Multi-Hour CTS with Changing Tidal Streams

On longer passages the tidal stream changes direction and rate each hour. At Yachtmaster level you must construct a multi-hour CTS by stacking tidal vectors end-to-end for the passage duration. Plot each hour's tidal set and drift consecutively from your departure point, then swing a single arc from the cumulative end of those vectors using your total distance through the water for the same period.

For example, on a three-hour passage at 5 knots: plot the tidal vectors for hours 1, 2, and 3 end-to-end. From the end of the third vector, swing an arc of 15 nm (3 hours × 5 kn). Where the arc crosses your intended ground track gives the water track — one single CTS that averages out the changing tide across the whole passage.

This construction gives one planned water track for the whole leg when the hourly streams and boat speed are reasonable forecasts. It is not automatically more accurate than a shorter calculation: monitor the actual track, compare it with fixes, and recalculate whenever the vessel's speed, leeway, tidal estimate, route, or safety margin changes materially.