
Lesson 7 - Course to Steer
What are Tidal Streams?​
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Tidal streams are like invisible rivers in the ocean.
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They are caused by the gravitational pull of the moon and the sun.
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Streams vary in direction and speed, depending on the tide.
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Failure to account for tidal streams can push your boat far off course.
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​​Example of Tidal Impact:
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A 2-knot southward tidal stream carried the boat 1 mile off course in just 30 minutes.
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Always account for tidal streams to stay on track.
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​​Tidal Stream Data:
To adjust for tidal streams, you need:
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Direction: The flow of the stream (e.g., southward).
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Speed: Measured in knots (e.g., 2 knots).Use tidal stream charts to find this information for your location.​
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Steps to Calculate Course to Steer (CTS):
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Draw your intended track on the chart.
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Add a vector for the tidal stream (direction + speed).
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Draw an arc from the tidal vector’s endpoint to intersect the track.
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Connect your starting point to the intersection: This is your CTS.
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​​For multi-hour trips:
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Account for changing tidal streams every hour.
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Plot vectors sequentially to create a comprehensive navigation plan.
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Keep measurements consistent to ensure accuracy.
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What is Leeway?
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Definition: Sideways drift caused by wind pressure on your sails and hull.
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Influenced by:
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Wind strength.
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Boat design.
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Sail trim.
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Leeway pushes your boat off course, requiring adjustments.
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How to Correct for Leeway:
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On a starboard tack, subtract the leeway angle from your heading.
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On a port tack, add the leeway angle to your heading.
Example:
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Heading: 270°
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Leeway: 10° on starboard tack
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Corrected course: 260° (270 - 10)
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​What is Estimated Position
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EP = Dead Reckoning (DR) + Corrections
Start with:
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DR: Based on course steered and distance traveled.
Add corrections for:
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Tidal streams.
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Leeway.
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Surface drift caused by wind.​​​​

​How to Plot Estimated Position (EP):
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Mark your DR position on the chart.
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Add a tidal vector to account for the stream’s movement.
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Add a leeway vector, if applicable.
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The final position = EP (Estimated Position).
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Sources of Navigation Errors:
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Compass and steering errors.
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Incorrect tidal data.
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Misjudged leeway.
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Inaccurate speed/distance logs.
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Lesson:
Errors grow over time. Cross-check your position regularly.
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Tips to Reduce Errors:
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Cross-check your EP with GPS or visual bearings.
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Use the One in Sixty Rule:
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1-degree error = 1 mile off course after 60 miles.
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Make regular position checks to avoid compounding inaccuracies.​​​