Few topics cause SBF Coastal (SBF See) candidates more stress than course conversion. Yet behind it lies just one formula and a clear sign rule. Once you understand both, the course questions in the navigation task become quick and reliable. This article explains converting between true course, magnetic course and compass course step by step – including the offset for wind and current.
Why there are different courses at all
Your magnetic compass on board does not point to geographic north. Two effects disturb it:
- Variation (Var): The magnetic north pole is not at the geographic north pole. This deviation depends on location and is printed in the chart’s compass rose.
- Deviation (Dev): Your own vessel (engine, steel parts, electronics) deflects the compass needle further. Deviation depends on the ship and the heading and is read from a deviation table.
That is why we distinguish three courses:
| Abbr. | Course | Reference |
|---|---|---|
| TC (rwK) | true course | geographic north pole (the “real” direction on the chart) |
| MC (mwK) | magnetic course | magnetic north pole |
| CC (MgK) | compass course | the reading of your on-board compass |
Note: In the German exam the labels are rwK (rechtweisender Kurs = true), mwK (missweisender Kurs = magnetic) and MgK (Magnetkompasskurs = compass). The logic is identical.
The one formula you need
From compass to chart you add both disturbances:
TC = CC + Deviation + Variation
The other way round – from chart to compass – you subtract both again:
CC = TC − Variation − Deviation
The magnetic course sits in between:
- CC + Deviation = MC
- MC + Variation = TC
Remember the order from inside out: CC → (Deviation) → MC → (Variation) → TC. Deviation always sits closest to the compass, variation on the outside toward the chart.
The sign rule: East plus, West minus
The most common mistake is the sign. The rule is always:
- East (E) = positive (+)
- West (W) = negative (−)
So a variation of “4° E” is +4°, a deviation of “3° W” is −3°. Just plug the values in with their sign – the result is then correct automatically, no matter which direction you convert.
Worked example 1: from compass to chart
Your compass reads CC = 120°. The chart gives a variation of 3° W, the deviation table for this heading 2° E.
- Apply signs: variation = −3°, deviation = +2°
- TC = CC + Deviation + Variation
- TC = 120° + 2° + (−3°) = 119°
The true course you draw on the chart is therefore 119°.
Worked example 2: from chart to compass
You picked a course of TC = 075° off the chart and want to know what to steer. Variation 2° E, deviation 4° W.
- Apply signs: variation = +2°, deviation = −4°
- CC = TC − Variation − Deviation
- CC = 075° − 2° − (−4°) = 075° − 2° + 4° = 077°
So you steer 077° on the compass to hold the chart course over ground – wind and current not yet included.
Offset for wind and current
The true course only gets you to your destination if neither wind nor current sets the boat off track. In practice – and in some navigation tasks – you therefore add the offset:
- Current offset: corrects for the set caused by current.
- Wind offset (leeway): corrects for wind drift.
The course you actually make over the seabed is the course over ground (COG). Schematically:
TC + offset (wind/current) → COG (or, in reverse, the course to steer in order to achieve a desired COG)
The East-plus, West-minus logic applies here too for the direction of the set. One thing to keep clear: variation and deviation are compass errors, while wind and current are external forces – but both are handled with the same sign principle.
The most common mistakes – and how to avoid them
- Signs swapped: Always write variation and deviation with their sign first, before you calculate. East +, West −.
- Wrong direction: Decide before you start whether you are going from compass to chart (+) or from chart to compass (−).
- Deviation table misread: Deviation depends on the heading – read the value for the correct course, not a fixed number.
- Beyond 360°: If the result exceeds 360°, subtract 360°; if it is negative, add 360°.
How to practise it properly
Course conversion is pure routine – it has to be second nature, so you can do it without thinking. The best approach is to work through dozens of variants with changing signs until the formula runs automatically. If you are still deciding which licence fits you, the comparison SBF Inland vs. Coastal helps, and you will find every German boating licence side by side on the licences overview.
Conclusion
Course conversion for the SBF Coastal is no mystery: one formula (TC = CC + Deviation + Variation), one sign rule (East +, West −) and a bit of practice are enough. Master both and you will solve the course questions in the navigation task in seconds – and walk into exam day relaxed.