Worked example: course to steer and ETA
1The lesson in plain words
Work one course to steer from start to finish with real numbers, then check it on the GNSS once you are under way.
The data
At 1000 you leave a buoy for a harbour entrance 12.0 M away on a ground track of 045°T. Your STW is 6.0 kn. The stream for the next two hours sets 120°T at 2.0 kn. The wind is from the east and gives 5° of leeway. Variation is 3°W, and the deviation card shows 1°E for headings near 034°M.
The triangle
Draw 045°T from the buoy, A. Lay off the stream, 2.0 M on 120°T, to C. From C, open the dividers to 6.0 M and cut the 045°T line at D. CD measures 026°T: that is the water track to steer. AD measures 6.2 M, so your SOG is 6.2 kn. The stream is partly fair (running with you), so you go faster over the ground than through the water.
ETA
Time = distance ÷ SOG = 12.0 ÷ 6.2 = 1.94 hours, about 1 h 56 min, so your ETA is about 1156. The passage is under two hours and the stream is forecast to stay the same, so one one-hour triangle is enough.
Leeway and compass
Heading roughly north-north-east, the east wind is on your starboard side and pushes you to port. Steer 5° into the wind, to starboard: 026 + 5 = 031°T. West variation is added going from true to magnetic: 031 + 3 = 034°M. East deviation is subtracted going to compass: 034 − 1 = 033°C. Give the helm 033°C.
Check under way
Check COG, SOG and XTE (cross-track error, your distance off the planned line) soon after departure and regularly afterwards. Expect about 045°T and 6.2 kn. COG shows direction, not which side of the line you occupy. Use your position and XTE trend to decide whether a correction is needed. Check the return track for dangers, make a small correction, recheck and log it.
2Key data
- Water track (triangle)
- 026°T
- SOG
- 6.2 kn
- Time and ETA
- 12.0 ÷ 6.2 = 1 h 56 min; ETA about 1156
- With 5° leeway
- 031°T
- Magnetic, then compass
- 034°M (3°W), then 033°C (1°E)
3Key points
- 1
Stream 2.0 M on 120°T, STW 6.0 M from its end: water track 026°T, SOG 6.2 kn.
- 2
ETA from SOG: 12.0 M ÷ 6.2 kn ≈ 1 h 56 min.
- 3
Leeway into the wind: 031°T; then 034°M and 033°C for the helm.
- 4
Check COG, SOG and XTE soon after you start, and adjust.
4Frequent mistakes
Wrong: Point the bow at 045°T and let the stream sort itself out.
Right: The stream would carry you south-east of the line all the way. Steer the course from the triangle.
Wrong: Give leeway the wrong way: 026 − 5 = 021°T.
Right: The east wind pushes you to port, so steer 5° to starboard, into the wind: 031°T.
Wrong: ETA = 12.0 ÷ 6.0 = 2 hours.
Right: That uses STW. Use SOG: 12.0 ÷ 6.2 ≈ 1 h 56 min.
5Practice case & exam keys
Same passage: 12.0 M on 045°T, STW 6.0 kn, east wind, 5° of leeway, variation 3°W, deviation 1°E. But the stream is now forecast to set 120°T at 3.0 kn. Work out the new course to steer, SOG, ETA from 1000 and compass course.
Show the answer
Lay off 3.0 M on 120°T, then 6.0 M from its end to cut 045°T. The water track is about 016°T and SOG about 6.0 kn: the stronger stream needs a bigger correction and gives almost no gain in speed. Time is 12.0 ÷ 6.0 = about 2 h, so the ETA is about 1200. With leeway, steer 021°T, then 024°M; with 1°E deviation still on the card for that heading, 023°C.
Exam keys
- Lay off stream from A, then STW from its end to cut the ground track.
- Read the water track from the triangle and SOG from AD.
- ETA = distance ÷ SOG.
- Leeway into the wind, then variation, then deviation.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
My notes for this unit
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