Small Boat Navigation I: Piloting

The capabilities of the Global Positioning System (GPS) are so obvious, beneficial and available at such a reasonable cost that I feel every boater should take advantage of this technology. But having said that, boats still get lost, run out of fuel and run aground. Just as radar did not make collisions a thing of the past, GPS will not prevent errors in navigation. It is no guarantee you will not get into trouble or suffer the consequences of bad seamanship. Like any other device, it is vulnerable to malfunction or power failure, especially in an environment as hostile to electronics as the sea. Remember, the infrastructure it relies on is highly complex, fragile and dependent on a massive government bureaucracy to maintain it. And as with any other system, if you don't trust your instruments or if you get confused and lose confidence in your equipment, it doesn't matter how well it's operating.

There is a navigational instrument even more important, valuable (and dependable) than GPS, the magnetic compass. It has only one moving part, requires no power source and depends solely on the earth for its operation. It is also extremely easy to use, it has no control panel and no controls. It provides navigators with a knowledge of direction, and at sea, direction is even more important than position. Not knowing where you are is not a problem as long as you know which way to go, and if you don't know what course to steer, a precise knowledge of your position does you no good anyway! It is alarming how many boaters today know their way around a GPS keypad but only have a vague idea of how to use a compass. Without some mechanical aid, only a knowledge of the sky can give you a true sense of direction and even an astronomer is lost when it is overcast.

Most boaters understand this, but many do not realize that precise navigation is also possible with a compass. Using one, along with a chart, is the basis of the ancient art of piloting, the science of navigating by means of landmarks. You cannot fully learn piloting from an article like this, but there are books, and websites which can teach you, and boating clubs and the US Power Squadron offer courses. Hopefully, this will inspire you to follow up on this vital safety skill. Piloting principles are straightforward and common sense and almost anyone who knows them will be delighted to share his knowledge with you. Old salts love to pass on this information to a new generation of boaters. It isn't just a backup skill for when your GPS is down, it is an art in itself and can provide great personal satisfaction and add to the joy of boating.

Compasses consist of a magnetized disk, or card, which pivots around a central balance point. Along the edge of the card are printed numbers, from 0 to 360 degrees. These numbers ( called collectively the compass rose because it is traditionally decorated with fancy artwork) represent direction so that 0 (or 360) is north. 90, 180, and 270 are east, south and west, respectively. All you need is one simple number to unambiguously specify a direction. So for example, north-east is the direction between N (0 degrees) and E (90 degrees), or 45 degrees. When you give a command like "steer 272" it means that the boat should be steered so that the bow is aimed in the direction of 272 and the steering compass (which is installed so that it reads what direction the boat is pointing) will read 272 (just a bit north of due west). The compass is built in such a way that the card always points to earth's North Magnetic Pole (0 degrees) and as the boat changes direction think of the card remaining still but the boat turning under it. What the mariner sees while steering is the numbers on the card flashing by until the boat is pointing in the direction he wants. Most steering compasses have the card immersed in a clear oil to damp out movement caused by the boat's motion and it is helpful to think of the card as being rigidly fixed to the earth's magnetic field with the boat freely swinging beneath it.

Compass roses are printed on nautical charts, too. In fact, there are usually several of them placed in various blank places about a chart for the convenience of the navigator and they contain a rose for true directions on the earth's surface, as well as another with the magnetic directions. They are both clearly labeled. Unfortunately, true geographic direction and magnetic direction do not always coincide and may differ by several degrees; they also vary from point to point on the Earth's surface, as well as over time. Since you will be using a magnetic compass, use the magnetic rose for piloting. The "true" directions are useful for celestial navigation and for GPS work, but when using the magnetic compass for steering, you will have to use the magnetic rose. It is usually printed on the chart inside the "true" rose. There are other issues with magnetic compass correction, such as variation, deviation, and adjustments which are outside the scope of this discussion and which must be understood before you're ready to solo; but the basic principles should be clear from what follows.

The fundamental task in piloting is laying out a course, or determining what course to steer to go somewhere. Here's the procedure: first locate on the chart where you are (for example, the marina where your boat is docked) and the destination you wish to go to (say, a secluded cove across the bay where you will rendezvous with friends). This is the equivalent of entering a destination or waypoint into your GPS. Next, draw a straight line on the chart in pencil between the two points. This will be your course, the invisible line on the water your boat must travel to reach its destination. Using your parallel ruler, or drafting triangles, "walk" this line (making sure to keep it parallel to the original course line you drew) to the nearest compass rose on the chart and read off the magnetic course, or angle, denoted by that line. This is the course that your steering compass will read once the boat is pointed at your destination. As a helmsman, your task is to get the boat moving and steer until your compass reads that course, then go. In theory, if you've done everything right, and if there are no external forces such as winds and currents acting on your boat, you will eventually reach your destination. Of course, if there are one or more obstructions between you and your goal, you will need to break up your course into several legs to get around them, with separate courses for each.

Incidentally, parallel rulers are a drafting instrument used to move a line around on a sheet of paper without changing its direction. They consist of two rulers connected by a mechanical linkage which keeps them parallel to each other. They are available at any marine store. A bit puzzling at first, once you've seen somebody use one you will understand exactly how they work. Some navigators prefer twin drafting triangles, a protractor, or a Breton plotter to do the same job.

