Reading Nautical Charts and Buoyage - Navigation Basics

Prerequisites

This is an introductory navigation lesson for the RYA Competent Crew course. No previous navigation experience is needed. You should already have:

• Learned basic boat terminology including port, starboard, bow, stern and abeam.
• Steered by compass and by landmark (from the Points of Sail and Steering lesson).
• Some familiarity with the ship's log and watch duties (from the Living Aboard and Watch Systems lesson).

Competent Crew does not expect you to be the navigator, but the syllabus asks you to understand the basics so that you can help the skipper, keep the log, steer a compass course, recognise buoys and help with position fixing. The full skills are taught on the RYA Day Skipper course.

Learning Objectives

By the end of this lesson you should be able to:

• Explain latitude and longitude and read a position from a chart.
• Measure distance on a chart using dividers and the latitude scale.
• Read common chart symbols: soundings, depth contours, drying heights, rocks and wrecks.
• Understand chart datum and calculate a simple depth of water from a charted depth and a height of tide.
• Identify IALA Region A buoyage: lateral, cardinal, isolated danger, safe water and special marks, by shape, colour and topmark.
• Understand compass bearings, reciprocals, variation and deviation, and convert between true and magnetic.
• Take a bearing with a hand-bearing compass and understand how two or more bearings give a fix.
• Understand dead reckoning and estimated position, and keep a log that allows them to be worked out.
• Choose a suitable chart for a job, explain what scale means, and check that a chart is up to date (syllabus: awareness of charts and navigational publications).
• Use the rule of twelfths for a rough height of tide, and work out under-keel clearance and clearance under a bridge (syllabus: tides and tidal streams, basic concepts).
• Read a light characteristic such as Fl(3)G 10s and state what the lights of lateral, cardinal, isolated danger, safe water and special marks look like at night.
• Convert between True, Magnetic and Compass directions when both variation and deviation apply, and steer a compass course accurately.
• Write a useful log entry, and describe the part of crew in the navigation routine on a passage.

RYA Competent Crew syllabus mapping: this lesson covers the Navigation items of the Competent Crew practical syllabus, namely awareness of charts (latitude, longitude, distance, symbols), chart datum and a simple height-of-tide calculation, recognition of IALA Region A buoyage, understanding of the compass, variation and deviation, taking hand-bearing compass bearings, and keeping the log so that the skipper can work out a DR and an estimated position. Where the syllabus says "awareness" you are expected to explain the idea in your own words and do the simple arithmetic, not to plot a full passage.

Charts

A nautical chart is a map of the sea. It shows the depth of water, the shape of the seabed, dangers, navigation marks, lights, the coastline as seen from seaward, and conspicuous features ashore. In the UK most yachts carry Admiralty (UKHO) charts, Imray charts or chart-plotter electronic charts. Paper charts remain essential: electronic systems fail, and a paper chart lets you see the whole picture at once.

Chart types, scale and what to carry

Charts come in different scales. The scale is a ratio, for example 1:75,000, meaning one unit on the chart is 75,000 of the same unit on the sea. A small-scale chart (such as 1:500,000) covers a big area with little detail and is used for passage planning. A large-scale chart (such as 1:12,500 or larger) covers a small area in great detail, and is what you need for entering a harbour. Remember it by the numbers: the smaller the number after the colon, the larger the scale and the more detail.

On passage the skipper normally uses the largest-scale chart available for the area, because small dangers drop off a small-scale chart. When you are asked to hold the chart or read a position, say which chart you are looking at, because the same name can appear on several charts of different scales.

Each chart has a title block and margin notes. Before relying on any chart check:

1. The edition date and the last Notice to Mariners correction (bottom left margin).
2. The units of depth: modern UK charts are in metres and decimetres, but older charts may be in fathoms and feet.
3. The chart datum for soundings (LAT on UK charts) and the height datum (MHWS).
4. The horizontal datum (WGS 84 or older such as OSGB 36), which matters when you transfer a GPS position onto the chart. Many charts say "positions obtained from satellite navigation systems can be plotted directly" and others give a correction to apply.
5. The buoyage note, which says whether the chart follows IALA Region A.

Paper charts cannot be hacked or run out of battery, and they show you the whole picture. A chart plotter is wonderful, but if the plotter is zoomed in too far you can miss a rock that is just off the screen.

Latitude and longitude

Position on the earth is given by two angles. The diagram below shows the grid.

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• Latitude is measured north or south of the equator, from 0 degrees at the equator to 90 degrees at the poles. Lines of latitude (parallels) run east to west. On a chart, latitude is read from the scale on the left and right edges.
• Longitude is measured east or west of the Greenwich meridian, from 0 to 180 degrees. Lines of longitude (meridians) run north to south. Longitude is read from the scale on the top and bottom edges.
• Each degree is divided into 60 minutes, and minutes into decimals. A position is written latitude first: for example 50 degrees 22.4 minutes North, 001 degree 45.0 minutes West, written 50 22.4N 001 45.0W.

Distance and the latitude scale

One nautical mile (nm) equals one minute of latitude (about 1,852 metres). Speed at sea is in knots: nautical miles per hour.

