Module 10 - Chartwork

Course to Steer (CTS)

What you will learn

Construct a true water track that counters tidal stream, then apply leeway and compass corrections in the correct direction.

A course-to-steer problem works backward from the desired ground track. The tidal vector is applied first; the boat-speed arc then reveals the water track needed to reach that track.

Draw the desired ground track from the starting position. For the chosen time interval, plot the tidal stream vector from the start in the direction the water flows towards, using drift equal to rate multiplied by time. From the end of that vector, swing an arc equal to the vessel's distance through the water for the same interval until it meets the desired ground track.

The line from the end of the tidal vector to the intersection is the required track through the water. Measure that direction as a true course. The distance from the start to the intersection is the expected distance made good over the ground; divided by time, it gives predicted speed over ground. If the boat-speed arc cannot reach the ground track, the assumed speed and interval do not produce a solution and the plan must change.

The water track is not necessarily the final compass heading. If leeway is expected, steer windward of the required water track by the estimated amount. Then convert the resulting true heading through variation and deviation when a compass course is required. Keep each stage labelled so tide, leeway and compass corrections cannot be applied twice or in reverse.

One-hour course to steer vector plotThe tidal vector is plotted first from the start. The boat-speed arc is swung from the tide end to meet the intended ground track. The joining water-track vector gives the true course to steer.Time base · 1 hourA · StartB · Tide-only pointC · After 1 hr

Construction order matters: tide first, boat-speed arc second, then measure the required water track.

Challenge the direction before the number

The intended ground track is east and the stream sets south, but a calculation produces a water track south of east.

  1. 1. Predict the answer

    To cancel a southerly stream, the boat's through-water motion must contain a northerly component.

  2. 2. Inspect the plot

    Check that set was plotted towards 180°T from the start and that the boat-speed arc began at the vector's end.

  3. 3. Measure the right line

    Measure from the end of the tidal vector to the ground-track intersection, not from the original start.

Sense check: A correction that points farther in the same direction as the unwanted set cannot cancel it.

Common mistake or limitation

  • Plotting the boat vector from the start changes the construction and gives the wrong water track.
  • The vector solution is true; silently calling it a compass course skips leeway, variation and deviation.

Recap

  • Tidal movement plus boat movement produces ground movement.
  • The tide-first construction solves for the required water track.
  • Leeway and compass conversion turn that water track into a heading to steer.

Optional quick check

Section 4 of 7

A desired track is east and the stream sets south. What broad feature must the water track have?

Choose one answer

Continue studying Chartwork

This public lesson is part of Revision Module 10. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.

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