Of course, your boat is not on rails; there are forces as well as steering errors which tend to push you off course. But the initial course line is your first estimate, it allows you to identify the landmarks and aids to navigation in your path that will allow you to correct your course over time, and it will alert you to hazards along the course to watch out for. Both hazards and landmarks, as well as much other useful information, is plotted on the chart. When using a GPS, the computer is constantly consulting the satellites to update your position, and continuously recommending a course to reach your waypoint. As a pilot, you are going to have to do this job yourself. Piloting provides a variety of techniques to help you correct your position as you travel, allowing you to alter your course as needed to ensure you arrive safely at your destination.

As your craft gets further from the familiar waters near your marina, compare the landmarks visible to you on shore or in the water (buoys, range markers, obstructions, lights, etc) with those on the chart. Charts plot many prominent objects carefully, and label them, so that the seaman has a selection of features to help with his piloting. These may be official aids to navigation such as buoys and lighthouses or perhaps prominent buildings, radio towers, smokestacks and so on. They are marked for identification (for example, "easternmost of three water tanks", or "steeple") and their positions carefully plotted . If you can identify several of these landmarks and locate them, it is possible to determine your position by taking bearings.

A bearing, or Line of Position (LOP) is like a course; it is a line of constant direction from you to a distant object, although it is not necessarily going to be a path for your boat. Let us say you are on the water and you visually identify several objects ashore which also can be located on your chart. If you were to aim your boat at these objects and read off your compass headings to them, it would give you a list of bearings to those landmarks. There is no need to actually use your boat and steering compass as an aiming device, most boaters carry a smaller bearing compass which can be aimed like a gun at those landmarks to read off the angles directly. You might determine that, say, the bearing to the "lighthouse" was 21 degrees, and the bearing to the "center span of the bridge" was 105 degrees. By locating these landmarks on the chart and using the compass rose on the chart to draw the LOPs, then "walking" them with parallel rulers to the appropriate landmarks, it is possible to draw these LOPs on the chart. Your boat is somewhere on each of these LOPs, so where they cross must be your position! Of course, we are assuming that you took all bearings very close together in time, so your boat's motion between the observations was negligible.

If you take three bearings on three objects and they cross at one spot on the chart, you are assured you have identified all three objects correctly and that your position is accurate. In other words, you are triangulating your position, doing the same thing with your sightings that your GPS unit does with radio waves from satellites. Two LOPs crossing will give you a position, but it may not be accurate if you misidentified a landmark or made a drafting error. If three LOPs do not cross near the same spot you know at least one was in error, but you won't know which one. If you shoot four bearings you can afford to make one blunder and still get a good position (or fix, to use the navigator's term). The three good ones will cross close together, forming a small triangle, the bad one will miss the intersection point of the other three altogether. In general, the quality of a fix is the size of the little triangle formed by the three LOPs. Sailors call it the "three-cornered hat". A small one tells you the fix is a good one. A big one tells you either misplotted, misread your bearing compass or mistook a landmark. Of course, two LOPs can give you a perfectly good fix, but you'll have no way of confirming that you didn't make a mistake. In other words, you won't trust it.

Piloting is a skill which depends on practice and experience, judgement and skill; but in practice it is based on the simplest of geometrical principles. The aids to navigation marked on a chart are identified and catalogued by number, and their characteristics are clearly written down to help you identify them, even in the dark. Every Coast Guard-placed light, Aid to Navigation (ATON) on the water has it's own flashing characteristics written down on the chart so you can clearly identify and locate it, even against the clutter and confusion of background lights on shore. This is what all those cryptic messages on the chart tell you ("7", flsh grn, 4 sec) . Even in a crowded harbor in the dark, you will be able to find that 4 second flashing green, number 7 buoy and be able to shoot your bearing.

The practice of navigation involves sailing along your course, taking frequent bearings and plotting your fixes on the chart as you go to ensure you are where you think you are and on schedule. If your boat starts to drift off course and you are confident your new positions are correct, you may have to lay out a new course to your destination, starting from your last good fix. The chart will show you if these new courses stand you into danger, and they will also suggest new landmarks for bearings up ahead, landmarks you may not even see yet and that you will need to know where to start looking for them. Perhaps the easiest way to visualize this is to consider how lighthouses were once used for coastal navigation. In the old days, before GPS and radar, dangerous coastlines were dotted with lighthouses placed far enough apart so that a ship offshore on a safe course parallel to the coast could shoot bearings and ensure it was still on course, neither too close to shore and standing into danger, nor too far offshore to have no lighthouses visible. The lights were spaced so that at least two were visible at any one time, sometimes three, so that navigators could keep up a running plot by triangulation to make sure they were on track. As one lighthouse would fall below the horizon behind them, one would always be reassuringly visible abeam and the navigator could glance at his chart and know the next one would soon be poking up it's beacon over the edge of the world ahead, He knew approximately what its bearing would be because his position was well known and constantly being updated. The height above sea level of each light was also printed on the chart, allowing the pilot to calculate how far away the light was across the curvature of the earth, as a check on his other calculations. After the fix was plotted, the distance to the light could be scaled off the chart to verify it was the correct one. If the fixes clustered along the course line, the situation was under control, and even the ship's speed could be determined. There is no feeling in the world like going up on deck and watching a distant lighthouse pop up over the horizon just where and when you expect it, right on schedule. Your crew will envy and admire this feat of mastery over arcane knowledge. They're not likely to mutiny, either, if you're the only one aboard who knows how. It's a feeling no soulless GPS screen can give you...

You owe it to your shipmates to master these skills before you go to sea and to avoid relying on one method of navigation alone. The tools needed for piloting, are simple: a steering compass, a bearing compass, a chart, and a few simple drafting aids like pencils, parallel rulers and dividers.. The basic skills are easy to learn, you can pick them up in an afternoon. But getting really good at them can take a lifetime.