ASA 105: Skill 13, Set and Drift, continued

 


13.Determine a course to steer which takes into account known current and leeway.


The use of "vector diagrams" solves set and drift problems.  It is back to Algebra 1 class.  The table on page 58 of Eldridge 2022 is also simple and useful.  Let's solve a problem graphically.

This can be quickly done graphically for any angle and speed of current on your chart.  The two images demonstrate that by using colored yarns to mark various elements of the problem.

Image 1

Image 2


The plan is to depart Menemsha Harbor and sail to Breton Point at a projected speed of 7 knots. The Rhumb Line distance is 27.3 nm. The current for the entire trip is 060 degrees True at 1/2 knot.

The dark blue yarn shows the Rhumb Line of 280 degrees True.
The blue arrow at midcourse is the current direction, 060d T
The red arrow is the destination R "2" Fl R 4 sec Whistle off Breton Point
The green arrow is the starting point at Menemsha Harbor 

What is the course to steer?

If we make no correction for current and steer the Rhumb Line heading of 280d T we will achieve the yellow arrow and be in treacherous ground.  The effect of the current over the 3h 54m trip is almost 1.95 miles off course.  The direction of the offset is 060d T.  The yellow arrow marks the point and the light colored line is the track and distance travelled on the wrong track.

To correct for current we measure the 1.95 nm displacement and project it in the opposite direction of the current.  That is the purple arrow.  Our course to steer is defined by the purple yarn.  The current is against us so the trip is longer as shown by the longer length of the purple yarn compared to the blue yarn.

The course to steer is 277d T and the distance (through the water) is 29 nm or 4h 8m.

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