Plotting a Sea Course

Open to everyone until Friday, October 2

To keep it, print it while it is open.

A navigator draws a line from the starting point to the destination, then adjusts it for wind, compass error and moving water. The final heading is the direction the boat must steer so it arrives at the planned place.

In the picture

The boat’s plotted position at the crossing of two course lines
The straight course line running to the destination island
The compass needle: magnetic declination, the angle between compass north and chart north
The boat’s wake angled away from its centreline: leeway caused by wind
The blue curved arrows: the direction and speed of a current

Examples

A ferry captain steers slightly up-current to dock at the correct pier.
A kayaker aims upstream to cross a river straight across.

Key takeaways

A course line shows the intended track; the heading is what you steer after corrections.
Wind, compass error and current can each push a boat off the planned line.