Compass, magnetic and true
A compass reading is affected by the vessel’s magnetic deviation. Magnetic north also differs from true north by the charted variation. For these examples, easterly corrections are positive and westerly corrections negative.
True = Magnetic + Variation
For example, compass 080°, deviation 2°E and variation 11°E give magnetic 082° and true 093°. Reverse the arithmetic when converting from true to compass. Use the relevant deviation table and chart variation, including its annual change, in real chart work.
Adding a current
Set is the direction towards which the current flows. Drift is its speed. This is different from the convention for wind, which is named for the direction it comes from.
In this simplified model, heading is true, speed through the water is constant, current is uniform and leeway is zero. The calculator adds east and north components to find course and speed over the ground. It does not calculate a course to steer to a destination.
Check your understanding
A boat heads due north at 5 kn while a current flows due east at 2 kn. Should the ground speed be 7 kn? Work it out before looking at the calculator.
Reveal the reasoning
The velocities are perpendicular, so their magnitudes do not simply add. Ground speed is √(5² + 2²) = 5.39 kn and the course over ground is approximately 021.8° true.
This model is for understanding vectors. Use proper passage planning, current observations and approved navigation equipment on the water.