Plotting your position

There are various ways of knowing your position from using a GPS to get the latitude and longitude of your location, to knowing where you are as you are next to a buoy.

Single and Three Point Fixes

This is particularly useful at the beginning and end of a passage i.e. passing a buoy at a river entrance. The requirements are:

  • You can see, positively identify and pass very close to the object
  • The position of the object is shown on the chart

This is the most common method of fixing your position. Using three objects in turn measure the bearing and then plot on the chart to see the intersection. You can also use transits and contour lines. Make sure you:

  • Use landmarks that are easily identified
  • Take the bearings as accurately as possible and quickly (as you will be moving)
  • Choose objects that are as spaced apart as possible and are not to far away

Estimating position

If you knew where you were you can calculate your position to take into account tide. This is known as estimating position or E.P. 1. First plot your last known position on the chart; 2. Plot the course steered through the water (CTW) as a True Bearing;
3. Measure off the distance you have travelled – this is your Dead Reckoning Position. This is also a great time to plot your Leeway; 4. Plot the Tidal Vector (using the same time span as you have taken for the course steered) at the END of your CTW (from point A to your Estimated Position (EP));

5. The Course over the Ground is from point A to your Estimated Position (remember: 2 arrows = 2 feet on the ground!)

Running fix

1. Take a bearing and draw it on the chart, together with the time and log reading;
2. You know you are somewhere on this line;
2. Sail for a set length of time (i.e. 15, 30 or 60 minutes) such that you can have a changed bearing of i.e. 30°;
3. Take a second bearing, and plot it on the chart. Note the time and log reading;
4. Choose a convenient point on the first position line and plot your Course steered Through the Water and the distance travelled;
5. At the end of this plot on your Tide (like when you do your Estimated Position - see above);
6. Plot a line parallel to the first position line that intersects your plot in step 5;
7. Where the second bearing (in step 4) and the transferred line meet is your position

Using a rising and dipping table

In the back of the Reeds Almanac, and also in the RYA Training Almanac you can find a rising and dipping table.

If the lighthouse you are looking at is far enough away that you can get it to just "dip" below the horizon, an you know the height of your eye above sea level you can use table and calculate the distance off the light house you are (or if you don't have the table use distance off = 2.08 × (√ (height of eye h metres + √ height of light H metres )

This range together with the bearing of the light house taken with a hand bearing compass can be used to calculate your position.

Using a sextant

A sextant is not just used for calculating the position of a star or the moon in astro-navigation.

With a sextant you can calculate the angle to the top of a chartered object - and from this the distance away from it you are.

  • use the sextant and measure the "vertical sextant angle"
  • correct for index error
  • read off the chart the height of the chartered object (i.e. a lighthouse), remembering it is above MHWSThe distance between you and the lighthouse = 1.856 x (height of chartered object taking into account tide / the angle in minutes)

i.e. if the measured angle is 1° 19', index error is +6': angle = 73'; the height to MHWS is 3m; the height of the lighthouse is 80m then 1.854 x (80-3 / 73) = 1.96 nm

This distance together with the compass bearing can be used to plot your position.

Course to Steer – Theory tips – RYA Day Skipper & Yachtmaster

Passage Planning and Tidal Streams

In our Theory Tips - Tidal Streams we told you how to look up the effect of tides. In this theory tip we are applying it to your proposed journey so we can write our passage plan.

1. Draw the Ground Track (from point A the start point through your destination point B);
(remember: 2 arrows = 2 feet on the ground!)

2. Estimate your boat speed;

3. Measure the distance so you can work out how long it is likely to take;

4. Look up the tide and draw on the Tidal Vector for an hour starting at the same place as our Ground track (from point A to point C);

5. Using your estimated boat speed place your dividers on the end of the Tidal Vector and see where is crosses the Ground Track (at point D);

6. This is known as the Water Track – and you can see what the bearing of the line which is the Course to Steer (do not forget to convert it to a compass heading {see Introduction to Chartwork for more information}).

Note 1: For us to work out our Course To Steer we must drawn a triangle, with each side representing the same length of time.

Note 2: The Course To Steer is the direction you point your yacht when accounting for tide to ensure that you reach your destination.

Remember:
1. Course
2. Tide
3. Steer

Air Masses – Theory tips – RYA Day Skipper & Yachtmaster

Weather is one of the more difficult areas of sailing theory to understand - the weather patterns we see are mainly due to where the air (known as air masses comes from.

Sources of air masses

The source of air mass depends on where they come from. Warm source regions are:

  • Sahara Dessert (dry and warm)
  • Tropical oceans (moist and warm)

Cold source regions are:

  • Arctic Ocean (moist and cold)
  • Siberia (dry and cold)
  • Northern Canada (dry and cold)
  • Southern Ocean (moist and cold)
A great video from the UK Met Office:

Stability of the air mass

Depending on where the air has come from also controls its stability. Unstable air produces strong winds and rain. Air masses (like polar air) are heated from below which makes them stable, and tropical air is heated from below which makes them unstable.

As air masses move towards us they are modified. As they move over:

  • the sea increases the moisture (as the water evaporates from the surface) - known as a maritime track
  • the land remains relatively dry - known as continental track
  • a warmer surface they cause a colder air mass to become unstable as the surface is warmed from below
  • a colder surface then a warm air mass is cooled from below it becomes stable.

Types of air mass

The combination of the source of the air mass and how it reaches us governs its type.

Tropical Continental

Originating over the Sahara this type of air mass reached the UK most often in the summer months.

Visibility is often moderate or poor due to the sand particles in the air (in the Canaries this type of air mass is known locally as a calima).

