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Lesson 7 - Course to Steer

What are Tidal Streams?​

  • Tidal streams are like invisible rivers in the ocean.

  • They are caused by the gravitational pull of the moon and the sun.

  • Streams vary in direction and speed, depending on the tide.

  • Failure to account for tidal streams can push your boat far off course.

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​​Example of Tidal Impact:

  • A 2-knot southward tidal stream carried the boat 1 mile off course in just 30 minutes.

  • Always account for tidal streams to stay on track.

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​​Tidal Stream Data:

To adjust for tidal streams, you need:

  1. Direction: The flow of the stream (e.g., southward).

  2. 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):

  1. Draw your intended track on the chart.

  2. Add a vector for the tidal stream (direction + speed).

  3. Draw an arc from the tidal vector’s endpoint to intersect the track.

  4. Connect your starting point to the intersection: This is your CTS.

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​​For multi-hour trips:

  • Account for changing tidal streams every hour.

  • Plot vectors sequentially to create a comprehensive navigation plan.

  • Keep measurements consistent to ensure accuracy.

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What is Leeway?

  • Definition: Sideways drift caused by wind pressure on your sails and hull.

  • Influenced by:

    • Wind strength.

    • Boat design.

    • Sail trim.

  • Leeway pushes your boat off course, requiring adjustments.

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How to Correct for Leeway:

  1. On a starboard tack, subtract the leeway angle from your heading.

  2. On a port tack, add the leeway angle to your heading.
    Example:

  • Heading: 270°

  • Leeway: 10° on starboard tack

  • Corrected course: 260° (270 - 10)

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​What is Estimated Position 

  • EP = Dead Reckoning (DR) + Corrections

Start with:

  • DR: Based on course steered and distance traveled.

Add corrections for:

  • Tidal streams.

  • Leeway.

  • Surface drift caused by wind.​​​​

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​How to Plot Estimated Position (EP):

  1. Mark your DR position on the chart.

  2. Add a tidal vector to account for the stream’s movement.

  3. Add a leeway vector, if applicable.

  4. The final position = EP (Estimated Position).

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Sources of Navigation Errors:

  • Compass and steering errors.

  • Incorrect tidal data.

  • Misjudged leeway.

  • 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:

  1. Cross-check your EP with GPS or visual bearings.

  2. Use the One in Sixty Rule:

    • 1-degree error = 1 mile off course after 60 miles.

  3. Make regular position checks to avoid compounding inaccuracies.​​​

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