Module 10 - Chartwork

Course to Steer (CTS)

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.Scale · 1 hourA · StartB · Tide endC · Ground point

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

  • Use one consistent time interval for tide and boat distance
  • Plot the tidal vector from the start before the boat-speed arc
  • The arc intersection reveals the required water track
  • Apply a leeway correction windward when converting water track to heading
  • Apply variation and deviation only after the true solution is clear

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

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