Polar Maritime

When the air mass originates from Canada (where it is cold and dry) it is known as Polar Maritime as it has a long passage across the North Atlantic picking up warm air from the sea and becoming unstable with frequent showers often heavy.

Returning Polar Maritime

Returning Polar Maritime is when the passage of the air mass goes further south giving it a longer track south over the North Atlantic (causing the air mass to become unstable as it picks up moisture) before turning north east towards the UK (where the sea track causes it to become more stable).

Arctic maritime

Arctic Maritime is similar to Polar Maritime but has colder and drier air due to the shorter sea track.

Tropical Maritime

Originating from the Caribbean (unstable air), and the crossing the warm waters of the Atlantic on South-Westerly winds, the air becomes stable as it passes over the cooler sea and picks up the moisture. This is the most common type of air for the British Isles.

This type of air mass typically has warm and moist air, with low cloud and drizzle.

Polar Continental

The Polar Continental air mass originates from Siberia (with its cold land mass) in the winter months (in the summer it is known as Tropical Continental as the land mass is warm). The weather produced depends on the sea track: in the south of the UK with the shorter sea track over the English Channel you get clear skies and frosts; when the air mass crosses the North Sea to Scotland and the North of England the air becomes unstable (with rain/snow) due to crossing the longer stretch of water.

Tidal Streams – Theory tips – RYA Day Skipper & Yachtmaster

Under international regulations (Safety of Lives at Sea - SOLAS) all boats have to do a passage plan (sometimes called a voyage plan) before leaving harbour. In tidal waters you need to be able to take account of the effect of the tide on your boat.

What are tidal streams?

Tides are the flow of water around the earth caused by a combination of the earths rotation and the effect on the oceans caused by the gravitational forces formed between the sun, moon and earth. This horizontal movement is called the tidal current, or tidal stream.

The largest movement (and hence the strongest tidal streams) usually is at the times of high and low water.

The shape of the land also affects the speed of the tidal stream (it's faster in tidal estuaries and narrow straits). The highest tide in the world is the Bay of Fundy in Canada - and its here you get the fastest tidal streams.

The weakest tidal streams are known as slack tides.

Where can I find the tidal stream data

Information on the tides that are flowing can be found in a Tidal Stream Atlas such as the English Channel Tidal Stream Atlas which is available at the RYA shop.

The tidal atlas gives you a pictorial representation of the tidal flow. You select the correct page for the tidal hour at the port listed at the top of the page, then select the arrow closest to where you will be. The direction of the arrow shows the direction of tidal flow, and the two numbers represent the speed of the tide (for example 09.14). The smaller number is for neaps and the larger number is for springs. This doesn't mean 14 knots for a spring tide in our example - they do not print the decimal point so its 0.9 knots for neaps and 1.4 knots for springs.

Another source of tidal information is from tidal diamonds. These are shown as magenta diamonds on the chart with a letter inside.

With tidal diamonds you again need to work out the tidal hour in relation to the nominated port. Each tidal diamond has data listed for direction of flow (dir) in degree, and then two columns for springs (Sp) and Neaps (Np).

If its not exactly springs or neaps you can take a figure in between for mid range for both tidal streams or tidal diamonds. If it doesn't line up with springs, neaps or mid range then you have to do "computation of rates". Details of how to do this will be in a future Theory Tip.

Coriolis Effect – Theory tips – RYA Day Skipper & Yachtmaster –

Weather is one of the more difficult areas of sailing theory to understand - and one of the reasons is due to the earth's rotation!n

What is the Coriolis Effect?

The earth rotates in an anti-clockwise direction on its axis. This rotation causes the winds to deflect to the right in the northern hemisphere and to the left in the southern hemisphere. This is what causes wind to rotate around low and high pressure systems in opposite directions if you are north or south of the equator!nn

This effect is named after Gustave-Gaspard de Coriolis who worked out why in 1835.nnn

How does the Coriolis Effect affect me?

If you stand in the centre of a spinning playground roundabout (spinning anticlockwise) and attempt to throw a ball it appears to curve to the right. Anyone who is not on the roundabout and watching will see the ball travelling in a straight line. Have a look at our video to see why.

What happens to the winds?

Winds in the northern hemisphere appear very much like a ball thrown on a children's roundabout - they are deflected to the right and are a major factor in explaining why winds blow anticlockwise around low pressure and clockwise around high pressure in the northern hemisphere and visa versa in the southern hemisphere. nnIf it wasn't for the Coriolis Effect the wind would flow directly from high pressure areas to low pressure areas.

The Coriolis Effect influences global wind patterns - and also gives the UK the prevailing south-westerlies, and the Canary Islands the north-easterlies.nn

Clouds – Theory tips – RYA Day Skipper & Yachtmaster

Weather is one of the more difficult areas of sailing theory to understand - and remembering the names of cloud types is one of the most tricky areas.

What are clouds?

Clouds are produced through condensation. The air rises, cools - and with the reducing temperature the air can hold less water vapour so condensation occurs. The condensation makes tiny drops (each m3 of air contains 100,000,000 droplets) of water or ice crystals which settle on dust particles - which form the droplets. Whether the droplets are ice or water depends on the height (and the temperature) of the cloud.

The water droplets (or ice crystals) then start sticking together - forming the clouds. Clouds are made up of either water droplets or ice droplets depending on the height and air temperature.

Great video from the UK Met Office

High clouds – cirrus / cirro

These are clouds over 6000m high. They include:

Medium height clouds – alto

The clouds that are between 2000m and 6000m are medium height are include:

Also at this level are Nimbostratus which are rain bearing layered clouds.

Low level clouds

These clouds are below 2000m and include:.