ASA 105: Skill 12, Estimated Position (EP)

 

12:  Allow for the effect of current and leeway to plot the estimated position.


Estimated Position or EP is DR plus outside influences factored into the outcome.  

Outside influences are current, steering error, and/or leeway to name the most common.

Continuing with the earlier example, factor in the current.  Using Eldridge, for example, we determine that a fair current occurs for our trip when we are in Buzzards Bay at the time Ebb Starts at Pollock Rip Channel.  See page 78.  We plan to arrive at Cleveland Ledge Light in time for the Ebb at Pollock Rip to start.

Assume we depart the Cleveland Ledge Light on 17 July 2022 at 1230.  See page 69.  We carry a fair current of about 1/2 knot down the Bay.  Turning into Quick's Hole we face a head current of 2 1/2 knots for the short distance.  Crossing to Menemsha Harbor the current is on the beam at about 1 knot.

LegBase speedCorrected speedCorrected transit
166.51h 52m
263.50h 34m
365.21h 1m
All Legs3h 27m

There is a convenient table in Eldridge on page 58 that quickly adjusts boat speed and heading to current.  On Leg 1 the current is directly astern so course is not affected, only speed.  On Leg 2 the current is from directly ahead so speed is reduced but course correction is not required.  For Leg 3 the current is abeam and the page 58 table shows a 9.5 degree course correction towards the current and reduction in speed by 0.8 knots.


This can also be done graphically on the chart.  To start, use the graphical current vector charts for the appropriate times of day.



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