This makes measuring distance easy, but only use the latitude scale (side edges), never the longitude scale at the top and bottom, because meridians converge towards the poles and a minute of longitude is shorter than a mile everywhere except the equator. On a Mercator chart the latitude scale also stretches towards the poles, so measure at the latitude level of the distance you are measuring.

The diagram below shows the method.

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1. Open the dividers so the points sit on the two positions on the chart.
2. Without changing the spread, move them to the latitude scale on the side of the chart, level with the area you are measuring.
3. Count the minutes between the points. 6.5 minutes of latitude = 6.5 nautical miles.

For longer distances, set the dividers to a convenient distance (say 5 miles) and "walk" them along the line, counting steps.

Directions on the chart: the compass rose

Directions are given in three-figure degrees measured clockwise from north: 000 is north, 090 east, 180 south, 270 west. Always say and write three figures: "zero four five", not "forty-five".

The chart has one or more compass roses printed on it. The diagram below shows one: the outer ring is aligned with true north (the geographic North Pole), and the inner ring with magnetic north, which is where a compass points. The difference between them, called variation, is printed across the rose.

To measure a direction on the chart, the navigator uses a plotter (such as a Portland or Breton plotter) or parallel rulers. The diagram below shows parallel rulers: lay one edge along the line, then "walk" the rulers across the chart without changing their angle until an edge passes through the centre of the compass rose, and read the direction from the rose.

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Using a Portland plotter

Most navigators now use a plotter instead of parallel rulers. A Portland plotter (a Breton plotter works the same way) is a clear plastic rectangle with a rotating dial in the middle. The dial carries a square grid. The base has an arrow marked for the direction of travel, an index mark at the top of the dial and a small variation scale either side of the index.

To measure the direction from A to B:

1. Lay the long edge of the plotter along the line from A to B.
2. Check the arrow points from A towards B, the way you are going. If it points the other way you will read the reciprocal, 180 degrees out.
3. Turn the dial, not the base, until the grid lines on the dial lie along the meridians and parallels of the chart, with N towards the top of the chart.
4. Read the true direction at the index mark. For magnetic, read against the variation scale at the chart's variation, or add west variation and subtract east.

To lay off a direction from a point, work the other way round: set the bearing on the dial first, turn the whole plotter until the grid lies along the meridians, slide it, still square, until the edge touches the point, and draw the line. The plotter is not a ruler for distance: always take distance off the latitude scale with dividers.

You can practise all of this on screen with the digital Portland plotter, which has a practice chart, dividers and drills. The exercises at the end of this lesson use it too.

Depths and soundings

The small numbers scattered over the sea area of a chart are soundings: the depth of water in metres (and decimetres, written as a subscript, for example 8 with a small 2 = 8.2 m) below chart datum. Chart datum on UK charts is Lowest Astronomical Tide (LAT), roughly the lowest level the tide falls to under normal weather conditions. Because of this, the actual depth is almost always more than the charted sounding, never less (except with unusually high pressure or strong offshore winds).

Depth contours join points of equal depth, like the height contours on a land map. The diagram below shows them: close-together contours mean a steep slope; widely spaced ones a gentle slope. Shallow areas (often the 0, 2 and 5 metre contours) are shaded in blue, and areas that dry at low water are shown in green.

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Drying heights are shown as underlined soundings in the green drying area: an underlined 2 with a small 1 (written 2.1) means the bottom there is 2.1 m above chart datum, so it is dry until the tide has risen more than 2.1 m.

Heights of land features, lighthouses and bridges are measured from a different level, Mean High Water Springs (MHWS) on UK charts, so that heights are the clearance you will normally have at the highest common tide.

Rocks, wrecks and other symbols

The diagram below shows the standard symbols for rocks and wrecks.

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Other symbols you will meet: anchorages (anchor symbol), prohibited anchoring (anchor crossed out), cables and pipelines (wavy magenta lines), lights (magenta flare), traffic separation schemes, and the seabed type (S = sand, M = mud, R = rock, G = gravel, Sh = shells). The full list is in the Admiralty publication Chart 5011, Symbols and Abbreviations, which every navigator should have.

Keeping charts up to date

Charts are corrected using Notices to Mariners (published weekly by the UKHO), and electronic charts by updates. A buoy that has been moved or a new wreck that is not on your chart is a real danger. Check the chart edition and the last correction noted in the bottom left margin.

Tides and Depth

The sea level rises and falls twice a day in most UK waters. Tide tables (for example in Reeds Nautical Almanac or the Admiralty Tide Tables) give the times and heights of high water and low water at standard ports, as heights above chart datum.

The diagram below shows how the depth of water is calculated.

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Depth of water = charted sounding + height of tide

For a drying area: Depth of water = height of tide minus drying height.

Worked examples:

1. Charted sounding 3.0 m, height of tide 4.0 m: depth = 3.0 + 4.0 = 7.0 m.
2. Drying height 1.2 m (underlined 1 with small 2), height of tide 3.5 m: depth = 3.5 minus 1.2 = 2.3 m. A yacht drawing 1.8 m would have only 0.5 m under the keel, which is too little in any swell.
3. You want to anchor in a spot charted 1.5 m. Low water tonight is 0.8 m. Depth at low water = 1.5 + 0.8 = 2.3 m. With a draught of 1.6 m, clearance at low water is 0.7 m: acceptable in a calm, sheltered anchorage, but the skipper might choose a deeper spot.

