Leeway and Current Allowance in the Plot

What the rule requires

A DR position is projected from the last fix using only course steered and distance run through the water, ignoring current and leeway Bowditch Ch. 8 §801. The boat manual says the same thing in working language: the key elements are the course steered and the distance travelled, without consideration to current, wind or other external forces, with distance from D = S x T/60 Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶5. A DR is therefore known to be wrong the moment there is any current in the area. The allowance is a separate, deliberate step.

Set is the direction toward which the current flows and drift is its speed Bowditch Ch. 8 §803. "Set 240° magnetic" means the boat is being pushed towards 240° magnetic, and "Drift 1.5 knots" means she is being pushed in that direction at 1.5 knots Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶8. On a boat plot that one vector is doing more work than the name suggests: set is the direction of these forces and includes wind, current and sea condition. Bowditch is explicit on the same point from the other side, defining the estimated position as a DR position corrected for the effects of leeway, steering error and current Bowditch Pub. 9 Ch. 9 §907.

To get the estimated position, lay off the current's set and drift for the elapsed time from the DR position; the end of that vector is the EP, and it is the best position available between fixes when only the current can be estimated .

The triangle runs in both directions: course steered plus the drift vector gives course made good, and inverting it gives the course steered required to make a given track over the ground Bowditch Ch. 24 §2402. Three forms come up, and the exam separates them by what it asks you to produce.

Track and speed made good. From A draw the ship's course and speed, then from B draw the set and drift; the direction and length of AC are the estimated track and speed made good. A ship on 080° at 10 knots in a set of 140°, drift 2 knots, makes good 089° at 11.2 knots Bowditch Pub. 9 Ch. 9 §908.
Course to steer at a given speed. Plot the current vector from the origin A instead of from B, then swing an arc of radius equal to the speed through the water from C to cut the desired course line at D. CD is the course to steer, AD the speed made good: to make good 095° through a set of 170°, drift 2.5 knots, at 12 knots, steer 083.5° and make good 12.4 knots.
Course to steer and the speed to use. AB is the course and speed to be made good, AC the set and drift from A, and the line C to B gives both answers: for 265° at 15 knots through a set of 185°, drift 3 knots, steer 276° at 14.8 knots.

Vector length is read as speed: drawn for one hour, the length of the intended-direction line represents the boat's speed in knots .

One reference-frame discipline before any of it. Compass roses carry true directions on the outside and magnetic on the inside Bowditch Pub. 9 Ch. 9 §901, boat courses are normally magnetic because a boat does not usually carry a gyrocompass , and tidal current directions are given in degrees true and must be converted to magnetic before plotting the set and drift problem .

Your Tidal Current Table gives the set as 118°, and your track is laid down in magnetic. What must happen before you draw the current vector?

Convert it. Table directions are true; apply variation to get magnetic, then plot both vectors in the same reference. Plotting a true set against a magnetic track puts the whole triangle out by the variation, and on a chart with 15° W variation that is not a rounding error.</details>

Telling it apart

The criterion is what information has been applied to the position, and the symbol on the chart announces it.

Fix — a position from two or more lines of position, enclosed by a small circle and labelled with the time to the nearest minute, written horizontally Bowditch Pub. 9 Ch. 9 §902.
Running fix — one LOP advanced parallel to itself along the course line for the distance run, crossed with a later LOP; circled, marked "R Fix", and labelled with the time of the second LOP Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶5.
DR position — course steered and distance run only, marked with a semicircle and the time written diagonally .
EP — that DR position after set and drift have been laid off, marked by a small square with the time written horizontally.

The DR plot is reset to each fix or running fix in turn Bowditch Pub. 9 Ch. 9 §904. It is not reset to an EP on the strength of an estimated current, and it is not continued from the old DR: the new course line is drawn from the fix, not from the DR Bowditch Pub. 9 Ch. 9 §903.

Working a question

Intended track to the destination 093° M, speed 5 knots. The Tidal Current Table for the area gives the current setting 265° T, drift 3 knots. Local variation 4° W. Find the corrected course to steer and the speed of advance Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶9.

Identify which of the three forms this is. Both the track and the speed to be made good are specified, so this is the course-to-steer-and-speed-to-use form, and the current vector is laid off from the origin, not from the end of the course line.
Convert the set. 265° T with 4° W variation is 269° M. Do this before anything is drawn.
Take the centre of the compass rose as the departure point. Draw the intended track 093° M out from the centre, 5 units long for 5 nautical miles in the hour, arrowhead at the outer end. This is the desired course and speed vector.
From the same centre draw the set and drift towards 269° M, three units long, arrowhead at the outer end.
Join the two arrowheads with a straight line. Measure its length against the nautical mile scale: 8.7 knots, the speed of advance.
Advance that line to the centre of the rose with the parallel rulers and read the inner circle: 088° M, the course to steer. Read it from the current arrowhead toward the track arrowhead; the reciprocal is a plausible-looking wrong answer.

