Sailing know-how / Navigation / Chartwork and courses
Chartwork and courses
Even with a plotter and phone, a paper chart belongs on board. If you can use it, you always know where you are, even if the electronics fail.
What a chart shows
- Depths in metres, referred to chart datum (in tidal waters the lowest low water).
- Hazards such as rocks, wrecks and shoals.
- Navigation marks with colour, shape and light characteristic.
- Along the side the latitude scale: one minute of latitude is exactly one nautical mile (1852 m).
From compass to chart
The compass does not point exactly to true north. Two errors must be allowed for:
- Deviation
- Error caused by iron and electrics on board, listed in the boat's deviation card.
- Variation
- Difference between magnetic and true north, shown in the chart's compass rose.
The sum is: compass course + deviation = magnetic course, + variation = true course. Easterly values are added, westerly subtracted. In the Mediterranean variation is currently small, mostly a few degrees east.
Dead reckoning
Dead reckoning means working out your position from course, speed and time: distance = speed × time. At 6 knots the boat covers one mile in 10 minutes. Wind (leeway) and current (set) shift the actual course over ground.
Fixing your position
With a hand-bearing compass you take bearings on two or three clear objects ashore, such as a lighthouse and a church spire. Draw the lines on the chart; where they cross is the boat. The closer the angles are to 90 degrees, the more accurate the cross bearing.
From my own experience
Planning the next day with the chart in the cockpit in the evening is one of my favourite rituals on board. Which bay, which wind, how far? Often everyone looks on, and the route takes shape together.