Heights of tide between high and low water are found from tidal curves or, roughly, with the rule of twelfths, which is explained below. You will learn the full method, with secondary ports, on Day Skipper.

Tide, tidal stream and the vocabulary you need

Two words are often confused and both matter:

• Tide is the vertical rise and fall of the sea. It is measured as a height above chart datum.
• Tidal stream is the horizontal movement of the water that goes with it. It is measured as a set (the direction it flows towards, in degrees True) and a rate (its speed in knots).

Other terms: high water (HW) and low water (LW); flood (the tide rising) and ebb (the tide falling); range (the difference between HW and LW heights); slack water (the short time when the stream is changing direction); springs (the biggest ranges and strongest streams, a day or two after new and full moon, roughly every two weeks) and neaps (the smallest ranges and weakest streams, around the quarter moons). Tide times in UK almanacs are normally in UT (GMT), so in summer you must add one hour for BST. The time of the tide does not match the time of the stream: in many places the stream keeps running for a while after high water.

Why crew should care: a tidal stream of 2 knots is a large fraction of a yacht's speed of 5 knots. A boat steering a straight line across such a stream arrives well off the line she was pointed along, and in a narrow channel or off a headland the stream can be much stronger.

Reading a tide table

A tide table (Reeds, Admiralty or a harbour publication) lists, for each day, the time and height of each high and low water. For example:

The range for this tide is 5.0 minus 0.8 = 4.2 m. Always check the time zone and whether the column is a standard port or a secondary port that needs corrections.

The rule of twelfths: a worked example

Assume the tide takes six hours to rise from LW to HW (it is closer to six hours 12 minutes, but this is good enough for estimates). The tide rises 1/12 of the range in hour 1, 2/12 in hour 2, 3/12 in hour 3, 3/12 in hour 4, 2/12 in hour 5 and 1/12 in hour 6. So the tide is slowest at the start and end and fastest in the middle two hours.

Using the table above (range 4.2 m, so one twelfth is 0.35 m):

The same rule works on a falling tide, counting from HW. In the third and fourth hours the tide changes by half of its range in only two hours, which is why bars and harbour entrances become usable (or unusable) quite suddenly.

Under-keel clearance

Safe depth is more than just "more water than the keel draws". The standard approach:

1. Required depth = draught + safety margin. A margin of 0.5 m is a common minimum in sheltered water, and more with any swell or if the bottom is rock. Note the boat's draught, and that she may heel or pitch.
2. Depth of water = charted depth + height of tide (or height of tide minus drying height).
3. Compare the two, and decide the time window.

Example: you want to cross a bar charted as drying 0.6 m. Your yacht draws 1.7 m and the skipper wants 0.5 m under the keel, so required depth is 2.2 m. Depth over the bar = height of tide minus 0.6, so you need a height of tide of at least 2.8 m. From the table above, the height is 2.90 m at 0900, so the earliest you should cross is 0900 (three hours after low water). On the falling tide you must be back across before about 1500 (three hours after high water).

Clearance under bridges and cables

Heights on a chart (bridges, overhead cables, lights) are measured above MHWS, not above chart datum. That means the actual clearance is usually more than the charted clearance, except at the highest tides. Clearance now = charted clearance + (MHWS height minus current height of tide).

Example: a bridge has a charted clearance of 10.0 m. MHWS at the port is 4.5 m. The tide now is 2.0 m. Clearance = 10.0 + (4.5 minus 2.0) = 12.5 m. If your mast is 11.2 m above the water (plus aerials) you have 1.3 m margin. Many skippers want at least a metre because a passing wake can raise the boat, and a swell can make her pitch. At high water on a big spring tide the clearance can be less than charted, so take care at the top of the tide. Always know the actual air draught of the boat including the masthead aerial.

The diagram below shows one important mistake: using the wrong reference level. Tide heights must be measured from the same chart datum as the soundings.

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IALA Region A Buoyage

Navigation marks (buoys and beacons) follow the IALA Maritime Buoyage System. The UK, Europe, Africa, most of Asia and Australia use Region A. The Americas, Japan, Korea and the Philippines use Region B, where the lateral colours are reversed. Marks are identified by shape, colour, topmark and, at night, light. Colour can be hard to see in poor light, so learn the shapes and topmarks too.

Lateral marks

Lateral marks show the sides of a channel. Their meaning depends on the direction of buoyage: normally the direction a vessel takes when entering harbour from seaward or going upriver. Around the UK coast in general it runs roughly clockwise around Great Britain; charts mark the direction with a magenta arrow where it is not obvious.

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The two diagrams above show the port and starboard hand marks. The map below shows where each system is used: Region A covers Europe, Africa, most of Asia and Australasia, while Region B covers the Americas, Japan, Korea and the Philippines. If you charter abroad, check which region you are in before you rely on colour.

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Image: Korakys, CC BY 4.0, via Wikimedia Commons. PNG rendering of the SVG, not otherwise modified.

When leaving harbour (against the buoyage direction), it is the other way round: red marks are on your starboard side. Marks are often numbered: even numbers on the port side, odd on the starboard side, counting up as you go in.

