Leeway allowance in plotting

Bowditch Ch. 23 §2302

Navigational Errors Set and drift describe the effect of current on the vessel's track. Set is the direction toward which the current is flowing; drift is the speed of the current. The vector sum of the intended course (course made good through water) and the current vector gives the actual course over ground.

Bowditch Ch. 24 §2401

Currents Surface ocean currents are driven primarily by wind, with major gyres in each ocean basin. Tidal currents are driven by tide rather than wind and reverse direction with the tide.

Bowditch Ch. 24 §2402

Currents The vector triangle for current correction is: course steered + drift vector = course made good. Inverting it: from course made good and drift, derive the course steered required to make a given track over ground.

Bowditch Ch. 8 §801

Dead Reckoning A dead-reckoning (DR) plot projects the vessel's position from the last fix using only course steered and distance run through the water, ignoring current and leeway. A new DR position is plotted at every course change, at every speed change, at the time of each fix, and at least once each hour — more often in pilot waters.

Bowditch Ch. 8 §802

Dead Reckoning DR plotting symbols are standardised: a fix is a dot inside a circle, a DR position a dot with a semicircle, and an estimated position a dot inside a square. The course line carries the true course as "C 090" above it and the speed as "S 8" below, and each position is labelled with the 24-hour time. Consistent labelling lets any watchstander read the plot at a glance.

Bowditch Ch. 8 §803

Dead Reckoning The estimated position (EP) is the DR position corrected for the estimated set and drift of the current. From the DR position lay off the current's set and drift for the elapsed time; the end of that vector is the EP, plotted as a dot inside a square. Set is the direction toward which the current flows and drift is its speed. The EP is the best position available between fixes when only the current can be estimated.