Module 10 - Chartwork

Course to Steer (CTS)

What you will learn

Construct and challenge a true water track that counters tidal stream before applying leeway, variation and deviation to obtain a heading.

A CTS problem works backward from the desired ground track. Plot the water's movement first; the boat-distance arc then reveals the through-water track needed to produce the ground result.

Draw the desired ground-track line from the starting position. For one stated time interval, plot the tidal set towards its true direction and drift equal to rate multiplied by time. From the tidal-vector endpoint, swing an arc equal to the vessel's distance through the water for the same interval until it meets the ground-track line.

The vector from the tidal endpoint to the arc intersection is the required track through the water. Measure it as a true direction. The distance from the original start to the intersection is the expected ground distance for that interval and therefore gives predicted SOG. If the boat-distance arc cannot reach the ground-track line, the assumed speed and interval provide no solution; change the plan instead of forcing an intersection.

The plotted water track is not automatically the compass heading. Correct the heading windward for expected leeway, then convert the resulting true heading through variation and deviation when a compass course is required. Label each stage to prevent tide, leeway or compass corrections being 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.

Predict the correction before measuring

The desired ground track is east and the stream sets south, but a plotted answer points the water track south of east.

  1. 1. Use the physical direction

    The boat requires a northerly through-water component to cancel the southward water movement.

  2. 2. Inspect vector origins

    Check that the tide starts at the original position and the boat-distance arc starts at the tidal endpoint.

  3. 3. Measure the solved vector

    Measure from the tidal endpoint to the ground-track intersection, not from the original start.

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

Common mistake or limitation

  • Plotting the boat vector from the original start changes the construction and gives the wrong water track.
  • Calling the true water-track result a compass course skips leeway, variation and deviation.

Recap

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

Optional quick check

Section 5 of 11

A desired ground track is east and the stream sets south. What must the required water track contain?

Choose one answer
Sources and factual review

Reviewed 2 September 2026 - Compass Revision editorial review.

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.

Go to full module
PreviousRunning Fix (Transferred Position Line)NextMulti-Hour CTS with Changing Tidal Streams