Lights on lateral marks. At night a port-hand mark shows a red light and a starboard-hand mark a green light, in any rhythm that is not the special composite group flashing (2+1) used for preferred channel marks. Typical lights are Fl R (single red flash), Q R (quick red) and Fl(2)G. The colour of the light, not the rhythm, tells you the side.

Preferred channel marks show a junction where a channel divides into a main and a secondary route. They are lateral marks with a horizontal band of the other colour, and a composite light Fl(2+1), meaning a group of two flashes then one flash.

Think of it as a normal lateral mark (the top colour) with a hint (the band) about where the other channel is. Lateral buoys are often numbered and lettered to match the chart, and the chart shows each buoy with a small buoy symbol, a magenta dot or circle for its position, and its light characteristics beside it.

The diagram below shows the most common mistake: getting this the wrong way round.

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Memory aids: "Red port wine" (port is red and on the left; port wine is red). And "Is there any red port left?" (port is red and on the left, entering).

Cardinal marks

Cardinal marks are placed north, east, south or west of a danger, and you pass on the side named by the mark. A north cardinal is placed to the north of the danger, so safe water is to the north of it: pass to the north. All cardinal marks are yellow and black, with two black cone topmarks. The diagram below shows the four marks around a hazard.

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In real life a cardinal buoy is often weathered and the topmark is the clearest clue. The photograph below shows a cardinal marker at Paignton, Devon. Read the topmark cones first, then confirm with the colour bands.

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Image: Derek Harper, CC BY-SA 2.0, via Wikimedia Commons

How to remember them:

• The cone points show where the black is: both up = black at the top (north); both down = black at the bottom (south); points out = black at top and bottom (east); points in = black in the middle (west).
• "W" for Wine glass and Waist (west: points together, black waist).
• Lights follow a clock face: east = 3 flashes (3 o'clock), south = 6 (6 o'clock), west = 9 (9 o'clock), north = continuous (12 o'clock). The south mark adds a long flash so it cannot be confused with east or west.

Isolated danger mark

Marks a small danger, such as a single rock, with navigable water all round it. The diagram below shows it.

Black with one or more broad red horizontal bands; topmark two black spheres; light white, group flashing two (Fl(2)). Think "two balls, two flashes, a danger underneath". Give it a wide berth.

Safe water mark

Marks safe water all round, for example the start of a channel (landfall) or the middle of a fairway. The diagram below shows it.

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Red and white vertical stripes; topmark a single red sphere; light white, isophase (equal on and off), occulting, one long flash every 10 seconds, or Morse "A".

Special mark

Not primarily for navigation: marks things such as a spoil ground, cable or pipeline, military exercise area, water-ski area, racing mark or an outfall. The diagram below shows it.

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Yellow, any shape (that does not conflict with lateral marks); topmark a yellow X; light yellow, any rhythm not used by other marks. Check the chart to see what it marks.

Emergency wreck marking buoys (blue and yellow vertical stripes, yellow upright cross topmark, alternating blue and yellow light) are used temporarily over new wrecks.

Lights and Light Characteristics

Many marks, and every lighthouse, are identified at night by the characteristic of their light: colour, rhythm and period. The chart or almanac gives it as shorthand. For example "Fl(3)G 10s 8m 5M" means:

• Fl(3): a group of 3 flashes.
• G: green.
• 10s: the whole pattern repeats every 10 seconds (the period).
• 8m: the light is 8 metres above MHWS.
• 5M: its nominal range is 5 nautical miles in clear weather.

At night, to identify a light: count the flashes, time the whole period with a watch (count seconds "one thousand and one..."), note the colour, and check these against the chart. A light with several colours in sectors, for example "Fl WRG 5s", shows white in the safe sector, red or green over the dangers on either side. If you are in a red sector, you are near a danger and must alter back into the white sector. Sector boundaries are shown on the chart as dotted lines and are excellent position lines.

Light range: the nominal range is how far the light can be seen in clear weather, whatever your height. The geographical range depends on the height of the light and the height of your eye above the water, because of the curve of the earth, and is the limit of how far you can see it even in perfect visibility. Roughly, the distance in nautical miles to the horizon is 2.0 times the square root of the height in metres. A light 25 m high would be seen from a boat with eye height 2 m from about 2.0 x (5 + 1.4) = 12.8 nm at best. A low light that you expect at 8 miles might not be seen until it is closer. Haze, rain and shore lights all cut the range further. When the light first appears over the horizon ("raising") or disappears ("dipping") you can work out the distance off, which Day Skipper students learn as a position line.

The Compass

Bearings

A bearing is the direction of one object from another, measured clockwise from north in degrees. The diagram below shows a lighthouse bearing 040 degrees from your boat.

A bearing you take to an object is the direction from you to it. To plot it on the chart, the navigator draws the line from the object back towards the boat: the direction from the object to you is the reciprocal, 180 degrees different. The diagram below shows the rule.

Examples: reciprocal of 045 = 225. Reciprocal of 300 = 120. Reciprocal of 180 = 000. In practice the line is drawn through the object at the bearing measured, and the boat is somewhere along it on the side away from the object.

