Your RYA course companion – Part 4

Push-button navigation has made every skipper's life easier, but there remains no substitute for paper charts, parallel rules and a soft pencil. Robert Avis guides you through the raw materials for traditional navigation.

Charts, books and drawing instruments

There are several chart suppliers who all present information similarly, but here we shall consider the format of Admiralty charts.

There are Admiralty charts for most parts of the world in a wide range of different scales. They come in three formats. The standard charts are printed on high-quality paper and usually come folded in half, measuring about 28in × 20in (710mm × 520mm). A range of those more commonly used by yachtsmen are available in the Small Craft series (prefixed SC). These are printed on paper that is not so heavy and are supplied folded to 81Ž2 in × 141Ž2in (215mm × 360mm), which are more manageable in the average pleasure craft. This year, the long-awaited Small Craft folio 5600, covering Poole to Chichester, was introduced. It contains ten A2 chartlets, and comes with a plastic wallet for easy use.

Flattening the earth

The shape of the earth is projected onto charts in two main ways, called projections. Either way, they are a compromise because they project a curved surface on to a flat format.

Mercator’s projection

(most yachting charts)

Lines of longitude (vertical) are parallel. To stabilise the projection, the lines of latitude are also parallel, but they get further apart the further from the equator they are. So it is important to measure distances off the latitude scale at the appropriate latitude because one nautical mile (one minute of latitude) closer to the equator will not be exactly the same as one nautical mile further away. Mercator projection is not ideal at high latitudes because land masses are greatly distorted.

Gnomonic projection

(pronounced no-monic)

This is used on large-scale harbour charts and on small-scale polar charts. On a gnomonic chart all great circles appear as straight lines.

Chart corrections

Unless you replace your charts every time you use a boat, it is important to keep them corrected and up to date. The Hydrographer of the Navy issues Weekly Notices to Mariners, which add up to more than 4,000 amendments a year. These are condensed into Small Craft Edition of Admiralty Notices to Mariners, which concentrates on those that are particularly interesting to yachtsmen, and can be obtained by subscription.

MBY’s Chart Update, edited by Dick Hewitt (December issue MBY), offers a concise list of amendments for popular yachting areas. If you are in any doubt about the last correction on a chart, look at the bottom left-hand corner at the ‘Small corrections’ note. This lists the year and corrections that have been appended to the chart. If you are using someone else’s chart, this is a good indication of how up to date the chart is.

Chart symbols

The authoritative guide to chart symbols is Admiralty Chart 5011, which, in fact, is not a chart, but an A4 booklet with a description of every symbol in use. Although not a particularly riveting read, it is a very useful reference book to keep on-board with your charts.

The majority of people miss the vast amount of information printed on a chart. Not only can it show obvious elements such as land and sea, it also shows depths of water and heights of land, speed and direction of tidal streams, dangers, safe areas, channels and so on.


There are three main type of lights used in navigation:

Flashing ­ where the period of light is shorter than the dark period

Isophase ­ where the period of light is equal to the dark period

Occulting ­ where the period of light is longer than the dark period

The time period of lights is given in seconds, and is timed from the beginning of the cycle until it starts again.

A light described as Fl 10s 10M indicates that every ten seconds it will flash, and it should be visible in good conditions for ten miles. If no colour is indicated, it is a white light. A light described as Fl (3) R 15s 15M indicates that every 15 seconds a red light will begin flashing three times, and it should be visible in good conditions for 15 miles. Note that ‘s’ indicates seconds, ‘m’ metres and ‘M’ nautical miles.

Depths and heights

All depths on a chart are measured below chart datum (CD), and drying heights are measured above it. This is a level derived from the lowest astronomical tide (LAT), which is the lowest tide predicted under average meteorological conditions. It should be noted that occasionally, tides are predicted to fall below CD or LAT. Heights of objects on the land are measured above ‘High Water’, which is usually determined as mean high water springs. Although this seems confusing, it allows an extra safety margin in case you forget and calculate heights above CD.

Dead reckoning

Having established your position you can then lay off your planned track: in this case, 110° true.

If you know your speed is ten knots you can predict where you will be by dead reckoning your position. At ten knots, you will travel one mile every six minutes. A dead reckoning or DR is shown by a small line at right angles to the track with the recorded time in minutes. At any time in the future, a quick glance at your watch will indicate your likely position from the DR.

A navigator needs some way to plot courses, some means of measuring, and some means of drawing.

Probably the most popular way to plot a course in a confined space is with the Breton or Portland plotter. The Douglas protractor is a smaller alternative but it cannot be used for drawing long lines. Parallel rules and Rolling rulers are not really suited to small chart table areas, although people from a Royal or Merchant Navy background will have difficulty in using anything else! Always use 2B pencils for navigation because it is easy to erase 2B lines, and charts will have a much longer life if they are not rubbed away.


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