The oldest, most basic – and perhaps most useful – instrument on a boat is the magnetic compass. No matter how much sophisticated electronic gear lines the dashboard, every boat should have one. When the batteries go dead or salt water fries the electronics, the magnetic compass will still work–it will still get you home.

Of course, you have to buy quality equipment, install it properly and maintain it if you plan to call upon your compass to bail you out of a jam. A compass that is not installed correctly can cause more problems than it solves. You also have to learn to use it, and keep those skills sharp. What follows is a primer than will help you do all of the above.Gene BjerkeFew boaters get excited about the compass anymore, but it will get you home when the electronics go dark.

Gaining fluency with a compass will put you in good company. It was the Chinese who discovered that a magnetized steel needle, free to move in a horizontal direction, will always line up on a north-south axis. It was the Europeans who developed the compass as we know it today. By about the 14th century, the modern nautical compass was essentially complete. Today, it is right up there with the rudder and the propeller as one of the few pieces of equipment that comes standard with almost every boat, power or sail.

The nautical compass differs from its land-based counterpart in several ways. Rather than a magnetized needle turning over a background with directions printed on it, the directions are printed on a compass card with small magnets attached underneath. The entire card turns to line up with the magnetic field of the earth. Because vessels pitch and roll, and the compass is secured to the vessel, the card is suspended in gimbals so that it remains level while everything else moves around it. Place all this in a case, add a reference line – called a lubber line – and fill it with fluid to damp the motion, and you have a modern ship's compass. Some compasses have additional lubber lines at 45 degrees either side of the main lubber line, which are useful for sailors who use a tiller and sit on either side of the cockpit.

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.

COMPASS ERRORS

The compass lines itself up with the magnetic field of the earth. However, the magnetic field does not operate in straight lines. Local influences can also modify it. That means that the heading your compass shows may not be the heading your boat is actually traveling on. These effects are collectively known as errors. This does not mean the compass is defective. Rather, compass errors – there are several kinds – are anomalies that change the instrument's performance, and they must be accounted for when you navigate.

The compass does not usually point to the North Pole, which is called true north. A GPS will indicate your course over the ground in terms of true north. A compass, by contrast, points instead to the north magnetic pole (called magnetic north), which is located in northern Canada. The difference between true north and magnetic north is known as variation. The exact amount of variation will differ at different locations and, if needed, you can compensate as you navigate. Your chart will have a number of compass roses printed on it. The outside ring of each rose will show true directions. The inner ring will show magnetic directions. In the center will be the amount of variation for that location. For example, if the variation is listed as "11 W," that means magnetic north is 11 degrees west of true north. It will also mention the rate and direction of change. Magnetic north moves around, though its movement is so slow that you can generally ignore it.

Variation is a function of location, and it affects all compasses the same. A second error you must be aware of is deviation, which refers to the effect that metals and other magnetic influences have on your compass . Deviation obviously differs from boat to boat, and at different headings. It can even change as ferrous material is added, subtracted, or moved about the boat from time to time. I once sailed as navigator on a boat going to Bermuda. In preparation for the voyage, the skipper had rearranged the stowage on the boat, and his changes had an effect on the deviation. We didn't discover this until we were putting to sea and I realized the compass was showing us the wrong course. It made for some very challenging navigation.

There are other sources of compass error but they are generally minor and of no great concern. There may also be some local anomalies, but these are usually noted on the chart.

COMPENSATING THE COMPASS

The presence of iron and steel on your boat can affect your compass. If there is not too much of it, many compasses can be adjusted – compensated – to neutralize the error. If the boat itself is made of steel, you can still compensate the compass using steel balls on either side, but this is a very technical process and requires the services of a professional.Gene BjerkeCompass, chart and electronics are the navigator's primary tools.

For other boats, the owner can often do the job. On some, usually older, boats, the compass has been compensated by placing small magnets around it. Most modern compasses come with compensating magnets installed, which allow you to cancel out your boat's deviation if it is not too large. They are usually set by turning slotted pins, one for N-S compensation and one for E-W compensation. The magnets should come zeroed out on a new compass.

There are a number of ways to compensate your compass, sometimes called "swinging ship." You can hire someone to do it for you, but I have had good luck with the following simple method. Make a shadow pin by inserting a dowel into the center of a straight line drawn on a flat board. The pin needs to be perfectly square with the board. You will also need a coin or a non-ferrous screwdriver – some compasses come with a plastic tool – to turn the pins. What you are going to do is use the sun as a reference to be able to run the boat in precisely opposite directions. It is true, the sun moves, but it takes four minutes to move one degree and you can make your adjustments in less time.

This is best done under power on calm water, with at least two people aboard. Have one person steer the boat due north by compass. Set up the shadow pin on a level surface in the sun and arrange it so that the shadow falls along the line on the board. Then the helmsman should turn the boat around and run in the opposite direction, steering to keep the shadow on the line. The compass should read due south. If it doesn't, slowly turn the N-S compensating magnet to remove half the difference. For example, if the compass reads 186 degrees instead of 180 degrees, adjust it to read 183 degrees.

That done, the helmsman should now run due south by the compass. Re-adjust the shadow pin so the shadow falls on the line, and repeat the process heading north. A couple of such runs should either remove any N-S deviation or demonstrate that all the deviation can't be dealt with.

Repeat the same process on east-west courses, and then make one more quick check on north-south to see if it has been affected by the east-west compensation. If the process successfully neutralizes any deviation, you can use the magnetic rose on your chart for courses and bearings. This simplifies life a great deal.

