Estimated Position from Current Set and Drift

What the rule requires

A DR position carries course steered and distance run from the last fix, and nothing else — current and leeway are excluded from it by definition Bowditch Ch. 8 §801. That exclusion is what creates the EP problem. The water the vessel is moving through is itself moving, and until that movement is applied the plot shows only where the vessel would be if the sea stood still.

The estimated position (EP) is the DR position corrected for the estimated set and drift of the current Bowditch Ch. 8 §803. The construction is fixed: from the DR position lay off the current's set and drift for the elapsed time, and the end of that vector is the EP. Nothing else is added, and the DR position itself is not erased or moved.

Two definitions decide the direction and the length of that vector. Set is the direction toward which the current flows. Drift is its speed Bowditch Ch. 23 §2302. The device in common use — a current is named for where it is going — is sound and worth holding onto, provided you also hold onto the second half: drift is a rate, so it must be multiplied by hours run before it becomes a distance you can step off with dividers.

The trigger for the whole construction is the appearance of an estimated current in the problem. Where set and drift are given or can be estimated and no fix is available, the EP is the best position available . Where no current information exists, the plot stops at the DR.

The vector triangle, and reading it backwards

Course steered plus the drift vector equals course made good Bowditch Ch. 24 §2402. The same triangle inverted answers the other half of the exam's interest in this topic: given the course you must make good over the ground and the current acting, derive the course to steer to achieve it. One triangle, two questions, and the difference between them is which side is unknown.

Stated as a track rather than a position, the vector sum of the course through the water and the current vector gives the actual course over ground .

Symbols and labelling

The plotting symbols are standardised and they are tested directly: a fix is a dot inside a circle, a DR position a dot with a semicircle, an estimated position a dot inside a square Bowditch Ch. 8 §802. The course line takes the true course above it as C 090 and the speed below it as S 8, and every position carries its 24-hour time. A watchstander relieving you reads the plot from those labels alone.

When a new DR position goes on the chart

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 . Note the second and third of those: a speed change with no course change still requires a new DR position, and a fix does not excuse the DR, it is one of the occasions for plotting it.

Surface ocean currents are wind-driven and set up the major gyres in each ocean basin; tidal currents are driven by tide instead and reverse direction as the tide turns Bowditch Ch. 24 §2401. Which of the two you are in decides whether one drift value can be carried through the whole run.

Your DR at 1400 is corrected for a set of 225°T, drift 2.0 knots, over 90 minutes. What distance do you lay off, and in what direction from the DR?

Lay off 3.0 miles in the direction 225°T from the 1400 DR position. Drift is a speed, so 2.0 knots for 1.5 hours gives 3.0 miles, and the set is the direction the current flows toward — 225°T is where the vector points, not where it comes from. The end of that vector is the 1400 EP, plotted as a dot inside a square and labelled 1400 .

Telling it apart

The criterion that separates the three positions on a plot is what information each one rests on.

Fix — rests on observed data external to the vessel. Dot inside a circle Bowditch Ch. 8 §802. This is the position that resets the DR, and DR accuracy must be confirmed by fixes whenever they are available Bowditch Ch. 1 §102.
DR position — rests only on course steered and distance run from the last fix, with current and leeway ignored Bowditch Ch. 8 §801. Dot with a semicircle. Most often misfiled by candidates who apply a given current to it and then call the answer a DR.
EP — the DR position after the estimated set and drift have been applied Bowditch Ch. 8 §803. Dot inside a square. Misfiled as a fix, because it is the position being navigated on; it remains an estimate, good only until the next fix.

Course steered against course made good

Course steered — the direction through the water, the course written above the course line as C 090 . It is one side of the triangle, not the answer to it.
Course made good — course steered combined with the drift vector Bowditch Ch. 24 §2402. This is the track over the ground, and it is what you compare against a charted hazard.
Course to steer — the unknown when the question fixes the track to be made good and gives the current. Same triangle, solved from the other end .

Working a question

You take a fix at 0800. You then steer 090°T at 8 knots and obtain no further fix. The estimated current sets 180°T at 1.5 knots. Determine your 1000 position.

Decide which position is being asked for. The problem supplies a set and a drift, so the answer is an EP, not a DR. If those two values were absent the plot would stop at step 3 and the DR would be the answer.
Plot and label the 0800 fix as a dot inside a circle, timed 0800 Bowditch Ch. 8 §802. Everything that follows is measured from here, because the DR plot projects from the last fix Bowditch Ch. 8 §801.
Advance the DR. Lay the course line 090°T from the 0800 fix, label it C 090 above and S 8 below. Two hours at 8 knots is 16 miles; step that off and mark the 1000 DR position as a dot with a semicircle, labelled 1000.
Size the current vector. The current has been acting since the fix, so the interval is 0800 to 1000 — two hours, not the time since the last course change. 1.5 knots for 2 hours is 3.0 miles.
Lay off the current vector from the DR position. Direction 180°T, length 3.0 miles, starting at the 1000 DR. The direction is the set as given; it receives no reversal and no compass correction.
Mark the EP. The end of that vector is the 1000 EP — a dot inside a square, labelled 1000 Bowditch Ch. 8 §803. It lies 3.0 miles due south of the 1000 DR.
Read the track you actually made. The line from the 0800 fix to the 1000 EP is the course made good, the vector sum of the course through the water and the current Bowditch Ch. 23 §2302. It falls south of 090°T, and that is the line to check against soundings and charted dangers, not the 090°T course line.

