Distinguish water-level predictions from stream information and estimate a simple time penalty.
What you’ll practise
- Adding directly aligned boat and stream velocities
- Calculating passage time from ground speed
- Understanding cross-stream vectors and local slack timing
Each question includes a custom diagram, the reasoning behind the answer, an explanation of the other choices and a follow-up exercise. Try the quiz first, or use the study view to read every answer.
01 / Learn
Height and movement are different
Tidal height describes water level. A tidal stream describes moving water, with direction and speed. Information about one does not automatically give you the other. Use the appropriate local stream information rather than treating high water as a universal promise of slack water.
02 / Learn
Use a deliberately simple model
Assume a yacht maintains 5 knots through the water. A constant 2-knot stream directly along its intended track gives 7 knots over the ground when favourable, or 3 knots when opposed. A fictional 21-mile leg then takes 3 hours with the stream or 7 hours against it.
03 / Learn
Know when the shortcut stops working
Those sums assume a straight line, constant speeds and a stream directly aligned with the track. A cross-stream needs a vector approach, not simple addition. Real streams change with place and time. Break a passage into useful stages and revisit your estimates as actual progress becomes available.
Your two-minute exercise
Calculate a 12-mile leg at 4 knots through the water with a constant 1-knot stream directly against you. Then explain why a cross-stream is a different problem.
Keep learning from the source
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