Variation

A magnetic compass points to magnetic north, which is not the same place as the geographic (true) North Pole. The angle between true north and magnetic north at your location is variation. It is printed on the chart's compass rose, with the year and the annual change, for example "Var 1 degree 30 minutes W (2020), decreasing 10 minutes annually". Around the south coast of England variation is currently about 1 degree west or less and decreasing; in other parts of the world it can be 20 degrees or more.

Deviation

The yacht's own iron, steel, engine, electronics and even a mobile phone near the compass bend the compass needle slightly away from magnetic north. This error is deviation. It changes with the boat's heading and is recorded on a deviation card for the steering compass, made when the compass is "swung" by an adjuster. On many yachts with a well-sited compass it is only 1 or 2 degrees.

The diagram below shows all three norths: true north, magnetic north (variation away from true) and compass north (deviation away from magnetic).

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Converting between true and magnetic

Directions on the chart are in degrees True (T). Directions on the compass are in degrees Magnetic (M) (or Compass, C, if deviation has not been applied). The rule in the diagram, "West is best, East is least", applies when converting from true to magnetic (chart to compass):

• True to Magnetic: add westerly variation, subtract easterly variation.
• Magnetic to True: subtract westerly variation, add easterly variation.

A matching memory aid for the other direction: "Error West, Compass Best; Error East, Compass Least", meaning the compass reading is the bigger number when the error is west.

Worked examples with 3 degrees West variation:

With 4 degrees East variation, a chart course of 200T becomes 196M, and a compass bearing of 090M becomes 094T.

Ignoring variation causes a steadily growing error: every 1 degree of error puts you about 1 nautical mile off for every 60 miles sailed (the "1 in 60 rule"). So a 5 degree error over 12 miles is about 1 mile off track: easily enough to put you on a rock in poor visibility.

The full conversion: True, Variation, Magnetic, Deviation, Compass

When a boat has both variation and deviation, the full chain is True, Variation, Magnetic, Deviation, Compass, remembered as "True Virgins Make Dull Companions". Going from the chart towards the compass (left to right): add West, subtract East at each step. Going from the compass back to the chart (right to left): subtract West, add East, which some remember as "Cadbury's Dairy Milk Very Tasty" (Compass, Deviation, Magnetic, Variation, True), with "Add Easterly" between the letters.

Worked example 1, chart to compass. The chart course is 075T. Variation is 3 degrees West. The deviation card gives 2 degrees East on a heading near 076.

1. True to magnetic: 075 + 3 (West, add) = 078M.
2. Magnetic to compass: 078 minus 2 (East, subtract) = 076C.
3. Steer 076 on the steering compass.

Worked example 2, compass to chart. The helm reports the boat heading 245C. Deviation on that heading is 3 degrees West and variation is 3 degrees West.

1. Compass to magnetic: 245 minus 3 (West, subtract) = 242M.
2. Magnetic to true: 242 minus 3 (West, subtract) = 239T. That is the true heading to plot.

If the numbers pass through 360 or 000, add or subtract 360 and keep three figures: 359 + 4 = 363, which is written 003.

The 1 in 60 rule gives the cost of a mistake: an error of 1 degree takes you 1 nautical mile off for every 60 nm run. For 20 nm and a 6 degree error (for example adding 3 West variation when you should subtract it) you are 20 x 6 / 60 = 2 nm off track, which is the difference between a safe offing and a rock.

The steering compass

The steering compass is mounted in front of the helm, in a binnacle or on a bulkhead, so the helm can read the heading against a lubber line: the fixed mark that represents the bow. Many have a red light for night use. Keep metal objects, phones, handheld radios and winch handles at least a metre away: they cause deviation.

Steering a compass course

The skipper will give a course such as "steer two-three-five". The helm's job:

1. Turn the boat until the compass card reading under the lubber line equals the course. Say it back: "Steering two-three-five."
2. Do not chase the card. The card swings as the boat moves, so steer on the average and make small, gentle corrections.
3. Steer by a distant object when you can: pick a cloud, a headland or a buoy that is on the right heading, steer at that, and check the compass every few seconds. This is steadier than staring at the card.
4. Under sail, if you cannot hold the course because the wind has shifted, tell the skipper and say what heading you are actually steering. The navigator plots from the headings actually steered, so a changed heading that is not reported spoils the plot.
5. If the wind or sea makes it impossible to keep the course within about 5 degrees, report it. Navigators plot a track from the course steered, so an unreported error accumulates.
6. At every change of course, note the time and the log reading.

Steering in a following sea and swell can cause large swings; practise the steering technique in the Points of Sail and Steering lesson and rely on the helm's feel as well as the compass.

The hand-bearing compass

A hand-bearing compass is a small compass with a sighting prism, used to take bearings of objects. The diagram below shows how it is used.

How to take a bearing:

1. Stand somewhere with a clear view of the object, at least a metre from the engine, steel rigging, winches and electronics, so deviation is minimal. The skipper will tell you good places on your boat; standing in the companionway near the engine is a bad one.
2. Hold the compass at eye level and look through the sight at the object.
3. Let the card settle, swaying with the boat, and read the bearing on the lubber line. Take two or three readings and use the average.
4. Call it out clearly with the time: "Lighthouse, two-seven-zero, at one-four-two-five."