The result passes inspection. A set of 269° M against a track of 093° M is very nearly a head current, so the speed you must turn through the water is well above the 5 knots you intend to make good, and the heading barely shifts off the track. A beam set would do the opposite: small change in speed, large change in heading. Walk the rulers carefully in step 6, because if they slip the original line of direction is lost Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶5.

Getting set and drift out of your own plot

The difference in position between a DR position and a fix taken at the corresponding time is due to the external forces acting on the boat, accounted for as set and drift Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶8. Measure the direction from the DR to the fix and that is the set. Measure the distance and divide it by the elapsed time to get the drift, which is the distance formula worked backwards: 0.75 mile of offset thirty minutes after the last fix is 0.75 x 60/30 = 1.5 knots. Apply that set and drift to every DR position from that fix until the next one to obtain an estimated position Bowditch Pub. 9 Ch. 9 §903. The limitation is in the method: it can only be done after some portion of the voyage has already occurred.

Your 1300 fix plots 1.2 miles in direction 070° M from the 1300 DR. You last fixed at 1200 and have held one course and speed since. State the set and drift.

Set 070° M, drift 1.2 knots. The direction runs from the DR to the fix, and the interval is one hour, so the distance reads directly as the drift. Then reset the DR: draw the new course line from the 1300 fix, and apply this set and drift to the DR positions that follow to get an EP.</details>

Where candidates lose the point

Laying the current vector from the wrong vertex. A candidate who has only ever worked the estimated-position problem puts the set and drift on the end of the desired track and reads the connecting line as the course to steer. That answer will be on the paper and it will be wrong by roughly twice the correction. For a course to steer, the current vector comes off the origin Bowditch Pub. 9 Ch. 9 §908.
Reversing the set. "Set 240°" means the boat is being pushed towards 240°, not that the current arrives from 240° Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶8. A reversed set produces a course to steer displaced to the wrong side of the track by the full correction, and it looks entirely reasonable.
Plotting a table set straight onto a magnetic plot. The tables are in degrees true.
Assuming a favourable current when advancing an LOP. To hold a margin of safety when advancing a bearing to a NAVAID on shore, always assume the current slows the vessel's speed over ground, which makes the running fix plot closer to the shore than the vessel actually is Bowditch Pub. 9 Ch. 10 §1019. Assume a following current and the fix plots further from the NAVAID than your true position, which is the error that puts you on the beach.
Treating an EP as though it were a fix. It is a square, not a circle, and the DR is reset to fixes and running fixes Bowditch Pub. 9 Ch. 9 §904.
Expecting the current triangle to absorb everything. Current, leeway and steering error combine into an error that grows with time on DR Bowditch Pub. 9 Ch. 9 §905, so an EP built from a tabulated current alone is still an estimate.

Check yourself

You are on course 080°, speed 10 knots, with an estimated set of 140° and drift of 2 knots. What are your estimated track and speed made good?

Estimated track made good 089°, speed made good 11.2 knots. This is the forward problem, so the course and speed vector goes down first and the set and drift vector is laid off from its end (B); the line from the origin to the end of the current vector gives both answers Bowditch Pub. 9 Ch. 9 §908.</details>

You must make good a course of 095° through a set of 170°, drift 2.5 knots, and you can turn 12 knots through the water. What course do you steer, and what will you make good?

Steer 083.5° and make good 12.4 knots. The current vector is plotted from the origin, then an arc of radius 12 knots swung from its end cuts the desired course line; the direction from the end of the current vector to that intersection is the course to steer, and the length along the desired course line is the speed made good.</details>

At 1400 you correct the 1400 DR position for the estimated set and drift. How is the resulting position drawn and labelled?

A small square with the time 1400 written horizontally, and it is an estimated position, not a fix or a DR Bowditch Pub. 9 Ch. 9 §902. Semicircle with the time diagonal is the uncorrected DR; a circle would claim two or more LOPs you do not have.</details>

The Tidal Current Tables give the set as 310° for the hour you will be in the passage. Variation is 6° W and your track is plotted magnetic. What set do you draw?

316° M. Table set is in degrees true and must be converted before the vector goes on the chart Boat Crew Seamanship Manual (M16114.5C) Ch. 14 — Piloting ¶8, applying the westerly variation as in the worked example, where 265° T became 269° M.</details>

You are running the coast and take two bearings on the same light to work a running fix, with shoal water inshore. You do not know the current. Which way do you bias the run between bearings?

Assume the current slows your speed over ground, so advance the first LOP the shorter distance. That plots the running fix closer to the shore than you actually are, which is the safe error Bowditch Pub. 9 Ch. 10 §1019. Assuming a following current plots you further offshore than the truth.</details>

Your 1500 fix is offset from the 1500 DR. What do you do with the plot, and what do you do with the offset?

Reset the DR: draw the new course line from the 1500 fix, not from the 1500 DR Bowditch Pub. 9 Ch. 9 §903. The offset itself gives you set and drift — direction DR to fix, distance divided by the elapsed time — and you apply those to each DR position after 1500 to produce an EP until the next fix.</details>