OUPV / Six Pack Captains License Course

PLOTTING - Set and Drift


Set and Drift

While dead reckoning (DR) uses speed, time, course and distance to find your vessel’s approximate position, it does not take currents into consideration.  In reality, your vessel will most likely be affected by currents when underway.  It is important to factor a current’s speed and direction into your trip.  The direction a current is moving is called set and the speed of a current is called drift.The compass rose on a chart is used read the degrees a current is running, and since there is no magnetism in currents, they are always read in true, or the outer ring of the compass rose.  For this section you will need your chart, parallel rulers and dividers, and calculator.

Example of Gulf Stream Miami to Bimini

Long Island Chart Plot Demonstration

You are at New Haven Light at 41 degrees 13.3 N,72 degrees 56.6 W heading to Stratford Shoals Middle Ground Light at 41 degrees 03.6 N, 73 degrees 06.1W. You are on a course of 218 degrees true at a speed of 7 knots. The current is setting at 180 degrees due south at a drift of 2 knots. What is your course to steer (CTS)?

You are at Branford Reef Light at 41 degrees 13.3 N, 72 degrees 48.4W. You are headed to New Haven Light at 41 degrees 13.3 N,72 degrees 56.6 W at 13:30 on a course of 270 degrees true at a speed of 3.05 knots. You calculate that you should arrive at New Haven Light at 15:30. Instead at 15:30 your position is 41 degrees 12.3 N, 72 degrees 55.7 W. What was the set and drift of the current?