Had the question instead specified the track to be made good — say, a required ground track of 090°T with the same current — step 5 reverses. The drift vector and the desired course made good are laid down first, and the course steered is derived from them Bowditch Ch. 24 §2402. Candidates who work every current problem in one direction only lose these outright.

Where candidates lose the point

Answering with the DR when the question gave a current. The distractor is the position 16 miles due east of the fix, and it is attractive because it is a correct DR position — just not the one asked for. A stated set and drift is the trigger for the EP construction; the DR by definition ignores the current Bowditch Ch. 8 §801.

Laying off the set 180° reversed. A candidate who thinks of a current as coming from a direction puts the EP 3.0 miles north of the DR and picks an answer that is wrong by twice the vector length. Set is the direction toward which the current flows Bowditch Ch. 23 §2302.

Using the drift value as a distance. Laying off 1.5 miles instead of 3.0 puts the EP short, and on a multiple-choice sheet there is usually an option waiting there. Drift is a speed; it becomes a distance only after multiplication by the elapsed time Bowditch Ch. 8 §803.

Timing the vector from the wrong event. Where the run includes a course change at 0900, a candidate will sometimes apply the current for one hour instead of two. The DR runs from the last fix, and so does the current's effect on it.

Naming the wrong symbol. Asked how an estimated position is shown, candidates reach for the circle because that is the symbol they draw most often. A circle is the fix, a semicircle the DR, a square the EP Bowditch Ch. 8 §802.

Navigating on the EP as though it were observed. The EP is the best position available between fixes when only the current can be estimated , and DR accuracy degrades with time until confirmed by an external fix Bowditch Ch. 1 §102. Any answer option that treats an estimate as removing the requirement to fix the vessel is wrong.

Check yourself

You depart a 1200 fix steering 000°T at 10 knots. At 1300 you increase to 14 knots, no course change. Are you required to plot a new DR position at 1300?

Yes. A new DR position is plotted at every course change, at every speed change, at the time of each fix, and at least hourly — more often in pilot waters Bowditch Ch. 8 §801. A speed change alone triggers it, and here the hourly requirement falls at the same moment in any case.

A question gives you a 0600 fix, a course of 270°T at 6 knots, and asks for the 0900 dead reckoning position. A current setting 000°T at 1.0 knot is charted in the area. What do you do with the current?

Nothing. The question asks for a DR position, and the DR is built from course steered and distance run alone, with current and leeway ignored . Plot 18 miles along 270°T and stop. Applying the current would produce an EP, which is a different position and a different symbol Bowditch Ch. 8 §803.

Your 1500 EP lies 4.0 miles from the 1500 DR in the direction 045°T. The last fix was at 1300. What set and drift were applied?

Set 045°T, drift 2.0 knots. The vector runs from the DR to the EP in the direction of the set, and its length is drift times elapsed time — 4.0 miles over the two hours since the 1300 fix gives 2.0 knots .

You are steering 315°T at 9 knots with a current setting 315°T at 2.0 knots. How does the EP differ from the DR, and what is your course made good?

The EP lies ahead of the DR along the same line, by 2.0 miles for every hour since the fix. Course made good is 315°T: course steered plus the drift vector, and with both in the same direction the triangle collapses to a straight line Bowditch Ch. 24 §2402. The current adds distance run over the ground without altering the direction of the track.

You must make good a track of 180°T through an area where the current sets 090°T at 2.0 knots. Your speed is 10 knots. Which side of 180°T is your course to steer, and why?

West of 180°T. The current sets toward 090°T, so it pushes the vessel east of the intended track; the course steered must be offset into the current so that course steered plus the drift vector equals the desired course made good . Construct the triangle with the 180°T track and the current vector laid down first, then measure the course steered.

A question describes a vessel working a tidal estuary and gives one set and drift for a six-hour transit. What should make you cautious about carrying that value through the whole plot?

Tidal currents reverse direction with the tide rather than running steadily like the wind-driven ocean currents Bowditch Ch. 24 §2401. A single set and drift will not hold across a turn of the tide, and the EP built on it degrades accordingly — one more reason the position must be confirmed by fix, especially in pilot waters where DR positions are required more often than hourly .

On a plot you are handed, a position is marked with a dot inside a square and labelled 2030. What can you say about how it was obtained?

It is an estimated position: a DR position corrected for the estimated set and drift of the current, the square being the standard EP symbol Bowditch Ch. 8 §802. It rests on no observation of anything outside the vessel, so it is the best position available only until a fix can be obtained .