Estimated Position from Current Set & Drift

TL;DR — Set is the direction toward which a current flows; drift is its speed. When you apply the current vector to your DR position, the result is an estimated position (EP), which is more reliable than a bare DR but less reliable than a fix from independent LOPs.

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What the Rule Says

Federal regulations impose explicit, affirmative duties on the person directing the movement of a vessel to know and account for current before and during every transit.

33 CFR §164.11 requires the master or person in charge to ensure that:

• Current velocity and direction for the area to be transited are known by the person directing movement 33 CFR §164.11 (paragraph (l));
• Predicted set and drift are known by the person directing movement 33 CFR §164.11 (paragraph (m));
• Tidal state for the area to be transited is known by the person directing movement 33 CFR §164.11 (paragraph (n));
• Speed is set with consideration for the strength and direction of the current 33 CFR §164.11 (paragraph (p)(7)).

These are not advisory suggestions. They are mandatory duties of the owner, master, or person in charge. Failure to know set and drift is a regulatory violation, not merely a seamanship lapse.

For towing vessels, 33 CFR §164.78 independently requires that each person directing and controlling movement knows the speed and direction of the current, and the set, drift, and tidal state for the area to be transited 33 CFR §164.78 (paragraph (a)(6)). The towing vessel regulation also requires that safe speed account for the speed and direction of the current 33 CFR §164.78 (paragraph (a)(7)).

For small passenger vessels (T-boats), 46 CFR §185.304 requires special attention to current velocity and direction of the transiting area and tidal state 46 CFR §185.304 (paragraphs (a)(1) and (a)(2)).

The navigational science behind these regulatory duties is grounded in Bowditch. 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 Bowditch Ch. 23 §2302. The vector sum of the course made good through the water and the current vector gives the actual course over ground Bowditch Ch. 23 §2302.

The vector triangle works in both directions. Given course steered plus the drift vector, you obtain course made good. Inverting the triangle — given course made good and drift — you derive the course to steer in order to make a desired track over ground Bowditch Ch. 24 §2402.

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The Mechanics: DR Position vs. Estimated Position

Understanding the distinction between a DR position, an estimated position (EP), and a fix is essential for the exam.

Dead reckoning (DR) is the determination of position by applying course steered and distance run from a known starting point Bowditch Ch. 1 §102. A DR position accounts only for what the vessel's own propulsion and helm have done — it does not account for current, wind, or leeway. DR accuracy degrades over time and must be confirmed by external fixes whenever possible Bowditch Ch. 1 §102.

An estimated position (EP) is obtained by applying the current vector (set and drift) to the DR position. It is more accurate than a bare DR because it incorporates the best available knowledge of external forces acting on the vessel, but it is still an estimate — not a fix — because it relies on predicted or observed current data rather than independent lines of position.

A fix is obtained from two or more simultaneous, independent LOPs (bearings, ranges, soundings, GPS, etc.) 33 CFR §164.11 (paragraph (d)). Buoys alone cannot be used to fix a vessel's position; they may corroborate a fix obtained by other means, or, if no other aids are available, establish only an estimated position 33 CFR §164.11 (note following paragraph (e)); 33 CFR §164.78 (paragraph (a)(3)).

A running fix is obtained when LOPs cannot be taken simultaneously — the first LOP is advanced along the DR track to the time of the second observation, and their intersection is the running fix Bowditch Ch. 7 §702. A running fix is less accurate than a fix from simultaneous LOPs Bowditch Ch. 7 §702.

Constructing the EP on the Chart

The procedure on the chart follows directly from the vector triangle Bowditch Ch. 24 §2402:

1. Start at the last known fix. Label it with time and the fix symbol (small circle with a dot). 2. Plot the DR track from the fix along the course steered for the elapsed time, using the vessel's speed through the water. Mark the DR position with a half-circle. This gives you the DR position at the time of interest. 3. Apply the current vector. From the DR position, draw a line in the direction of the set (the direction toward which the current flows) Bowditch Ch. 23 §2302 for a distance equal to drift × elapsed time (in hours). Drift is the current's speed Bowditch Ch. 23 §2302. 4. The end of the current vector is the EP. Label it with time and the EP symbol (small square).

The EP represents your best estimate of where the vessel actually is, accounting for the predicted effect of current on the DR track.

Inverting the Triangle: Course to Steer

When you need to make good a specific track over ground in the presence of a known current, you invert the vector triangle Bowditch Ch. 24 §2402:

1. From the departure point, draw the desired track (course made good) to the destination. 2. From the departure point, draw the current vector (set and drift for the planned transit time). 3. From the tip of the current vector, swing an arc equal to the vessel's speed through the water until it intersects the desired track line. 4. The line from the tip of the current vector to that intersection is the course to steer.

The person directing movement of the vessel is required to know predicted set and drift 33 CFR §164.11 (paragraph (m)) precisely so this correction can be applied before the vessel departs the fix, not after it has already been set off track.