DEVIATION TABLE

Gene BjerkeGraph 1
Gene BjerkeGraph 2If there is any remaining deviation, you need to make a deviation table for your boat. A deviation table is simply a table of corrections to be applied to your compass to convert between magnetic courses and what you will actually steer using your compass. For example, if the magnetic rose on the chart indicates that your course should be 135º magnetic and your deviation table shows you have a deviation of 5º West at that course, you would add the two numbers together to get a course of 140º, and steer 140º by your compass.

There are two basic ways to generate a deviation table, by direct observation or by taking bearings. Since compasses on most small boats are not equipped to take bearings directly, we will not go into that technique (if your compass is so equipped, you can find the technique described in several references, including Chapman Piloting and Seamanship).

To create a deviation table by direct observation, first lay out your chart and find as many ranges as you can, a range being two fixed objects, on land or water, that you can line up visually from your boat and know you are running in a specific direction (magnetic). For example, you may find that if you line up with a beacon on the water and a steeple on land you will be running a magnetic course of 090 (and 270 in the opposite direction). That will give you two known magnetic directions to run.

Try to get as many different directions as possible. If you can find courses for every 15 degrees of the compass, great; if not, do the best you can. Then go out on your boat and run each of the ranges in both directions, taking careful note of what your compass says each time.

You can tabulate your results in three columns, either on paper or using a spreadsheet. The first column will list the correct magnetic course; the second column will contain what your compass read on that course; and the third column will list the difference between the two, which is the deviation for that course. For example, if the course you were running was 090 magnetic and the compass showed 092, then your deviation would be 2W, meaning two degrees west, because the proper course was west of what your compass showed (See Table 1).

If you were able to find a large number of ranges, the raw table might be good enough. But to make a neater table, and get a sense of the deviation for your boat, you can make a simple diagram. On a piece of graph paper, draw a vertical line and label it in appropriate increments, say every 10 degrees, to represent magnetic courses. Label the deviations every one degree, with easterly deviations to the right of the line and westerly deviations to the left (see Graph 1). Now plot the deviations from your raw data table, and draw a fair curve through all of them (see Graph 2). If any points are outside the curve, they should be treated with suspicion.

The result is that you can now create a more complete deviation table, because the graph gave you many more numbers than you could collect on your own. You can use any increment that you find convenient. In Table 2 we have used 15 degrees. To fill in the right column, you can pick the nearest whole degree from the fair line you have drawn.

USING THE COMPASS TO NAVIGATE

This may seem like a great deal of work, but it is necessary in order for your compass to be a useful and accurate guide. The day-to-day use of the compass involves two things: determining the course to steer and then steering that course.

Once you lay out a course on your chart, you need to convert that course to the proper compass course to follow. You may need to do some conversions between true, magnetic and compass courses. Fortunately, this is not difficult. The basic conversion formulas are intimidating when written out, but there is a user-friendly way to make calculations that involves making a list, filling in the values and doing some simple math. Here is the list:

True
Variation
Magnetic
Deviation
Compass

There are a variety of mnemonics to help remember this, such as "True Virtue Makes Dull Company." The idea is to fill in values by pulling information from charts and tables, and calculate the rest. When proceeding down the column, add westerly and subtract easterly variation and deviation. Obviously you do the opposite when working up the column. For example:

True – 145
Variation – 10E
Magnetic – 135
Deviation – 5W
Compass – 140

Gene BjerkeTable 1
Gene BjerkeTable 2This means you start with the true heading of 145 degrees and subtract 10 degrees to account for the variation (both pieces of information gleaned from your chart) to arrive at the magnetic course of 135 degrees. You then add 5 degrees to account for deviation (information that is specific to your boat, and comes from your tables) to arrive at a compass heading of 140 degrees. Thus you would steer a compass course of 140 to maintain a magnetic course of 135 or a true course of 145. These are all exactly the same course, just measured in different ways.

STEERING WITH THE COMPASS

When steering by compass, the first thing is to trust your unit. In my early days, I sailed into a bay and looked around. The compass said to go one way, but it looked to me like I should go a different way. So I went the way I thought I should go and ended up on the opposite side of the bay too late in the day to make it to my destination. Not a good scene.

To make sure that you can trust it, keep anything that might attract the magnets several feet away from the compass. That includes tools, knives, flashlights, portable radios – anything that might include iron or magnets. When in doubt, keep it away.

A modern compass is a robust instrument. With a little care it will last for many years. Cover your compass to protect it from the sun when you are not using it. As you are cruising about, check that it reads correctly on your familiar courses. If the card sticks or a bubble develops, take it to a proper compass repair shop.

Next, remember that although the card appears to move, it is really the boat changing directions – the card generally stays steady. This may make it easier to know which way to turn in order to get back on course. But don't correct too much. Beginners tend to try to correct every little deviation from the course. That will lead to endless sawing away at the helm, or what a mate once referred to as "writing your name with the wake." Know that a boat will tend to wander somewhat as it gets pushed around by the waves, but it will wander in both directions. The best helmsmanship consists of doing the minimum amount of steering to make the boat stay on the required course.

So relax, look around and pay attention to other things beside the course. Watch the tides, the winds and the bow against the horizon. Getting a feel of the world around you helps give you a sense of what is happening to the boat. And check the compass at intervals to make sure you are still going where you want to go.

Gene Bjerke, whose work has appeared in Cruising World, Chesapeake Bay, Good Old Boat and Multihulls magazines, regularly crews on square riggers near his home in Virginia. He has been boating for 45 years.