Navigation

Navigation is the science of determining the position of a ship or aircraft and charting a course for guiding the craft safely and expeditiously from one point to another. The practice of navigation requires not only thorough knowledge of the science of navigation, but also considerable experience and judgment.

The science of navigation is divided into Four Principal Techniques: (1) so-called Dead Reckoning which is derived from the phrase deduced reckoning, and estimates the approximate position of a craft solely from its course and speed; (2) Piloting, which involves guiding the craft by frequent reference to geographical landmarks and navigational aids and by use of soundings(3) Celestial Navigation, which uses the observation of celestial bodies to determine position on the surface of the earth; and (4) Electronic Navigation, the most important and advanced system of navigation today, using radio and electronic equipment

Position, Direction & Speed on the Earth's Surface

The basic problems of navigating any craft involve the determination of its position and direction and the measurement of speed, distance, and time in proceeding from one point to another. Position is a point on the earth's surface that can be recognized as a set of coordinates, such as Latitude and Longitude. Direction is usually indicated as the angular measurement in degrees of arc, from the direction of true north between one place relative to another without reference to the distance between them. Speed is the rate of travel expressed in nautical miles per hour (1 knot = 1.853 km/h), and distance is the spatial length between two places without reference to the direction between them

The basic concept in Piloting is known as the Line of Position - a line indicating a series of possible positions of the craft and determined usually by observation. The line may be straight, curved, or irregular, such as a line produced by plotting a series of soundings taken over a period of time. One line of position is not sufficient to determine the exact position of a ship. The point of intersection of two or more lines of position, taken simultaneously or adjusted for time lapse, is a positive position known as a Fix. Fixes serve as reliable guideposts for future movements or decisions. The navigator in pilotage waters strives constantly to arrive at such intersections of lines.

Lines of position may be obtained by several methods. A line of position may be obtained by any one of the following methods: a range within which two known fixed objects appear in line, and the ship is placed somewhere on this line; a compass bearing of an object observed visually or by radar; a range by range finder or by radar; a single sounding or a series of soundings of the bottom (usually referred to as a chain of soundings); a horizontal angle, measured by a sextant, between two known objects; a vertical angle, measured by a sextant, of an object of known height; an echo of the ship's whistle or siren; the use of synchronous radio and sound transmissions from a known fixed object; a radio direction-finder bearing; lines of position derived from one of several electronic systems; and astronomical lines of position.

Piloting is the most exacting form of navigation because it entails the movement of yachts under many potentially dangerous conditions. The greatest care and exactness is necessary for success in piloting, especially in poorly charted coastal waters or under unfavorable weather and visibility conditions. One of the chief concerns of the navigator in pilotage waters, where traffic is heavier than at sea, is to avoid collision with other ships.

Dead Reckoning (DR) and Estimated Position (EP)

Yachting in coastal waters in good visibility usually requires no more than pilotage. There are times, however, when the cruising sailor will need to know how to establish their position by Dead Reckoning - that is reckoning deduced from log and compass.

The principle is simple if you know the direction and distance you have sailed from a given start position, you can quickly establish where you are.

To convert this simple dead reckoning into something more useful, it is necessary to add two corrections - leeway and tidal streams. Leeway must be estimated by comparing the yachts wake with her course. In practice, leeway will be negligible when reaching or running but could be as much as 10 degrees when beating.

The direction and rate of the tidal stream comes, as outlined earlier, from the chart or tidal stream atlas. Once leeway and tide have been taken into account, the position by dead reckoning can be converted to an Estimated Position, as shown on diagram.

The basic principles outlined above show how to find your position by dead reckoning; in other words working from history. The opposite task for the navigator is to establish a course to steer allowing for tide and leeway.

This will not apply when beating, as you will try to sail as close to the wind as possible, but will be useful when planning the offwind legs of your journey.

1. You work the above Dead Reckoning procedure in reverse.
2. Estimate the expected tidal stream.
3. Estimate your expected speed and, using the construction shown, work out the required course.
Add a correction for leeway (upwind).

That is the basic principle of all the navigation required for Inshore and Coastal cruising. How much of it you actually put into practice depends on your patience and prevailing weather conditions.

Any combination of these methods of determining a line of position permits fixing the position of a yacht. Fixes may be arrived at by cross bearings, by a bearing and distance of the same object, by a bearing and a sounding taken simultaneously, by horizontal sextant angles, and by two bearings of a single object taken at different time intervals but adjusted for time lapse when plotted known as a Running Fix.

In addition to these graphic methods, a yachts position can be deduced by the use of Horizontal Angles in conjunction with a Three-arm Protractor. Such a protractor consists of a circle, graduated in degrees, to which is attached one fixed arm and two arms pivoted at the center. If horizontal angles taken on three identifiable fixed objects shown on a chart are set on the protractor and the latter is positioned on the chart with the objects lined up on the three arms, the position of the ship is fixed at the center. Navigation Aids may consist of various types of buoys, lighthouses, and light vessels, with characteristic shapes and colors providing significant daytime identification and characteristic phases and colors of lights providing identification at night. Where these aids are absent, the navigator must resort to taking bearings of mountain peaks and of charted structures such as water tanks or church spires, and taking tangent bearings of islands or points of land.

The hand-bearing compass allows you to measure the magnetic bearing from other objects to your position. If you take the bearing of a known object using a hand bearing compass and then relate that to the chart, you must be on a given Position Line from that object.

By taking the bearings of two objects, you can establish your position with certainty as the point where the position lines meet. For the smallest error, choose objects as nearly as possible at right angles to each other. Ideally, a third object will confirm your fix exactly, but in practice you will find that a fix from three objects will give a 'Cocked Hat' area of uncertainty. Assume you are in the center of that area unless you need to play safe by assuming that you are closest to your nearest point of danger

Practical pilotage in a small yacht is difficult. You might prefer to work in Magnetic bearings all the time, to avoid possible errors in converting to True. You will also find that a plotting instrument like a Breton Plotter will be much easier to use than the parallel rules of formal navigation.

Even a plotter may be difficult to use at times, so part of your planning might include drawing radial lines from known objects at, say, 1 to 10 degree intervals in order to make life easier when afloat. Equally, a few notes written on the chart before you slip it into its protective covering might help you remember important details.