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Why It Matters on the Exam

OUPV and Master 100 GT written exams test this topic from several angles:

Regulatory knowledge questions ask which paragraphs of 33 CFR §164.11 require knowledge of current, set, drift, and tidal state. The answer is paragraphs (l), (m), and (n) 33 CFR §164.11. Towing vessel candidates must also know 33 CFR §164.78(a)(6) 33 CFR §164.78.

Definition questions test the precise meaning of set and drift. Set is the direction toward which the current flows — not from which it flows (that is the convention for wind) Bowditch Ch. 23 §2302. Drift is the current's speed Bowditch Ch. 23 §2302. Confusing these two definitions is the single most common error on this topic.

Position hierarchy questions ask candidates to rank DR position, EP, running fix, and fix by reliability. From least to most reliable: DR → EP → running fix → fix from simultaneous LOPs Bowditch Ch. 1 §102; Bowditch Ch. 7 §702.

Buoy questions ask whether a mariner can use buoys to fix position. The answer is no — buoys may corroborate a fix or establish only an estimated position 33 CFR §164.11 (paragraph (e) and note); 33 CFR §164.78 (paragraph (a)(3)).

Chart plotting questions present a scenario with a known fix, a course steered, a speed, an elapsed time, and a current (set and direction), then ask for the EP or the course to steer to make good a track. The vector triangle from Bowditch is the tool Bowditch Ch. 24 §2402.

Marine navigation integrates all four methods — dead reckoning, piloting, celestial, and electronic — with electronic navigation as the principal real-time reference and the others as cross-checks Bowditch Ch. 1 §101. The EP from current is a piloting tool that bridges the gap between fixes.

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Common Pitfalls

**Confusing set with the direction current comes from.** Wind is named for the direction it comes from; current is named for the direction it goes toward. A current with a set of 090° flows eastward Bowditch Ch. 23 §2302.

Treating an EP as a fix. An EP incorporates predicted current data — it is still an estimate. Only independent LOPs produce a fix 33 CFR §164.11 (paragraph (d)).

Using buoys to fix position. Buoys are placed in approximate positions and can be moved by currents, ice, or vessel collisions. They cannot fix a position; at best they establish an EP when no other aids are available 33 CFR §164.11 (paragraph (e) and note).

Forgetting to apply elapsed time to drift. Drift is a speed (knots). The distance the current displaces the vessel equals drift × elapsed time in hours. Applying drift directly as a distance without multiplying by time is a frequent arithmetic error on chart problems.

Applying current in the wrong direction. The current vector is drawn in the direction of the set — the direction the current is moving toward — from the DR position Bowditch Ch. 23 §2302. Drawing it in the reciprocal direction produces an EP on the wrong side of the DR track.

Neglecting magnetic variation and deviation. The person directing movement must know and correctly apply magnetic variation, deviation, and gyrocompass errors 33 CFR §164.11 (paragraph (i)). A course steered plotted on a chart must be converted to true before it can be plotted; failure to do so corrupts the entire DR track and any EP derived from it.

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Quick Check

Q1 — What is the regulatory term for the direction toward which a current flows, and what is the term for its speed?

Set is the direction toward which the current flows. Drift is the speed of the current. Bowditch Ch. 23 §2302

Q2 — Under 33 CFR §164.11, which specific paragraphs require the person directing movement to know current information?

Paragraph (l) — current velocity and direction; paragraph (m) — predicted set and drift; paragraph (n) — tidal state. 33 CFR §164.11

Q3 — A vessel departs a fix on course 045°T at 8 knots. After 1.5 hours, the navigator applies a current with set 180° and drift 2 knots. Where is the EP relative to the DR position?

The DR position is 12 nautical miles (8 kts × 1.5 hrs) along the 045°T track from the fix. From the DR position, the current vector is drawn 3 nautical miles (2 kts × 1.5 hrs) toward 180° (due south). The EP is 3 nm due south of the DR position. Bowditch Ch. 23 §2302; Bowditch Ch. 24 §2402

Q4 — Can a mariner use buoys alone to fix the vessel's position? What position quality do buoys alone provide?

No. Buoys cannot be used to fix a position because they are placed in approximate positions and may be displaced by currents, ice, or collisions. If no other aids are available, buoys alone may establish only an estimated position. 33 CFR §164.11 (paragraph (e) and note); 33 CFR §164.78 (paragraph (a)(3))

Q5 — Rank the following from least to most reliable: fix from simultaneous LOPs, DR position, running fix, estimated position.

Least reliable to most reliable: DR position → estimated position → running fix → fix from simultaneous LOPs. Bowditch Ch. 1 §102; Bowditch Ch. 7 §702

Q6 — A towing vessel operator is planning a transit. Which regulation requires that person to know set, drift, and tidal state, and which paragraph specifically?

33 CFR §164.78(a)(6) requires each person directing and controlling movement of a towing vessel to know the speed and direction of the current, and the set, drift, and tidal state for the area to be transited. 33 CFR §164.78