A hand-bearing compass is also the best way to check for risk of collision: if another vessel's bearing stays steady while it gets closer, you are on a collision course.

Finding Your Position

Position lines and fixes

A single bearing tells you that you are somewhere on a line, but not where. The diagram below shows why one bearing is never enough.

Take two or more bearings of different, identified objects, ideally about 60 to 90 degrees apart, and plot them: where they cross is a fix. Three bearings are better: they usually form a small triangle (a "cocked hat"); the smaller it is, the more accurate the fix. Take the bearings as close together in time as possible. Bearings of objects near the beam change fastest as the boat moves, so take objects ahead or astern first and the one nearest the beam last.

Other position lines: a transit (two charted objects in line, which is very accurate and needs no compass), a depth contour (crossing the 10 m line), or a distance off measured by radar. A GPS position should always be cross-checked against at least one of these.

Transits and clearing bearings

A transit is when two charted objects appear in line, for example a church spire and a lighthouse. You are somewhere on the line through both objects, and the line is accurate to a fraction of a degree because no compass is involved. If you cross a transit while sailing, note the time and the log reading: it is a precise position line to use with another bearing. A leading line is a transit marked on the chart for entering a harbour: keeping the two marks in line keeps you in the dredged channel. A clearing bearing is a bearing the skipper writes on the chart, such as "keep the lighthouse bearing more than 270M": if the bearing goes below it, you are heading into danger. The crew can help by watching the bearing and calling it.

GPS and the chart plotter

GPS gives latitude and longitude, usually accurate to a few metres. Learn to read the position, the course over the ground (COG), speed over the ground (SOG), and how to put in a man overboard (MOB) mark in an emergency. Limitations: the plotter shows your position on whichever chart is loaded, which may be out of date or uncorrected; the screen can fail; and the datum of the paper chart may differ. For this reason the skipper checks the GPS against another source (a bearing, a transit or the depth) and keeps a paper log. When asked for the position, give it as latitude then longitude, to a tenth of a minute, and say the time.

Dead reckoning

If you know where you were and what course and speed you have sailed since, you can work out where you probably are. This is dead reckoning (DR). The diagram below shows it.

DR = last fix + course steered + distance run (from the log). Example: fix at 1000; steering 040T at 5 knots; at 1100 the DR is 5 nm along 040T from the fix. The log distance (not speed times time) is the best measure of distance run, which is why accurate hourly log entries matter.

Reading the log

The log instrument measures speed and distance through the water. The diagram below shows the instrument and a ship's logbook page.

Click to zoom

Distance run between two entries = later log reading minus earlier log reading. In the logbook shown, from 1100 (5.2) to 1200 (10.1) the boat ran 4.9 nm, and from 1200 to 1300 (14.8) it ran 4.7 nm. When you keep the log on watch, record the total or trip log reading (not the speed) at every entry and every course change, so the navigator can do this sum.

What goes in each column of a useful log. On most yachts the log is a printed book or sheet. Every hour, and at every change of course or sail plan, record:

Here the distance run between 1000 and 1100 is 17.5 minus 12.3 = 5.2 nm, and between 1100 and 1200 is 22.4 minus 17.5 = 4.9 nm. A falling barometer (for example 3 hPa or more in three hours) is an early sign of deteriorating weather and should be reported to the skipper. Write legibly and in pencil or waterproof pen, and never leave a gap: the log may be needed in an emergency or an investigation.

Estimated position

DR ignores the tidal stream (the horizontal movement of the water) and leeway (sideways drift from the wind). An estimated position (EP) adds them. The diagram below shows the DR position moved by the tidal set and drift to give the EP, and the resulting course over the ground.

Example: DR at 1100 as above; tidal stream for the hour is 1.5 knots setting 090T (the direction the stream flows towards). Plot 1.5 nm in direction 090 from the DR: that is the EP. A fix is marked with a circle and the time; a DR with a short line across the track; an EP with a triangle. EPs are a Day Skipper skill, but as crew you should understand why the boat does not always go where it is pointed.

Worked Example: Helping With a Fix

You are crewing on a passage along the coast. Variation on the chart is 3 degrees West. The skipper asks you to take bearings of a lighthouse and a church spire, both marked on the chart.

1. You go to the windward shrouds, well away from the binnacle and the engine. You identify the lighthouse (white tower, matching the chart description) and the church spire (marked "Spire" on the chart).
2. The spire is roughly ahead and the lighthouse almost abeam, so you take the spire first: 315M. Then the lighthouse: 270M. You call out: "Spire three-one-five, lighthouse two-seven-zero, time one-four-two-five. Log reading 23.6."
3. The skipper converts to true. Variation is West, so going from magnetic to true it is subtracted: spire 315 minus 3 = 312T; lighthouse 270 minus 3 = 267T.
4. She plots a line through the lighthouse at 267T and a line through the spire at 312T. They cross at the boat's position, about 2.3 miles from the lighthouse. She marks it with a circle and "1425".
5. She checks it: the echo sounder reads 12 m, and with about 2 m of tide the chart should show around 10 m at that point. It does. The fix is confirmed.
6. Had she wrongly added the variation (273T and 318T), the lines would have been rotated 6 degrees: at 2.3 miles that puts the fix about a quarter of a mile out, and the error grows with distance.

