Skysail Training            Keith Bater

(inc Chartwork

Navigation - Chartwork, Dimensions and Conventions          RYA Books   

Click here to buy a 2 page laminated summary of all the chartwork, only 2.50:   SKYSAIL NAVIGATION  SKILLS CHARTS


Always write the time as 24 hour clock nnnn, eg  0356 with no decimals.

The Standard Time is:

Coordinated Universal Time (UTC) - also known as UT, the International Convention the same as GMT  ie 1330 UT

Greenwich Mean Time (GMT)  May be called Zulu or Z time by the military and Coastguard, and on weather charts  ie 1330Z

Tide tables in almanacs for UK ports are always UT. In the summer months (non-shaded areas) you must add 1 hour (0100) to the time shown to obtain British Summer Time (BST).  Summer Time can also be known as Daylight Saving Time (DST).

 Click here - Time Zones - For more detail on the RYA Training Charts, UT, Time Zone -0100, SPDST.


 see Lat & Long     Measured in Degrees, Minutes ( 60 minutes in a degree) and hundredths of a minute.

Latitude denotes North / South from the Equator: the sides of the chart.  eg Latitude 51 55' .70 N
Longitude denotes East / West from Greenwich: top and bottom of the chart. Longitude degrees are given in three figures eg 004 35'.35

NB Parallel of Latitude, Meridian of Longitude. Greenwich is the Prime Meridian ( 0 degrees )


1 Nautical Mile (M)   = 1 minute of arc of latitude
                                = 1852 metres
                                = 2000 yards

Speed is measured in Knots (kt).  1 knot is one nautical mile per hour.

1 Degree of Latitude = 60 M
Always measure distances on the chart on the Latitude scale (North - South), at the side, opposite the distance required.  
(A minute of Longitude varies from approx 1 mile at the Equator to zero at the Poles, so cannot be used for measurements).  See Mercator Projection below.

One tenth of a nautical mile may be referred to as a Cable = 200 yards. Abbreviation - ca

Charted Depths are always in metres (m)  eg 15 6  is 15.6 metres below Chart Datum. (LAT, Lowest Astronomical Tide).  Depths in brackets are not at the charted position. See  Tide definitions

Tide Heights in the tide tables are above Chart Datum, so depth = Charted Depth + Tide Height

Tidal Streams are in the Tide Diamonds on the chart or in a separate Tidal Atlas. The information is Direction (known as Set)  as a True Bearing, speed at Springs and speed at Neaps.  Speed is known as Rate. See Tidal_stream_interpolation    and  TIDAL STREAM RATES CALCULATION

Drying Heights (underlined, usually on a green area eg  3 5 is 3.5 metres above Chart Datum. Depth then is Tide Height - Drying Height

Heights eg Lighthouses and hills are above Mean High Water Springs (MHWS).

Vertical Clearances ie under bridges are above Highest Astronomical Tide (HAT)  see Moire light and HAT Vertical clearances

Measured in degrees 0  to 359 .  000 is North.     All bearings that you draw or measure, or are shown (eg tides or transits) on charts are True.

T = True             M = Magnetic            C = Compass     (see Variation pages)

T   Variation  = M 

M Deviation = C

Wind direction relates to where it comes from (a North Westerly wind comes from the NW).  Tide direction, known as set, relates to where it is going to (a set of 270 T is going to the West).  Tide speed is Rate, in knots.  Distance travelled by tide is 'Drift'.


Always note the time eg '2318' - no brackets;  and log reading eg (19.7) in brackets next to a fix.  A fix is of little use without this extra information.

Chart projections

The charts most suitable for navigation are the Mercator and Gnomonic projection.

Mercator Chart

The Mercator Projection transforms the spherical earth onto a cylindrical flat sheet of paper. It distorts the latitude and longitude scales towards the Poles, so land masses near the poles are grossly exaggerated in area. But it has the great advantage that lines of a constant compass bearing are straight on the chart, so a navigator can plot a course once and maintain that heading over a long distance. This is called a Rhumb Line, and it crosses all meridians of longitude at a constant angle.  It is not the shortest course.

The shortest distance between two points on the earth's surface is a Great Circle route, but this requires regular changes in the compass course.  A Great Circle is a straight line on a Gnomonic Projection chart.  Meridians of longitude, and the Equator, are all Great Circles, as is any plane surface which passes through the centre of the Earth.



Your chart instruments are a Portland Plotter and single handed dividers.  

The plotter has a large blue arrow which you point from your position towards your destination or bearing, using the edge of the plotter.  Opposite the large arrow is an error scale from 30 deg East to 30 deg West (this appears to be the wrong way round, but it is not!)

In the centre is a rotating compass rose and grid with 2 small blue arrows - you always set these to point North, using the squared grid on the central rose and aligning it with a convenient N-S or E-W line on the chart.

You either read a bearing to a target from the plotter, using the rotating compass rose opposite the small black arrow, or set the bearing first and draw it on the chart.  If you set the net compass error in degrees East or West (variation + / - deviation - see  variation ) on the error scale, and read off the True bearing opposite the net error, this automatically applies variation and deviation with minimum arithmetic.

Put your pencil point on the chart at the position or mark, and rotate the plotter round the pencil point.  Draw visible pencil lines, no longer than you need.  Use the dividers to measure distance on the Latitude (left or right) scale. You can use plotter or dividers to check Latitude and Longitude on the relevant scales, the dividers are more accurate

Accuracy (courtesy of Ian Malcolm) 

'What accuracy is required?' is a common question.  Remember the course is part final assessment, part continuous assessment, so the same standards should be applied throughout.   For Day Skipper and CS/YM Theory the accuracy to aim for is:

This is not practical at sea unless you have a 'big ship' chart table on a vessel of corresponding size and stability, and ALL you are responsible for is the navigation.  However as you are sailing a desk which is wide, horizontal, dry, and well lit, (ie totally unlike a small yacht), that's what the RYA is looking for.  In reality it will be a lot worse so its sensible to try for the best on land.  Practice does make perfect.  But getting the principles right is still more important than outright accuracy.

For chart work,  the instructor checks with the aid of an acetate overlay with the 'official' RYA plot copied on it. As for full marks you always have to give a Lat and Long answer measured from your plot, the accuracy required is comparable. Accurate plotting with a well sharpened 2B or 3B pencil is required, while on a practical course, a blunt chinagraph on the plastic cover of the Small Craft Folio is plenty good enough and on leisure vessels clarity wins over chart life.

When drawing in the classroom, keep the pencil line medium to light in pressure and really fine, you will be doing a lot of rubbing out and the training chart is NOT on good quality paper and you need it to last in shape till your assessments. Try to keep lines only as long as necessary.  Use a good quality plastic rubber such as a Staedtler.  

The questions always indicate which tidal diamond to look up the set and drift from, how much leeway, what variation etc. to eliminate the real life uncertainty of being in between 3 tidal diamonds on the chart and interpolating by educated guess.

The objective seems to me to be to require you to work to a standard that you will:

A,  aim to achieve until you have the experience to know how much and when you can safely relax it, and

B, allow the instructor to easily detect slip-ups like reading off the wrong hour of tide.

Posted 18th April 2011  Keith Bater