Compass Course with a Heading-Referenced Deviation Card
At 0600 you are officer of the watch on a 35 m steel-hulled training vessel in open water, making 9 knots. The planned true course to the next waypoint is 084°T. The chart notes magnetic variation 6°W for the area and year. The ship’s deviation card, graduated by compass heading, gives: 060°C, 4°E; 090°C, 2°W; 120°C, 1°W. The helmsman has been averaging 090°C. A bearing of a distant radio mast taken with the steering compass reads 147°C; its charted true bearing is 139°T. Visibility is good and no local magnetic anomaly is charted. The master wants a compass course that will hold the true track and asks whether the recent fitting of a stainless-steel guard rail near the binnacle has invalidated the card. You must give the course and decide what the bearing check proves.
1. What compass course should you give the helmsman? Show each conversion and explain why the deviation is taken at the compass heading rather than at 084°.
Read the worked answer
Use the relation True = Compass + Deviation + Variation, with east positive. Variation is 6°W, so V = −6°.
- True to magnetic: M = T − V = 084 − (−6) = 090°M.
- First compass estimate: at 090°C the card gives D = −2° (2°W), so C = M − D = 090 − (−2) = 092°C.
- Interpolate at 092°C: between 090°C (−2°) and 120°C (−1°), D ≈ −1.9°. Then C ≈ 090 − (−1.9) = 091.9°C, i.e. steer 092°C.
Check: 092°C + (−1.9° deviation) = 090.1°M; 090.1°M + (−6° variation) = 084.1°T, within a tenth of a degree. The deviation is read at the compass heading because the card is graduated that way; using 084° would apply the wrong tabulated value and ignore the fact that deviation changes with heading.
2. The mast bearing is 147°C. Does it confirm the card at the steering heading? What would a reading of 152°C imply, and what would you do?
Read the worked answer
Expected compass bearing: Cb = T − V − D. With T = 139°, V = −6°, and D at 090°C = −2°, Cb = 139 − (−6) − (−2) = 147°C. The observed 147°C therefore confirms the card at that heading, not at all headings.
If the bearing read 152°C, the discrepancy would be +5° on the compass. The implied deviation would be D = T − V − Cb = 139 − (−6) − 152 = −7° (7°W), compared with −2° on the card. That is a 5° change and means the card cannot be trusted for steering at that heading. A single bearing at one heading does not validate the whole card; after fitting steel near the binnacle the compass should be re-swung through all headings with electrical equipment on and off. Until then, use the gyro/GPS track as primary and treat the magnetic compass as a steering reference only, after checking another bearing on a different heading.