Navigation

Chart, position and units

• Positions are given in latitude (N/S) and longitude (E/W), for example 43°31′ N, 16°15′ E for Trogir.
• 1 nautical mile (nm) = 1852 m. It equals one minute of arc of latitude, so distances are measured on the side scale of the chart.
• 1 knot (kn) = 1 nautical mile per hour, about 1.85 km/h.
• Depths on the chart refer to chart datum, not to the current water level.

Courses and the compass

A magnetic compass does not point exactly to true north. Two errors are taken into account:

• Variation: the difference between magnetic and true north. It is printed on the chart and changes slowly with place and year.
• Deviation: the error caused by iron and electronics on board. It is listed in the boat's deviation card.

Compass course + deviation = magnetic course
Magnetic course + variation = true course
Easterly values count positive, westerly negative.

Dead reckoning: distance, time and speed

Without GPS, position is worked out by dead reckoning from course, speed and time. The basic formula is distance = speed × time.

Example: 6 kn for 1.5 hours gives 9 nm. Tidal stream and leeway shift the real position, so it is checked regularly with bearings or GPS.

Taking bearings

A bearing is the direction to a landmark. Two bearings on different objects, plotted on the chart, cross at your position (a cross bearing). A third bearing shows how accurate the fix is.

Buoyage (IALA Region A)

Europe belongs to IALA Region A. Lateral marks show the sides of a channel, seen coming from seaward towards the harbour:

• Port side: red can-shaped buoy, can topmark.
• Starboard side: green conical buoy, cone topmark pointing up.

Cardinal marks are black and yellow and show on which side of a danger the safe water lies. The two cones of the topmark point: north both up, south both down, east apart, west together. Pass on the side the mark is named after.

Rules of the road

The Collision Regulations (COLREGs) define who gives way. The most important ones for sailing boats:

• Two sailing boats with the wind on different sides: port tack gives way to starboard tack.
• Two sailing boats with the wind on the same side: windward gives way to leeward.
• An overtaking vessel always keeps clear.
• A power-driven vessel gives way to a sailing vessel, except in narrow channels, towards vessels restricted in their ability to manoeuvre and towards fishing vessels.
• Under engine you are a power-driven vessel, even with sails up.