Basic Inshore Navigation
Interpretation of Charts
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Nautical Charts are a projection of a curved surface of the earth onto a flat sheet of paper. As a result charts are drawn using a system of Latitude (vertical lines) and Longitude (horizontal lines) as a grid reference and a Scale. Distance in nautical miles is always measured on the latitude scale (up the side of the chart) where one minute of latitude is equal to one Nautical Mile. The vertical lines on the chart all relate to true north.

Charts vary in scale according to their purpose. The Large-Scale Charts cover a relatively small area in great detail and are the most useful for navigators operating in crowded waters.

Every chart is full of information, most of it given by means of a wide range of symbols. They are all listed in Chart 5011 (which is actually a booklet). To keep charts updated with changes in lights, buoys or corrections to other important features use the weekly Notices to Mariners or quarterly Small Craft Edition of the Notices to Mariners published by the Admiralty.

Dead Reckoning (DR) and
Estimated Position (EP)

Yachting in coastal waters in good visibility usually requires no more than safe 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 in diagram.

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 off-wind 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.

Fixing Position by Bearings

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.

Fixing Position by Transits

Although buoys form the principal aid to safe pilotage, sailors will find the visual use of transits extremely useful. A transit is formed when two identifiable objects are brought in line with each other. If those objects are in line, you know you must be on a Position Line extending through the objects. This gives a very accurate position line without reference to the compass or any other navigational aid.