Worked Example: Entering Harbour at Night

You are on the foredeck lookout of a yacht approaching a harbour at night. The skipper has told you the approach lights and you have the chart in the cockpit.

1. Far off you see a white light: you count Fl(2) every 10 seconds with a watch, and your counts agree with a lighthouse, "Fl(2)10s", on the chart. You tell the skipper "Light identified, group flashing two, period ten seconds, bearing zero-nine-five magnetic."
2. Closer in you see a red flashing light and a green flashing light ahead. You are entering from seaward, so the red is the port-hand mark and the green is the starboard-hand mark. The channel is between them: keep the red on your port side and the green on your starboard side.
3. A flash of white, fast, with 3 flashes appears off to one side of the approach: three flashes is an east cardinal (VQ(3) or Q(3)), so the danger lies to its west and you must pass to its east. You check your position against it and make sure you stay east of it, never between the mark and the danger.
4. A yellow light flashes near the entrance: you check the chart, which shows it is a special mark for a cable area, so you keep clear.
5. Earlier in the approach you passed a safe water mark (red and white stripes, white light isophase or Mo(A)) marking the start of the fairway, and from there you headed for the red and green pair.
6. At each stage you call the light, its colour and the count to the helm, and the skipper confirms each against the chart. The chart shows depths: with a charted depth of 2.0 m and a tide of 3.0 m you expect 5.0 m and you read this to the skipper from the sounder. Because it matches, the skipper is happy.

The skills here are patience (counting a light properly), calling out clearly, and not accepting an identification on one clue.

More Worked Examples and Exam Practice

Reading a position from a chart

Example: a buoy sits where the 50 degrees 30 minutes North parallel and the 001 degrees 20 minutes West meridian cross. You would say "Fifty degrees thirty minutes north, zero zero one degrees twenty minutes west". Always give latitude first, then longitude. To find a position, put one edge of the plotter or a pair of dividers on the latitude scale at the side, slide it across to the object, and read the minutes and tenths. A tenth of a minute is 0.1 nautical mile, about 185 metres, so a position read to the nearest tenth is accurate enough for crew work.

Distance, speed and time

Distance = speed x time. At 5 knots for 2 hours 30 minutes you travel 5 x 2.5 = 12.5 nautical miles. The time to a headland 9 miles away at 6 knots is 9 / 6 = 1.5 hours, so 1 hour 30 minutes. Speed is distance divided by time: 14 miles in 3 hours is 4.7 knots. A knot is one nautical mile per hour, so there is no unit conversion. For a long leg, open the dividers to 5 or 10 miles and step along the track.

A complete depth check

The echo sounder reads 3.2 m, the transducer is 0.4 m below the waterline and the keel draws 1.6 m. If the sounder shows depth below the transducer, the depth under the keel is 3.2 + 0.4 - 1.6 = 2.0 m. Whether the sounder shows depth below transducer, keel or waterline depends on its set-up, so ask the skipper once at the start of the trip. Now compare with the chart: the charted sounding is 2.4 m and the tide height at that time is 1.1 m, so you expect 2.4 + 1.1 = 3.5 m of water. The sounder, corrected to the waterline, says 3.6 m: that agrees to within 0.1 m, and the position is plausible.

Converting both ways, with three figures

Variation 5 degrees East: a true bearing of 120T becomes magnetic by subtracting East, 120 - 5 = 115M. A magnetic bearing of 060M becomes true by adding East, 060 + 5 = 065T. Variation 2 degrees West: 350T becomes 352M, and 358M becomes 356T. Take care when a number passes through 360: 358 + 4 = 362, written 002. Always keep three figures, so 5 degrees is written 005.

Two checks. "West is best": going from true to magnetic you add West. And if the variation is West, the magnetic number is always the larger of the pair.

Deviation and the deviation card

The steering compass is affected by iron on board: the engine, a steel winch, a mobile phone placed beside it. The error is deviation, and a card at the navigation station lists it for each heading. If the card says deviation is 3 degrees West on a heading of 090 compass, and variation is 4 degrees West, the total compass error is 7 degrees West. Going from compass to true you subtract West: 090C is 083T. The memory aid for the order is "True Virgins Make Dull Companions": True, Variation, Magnetic, Deviation, Compass. Going left to right (true towards compass) add West; going right to left (compass towards true) subtract West. Crew are not expected to swing a compass, but you should know why a phone or torch beside the compass can send the boat off course.

An exam-style fix

At 1100 the log reads 22.0. You take a bearing of the lighthouse (045M) and the church (140M) with 3 degrees West variation. True bearings are 042T and 137T. The skipper plots both lines and the fix is where they cross. At 1130 the log reads 24.8 and the course steered was 095T, so the distance run is 2.8 miles and the DR at 1130 is 2.8 miles along 095T from the fix. If the tidal stream is setting 180T at 1.5 knots, then in half an hour it moves the boat 0.75 miles south, so the estimated position is the DR position moved 0.75 miles along 180T. This is why a DR alone can be wrong in tidal waters, and why the skipper wants frequent fixes.

