, pick each one up, move it away from the others to eliminate interaction, and simulate the expected motions of your boat. If you are going to mount it on a sailboat, tilt it one way and the other to see if it will continue to read at the expected angles of heel. Turn it to see if there is any lag in its motion. Imagine it in place on your boat. Will it be easy to read? Will it fit in the available space? Most modern compasses are made of sturdy plastic materials. They will stand up well to the weather, but it is prudent to protect them from sunlight when the boat is not in use. Once you have a model in mind, comparison shop online to find the lowest price.
There are several well-respected manufacturers, including Ritchie, Aqua Meter, Suunto and Plastimo. All make compasses in a great variety of sizes, mounts, and configurations to fit any situation. In general, you can assume that the highest quality instruments will cost more, with prices ranging across the spectrum. Buy the best instrument you can afford. It is an investment in safety and security.
MOUNTING THE COMPASS
When you get the compass to the boat, don't immediately start drilling holes. You want the compass to be in front of the helmsman, but look around and see what might affect it. Analog instruments often have small magnets in them that could impact the compass. Electricity going through wires creates a magnetic field around the wire. Hold the compass temporarily in place where you want it. Now turn on all your switches one at a time and see if anything affects the compass. Start the engine and see if the instruments have any effect. Electric wires in the vicinity –including the wires to the compass, light if it has one – should be twisted to neutralize the magnetic field that they create.
Once you have decided on the best place, the compass must be mounted so that a line from the lubber line to the center of the card would be perfectly parallel with the centerline of the boat. Determine the centerline, either by measuring from the sides of the boat or lining up things that you know are mounted on the centerline. If the compass will be off-center, create a line parallel to the centerline and line the compass up on that. If the compass is not perfectly aligned with the boat, it will never read correctly.
Naturally, you will fasten the compass down with stainless or other non-ferrous fasteners.