What the examiner or instructor expects

• You can point out the latitude and longitude scales and say which one is for distance.
• You can name the buoys on a chart or on the water and say which side to pass them.
• You can do the variation arithmetic quickly and without error.
• You can take a bearing safely away from metal, call it clearly with the time and log reading, and write it in the log.
• You know the limits of your role: you are crew helping the navigator, not guessing the position.

Common Mistakes

• Using the longitude scale to measure distance. Always use the latitude scale at the side, level with where you are measuring.
• Mixing up True and Magnetic. Always write T or M after every direction. "West is best" applies going from true to magnetic.
• Applying variation the wrong way. Check: with westerly variation, the magnetic number is bigger than the true number.
• Relying on one bearing. One line is not a position. Take at least two, preferably three.
• Taking bearings near the engine or steel. Move away from metal and electronics.
• Misidentifying the object. Check the light characteristic, shape or description against the chart before using it.
• Confusing port and starboard lateral marks when leaving harbour. The colours relate to the direction of buoyage, which is towards the harbour.
• Passing the wrong side of a cardinal. Pass on the side the mark is named after: north of a north cardinal.
• Forgetting that soundings are below chart datum. Add the height of tide; and remember that drying heights are subtracted.
• Writing speed instead of the log distance in the logbook. The navigator needs distance run.

Safety Notes

• Electronic chart plotters are excellent, but they can fail or show charts at too small a scale to reveal dangers. Always have paper charts and know roughly where you are.
• In poor visibility assume your position is less accurate than it appears and keep well clear of dangers.
• Never pass close to an isolated danger mark or between a cardinal mark and the danger it marks.
• Check your chart is the latest edition and corrected.

Summary

• Latitude (north and south) is read on the side scales; longitude (east and west) on the top and bottom. One minute of latitude equals one nautical mile; measure distance only on the latitude scale.
• Soundings are depths below chart datum (LAT); actual depth = sounding + height of tide. Drying heights are underlined and subtracted from the height of tide.
• IALA Region A: red can to port, green cone to starboard, entering harbour. Cardinals: pass on the named side; cones point to the black. Isolated danger: two black balls, pass either side at a distance. Safe water: red and white stripes, red ball. Special: yellow X.
• Bearings are in three figures clockwise from north. Reciprocal = plus or minus 180.
• Variation (from the chart) and deviation (from the deviation card) separate true, magnetic and compass north. True to magnetic: add West, subtract East. Magnetic to true: subtract West, add East.
• Two or more bearings give a fix. DR uses course and log distance; EP adds tidal stream and leeway.
• Keep the log accurately: time, log reading, course, wind, barometer and events.

Check Your Understanding

1. On a chart, where do you measure distance, and why?

Answer: On the latitude scale on the side edges, level with the area you are measuring, because one minute of latitude equals one nautical mile. The longitude scale cannot be used because minutes of longitude get shorter away from the equator.

2. The chart shows a sounding of 4.3 m and the height of tide is 2.6 m. What is the depth of water?

Answer: 4.3 + 2.6 = 6.9 m.

3. A sounding shown underlined as 1.5 m is in a green area. The height of tide is 3.0 m. What is the depth?

Answer: It is a drying height, so depth = 3.0 minus 1.5 = 1.5 m.

4. Entering harbour in IALA Region A, you see a red can-shaped buoy. Which side do you leave it on? Which side when leaving harbour?

Answer: Entering, keep it on your port side. Leaving, it will be on your starboard side.

5. Describe a west cardinal mark and say which side of it you pass.

Answer: Yellow, black, yellow horizontal bands (yellow with a black middle band); two black cones point to point; white light flashing 9 times (very quick or quick). Pass to the west of it.

6. What is an isolated danger mark and how do you identify it?

Answer: It marks a small danger with navigable water all round. It is black with a red horizontal band, has two black sphere topmarks, and shows a white light group flashing two. Pass either side, keeping well clear.

7. Variation is 4 degrees West. The chart course is 085T. What magnetic course should be steered?

Answer: True to magnetic, add West: 085 + 4 = 089M.

8. Variation is 3 degrees West. You take a bearing of 270M on a lighthouse. What true bearing does the navigator plot?

Answer: Magnetic to true, subtract West: 270 minus 3 = 267T.

9. Why does a single bearing not give your position, and how many bearings should you take?

Answer: A single bearing only shows that you are somewhere along one line. You need at least two bearings of different objects that cross at a good angle; three is better because the size of the resulting triangle shows the accuracy.

10. At 1300 the log reads 14.8 and at 1400 it reads 20.3. You steered 055T throughout. How far did the boat travel through the water, and where is the 1400 DR?

Answer: 20.3 minus 14.8 = 5.5 nm. The DR is 5.5 nm along 055T from the 1300 position.

11. A buoy has black and yellow bands with two black cones pointing down. Which cardinal mark is this and which way do you pass it?

Answer: A south cardinal mark (yellow above black, cones point down). Pass to the south of it. At night it shows a white light flashing six times plus one long flash.

12. Variation is 5 degrees East and you take a magnetic bearing of 060M. What true bearing is plotted, and what is its reciprocal?

Answer: Magnetic to true, add East: 060 + 5 = 065T. The reciprocal is 065 + 180 = 245T.