5.3 Set, Drift & Course-to-Steer

Current Vocabulary That Wins or Fails Q163

Tidal height is vertical; tidal current is horizontal water motion. Chart Plot cares about the horizontal push:

Wind naming trap: wind is named for where it comes from ("north wind" from the north). Current set is named for where it goes. A set of 180°T is a southward-setting current.

Finding Set and Drift From DR vs Fix

When you have a DR and a fix for the same time, the current’s effect is the vector from DR to fix:

• Set = direction of that vector (true).
• Drift = length of that vector ÷ elapsed time since the last common reference (usually hours since the fix that started the DR run).

Worked example — set and drift

At 1200 you fix and then steer C 000°T at S 12 (through the water).
At 1300:

• 1300 DR is 12.0 NM due north of the 1200 fix (12 kn × 1 h).
• 1300 fix (two bearings) plots 1.5 NM due east of the 1300 DR.

Set = direction from DR → fix = 090°T.
Drift = 1.5 NM / 1 h = 1.5 kn.

Current: set 090°T, drift 1.5 kn.

Critical: DR and fix must share the same clock time. Comparing a 1200 DR to a 1300 fix mixes run distance into the “current” and corrupts set/drift.

Course and Speed Made Good

Using the same run: from the 1200 fix to the 1300 fix you moved 12.0 NM north and 1.5 NM east.

• Straight-line distance ≈ √(12² + 1.5²) = √146.25 ≈ 12.1 NM.
• SMG / SOG ≈ 12.1 kn.
• CMG / COG ≈ 007°T (slightly east of north).

You steered 000°T at 12 kn through the water but made good about 007°T at 12.1 kn over the ground. Exam questions may ask any one of: set, drift, CMG, SMG, or “where is the EP relative to the DR.”

Course to Steer — The Current Triangle

Now reverse the problem. You know:

• Desired track / course to make good (e.g., stay on a range or channel centerline).
• Boat speed through water.
• Current set and drift.

Find CTS and SOA.

Construction (one-hour vectors)

1. From start point, draw a long line in the desired track direction.
2. From start, lay off the current vector: direction = set, length = drift × 1 h (NM in one hour = knots of drift).
3. From the head of the current vector, swing an arc of radius = boat speed × 1 h (NM the vessel travels through the water in one hour).
4. Where that arc cuts the desired-track line is the one-hour ground position along the intended track.
5. Line from current-vector head to that intersection = course to steer (true).
6. Distance from start to intersection along the track = SOA (NM per hour).

You always aim into the component of current that would set you off track—crabbing so the ground track stays on the desired line.

Worked example — CTS and SOA (classic numbers)

Make good 270°T. Current sets 180°T at 3 kn. Boat speed 6 kn through water.

Current pushes south, so head north of west.

For a pure cross-current:
sin θ = drift / boat speed = 3 / 6 = 0.5 → θ = 30°.
CTS = 270° + 30° = 300°T.
SOA = √(6² − 3²) = √27 ≈ 5.2 kn along 270°T.

Steer 300°T; ground path goes west at about 5.2 kn.

ETA with current — use SOA

Waypoint 10.4 NM ahead on the 270°T track; SOA 5.2 kn.
T = 60 × 10.4 / 5.2 = 120 min = 2 h.
Depart 0800 → ETA 1000.

If you wrongly used 6 kn water speed: 60 × 10.4 / 6 ≈ 104 min → predicted 0944—you arrive late and may also be south of track if you never offset. On passenger runs through a current-swept inlet, that is how you leave the channel; on Q163, that is how you miss the only allowed error.

Second worked triangle — oblique current

Desired track 000°T, boat 10 kn, current set 045°T, drift 2 kn.

You cannot use the simple right-triangle shortcut without resolving components. Plot one-hour vectors:

• Current: 2 NM toward 045°T.
• From end of current vector, 10 NM radius arc to cut the 000°T track line.
• Read CTS from the water-speed leg; measure SOA along 000°T.

Expect CTS slightly west of north (into the northeasterly set’s easterly component) and SOA less than 10 kn. On the exam, multiple-choice options usually match a carefully plotted triangle within about 1°–2° and 0.1–0.2 kn—another reason neat dividers matter.

Compensating When Crossing Channels

Master LT 100 operations often cross ebb/flood sets in inlets, rivers, and ferry routes:

1. Identify the desired ground track (range, channel axis, rhumb between buoys).
2. Estimate or take published set/drift (current tables / known local knowledge as given in the problem).
3. Solve CTS so the COG stays on the track.
4. Compute ETA with SOA.
5. Take frequent fixes; if set changes with tide stage, re-solve—do not cling to one CTS all day.

Exam version: the stem states set/drift and desired course to make good; you supply CTS and sometimes SMG/SOA or ETA.

Exam Phrasing: “Set and Drift” vs “Course to Steer”

Candidates who automatically “always add 30° for current” without reading which corner of the triangle is asked will miss an easy 90% point.

Leeway

Leeway is downwind sideways slip from wind on the hull/superstructure. On exam plots it is treated like an additional set/drift (or already folded into a stated “current”). Do not invent a separate mystical correction beyond the problem data.

Arithmetic Traps

Quick numeric self-check set

1. DR 6 NM west of start after 1 h on 270°T @ 6 kn; fix 1 NM south of that DR → set 180°T, drift 1 kn.
2. Want 090°T, set 000°T @ 2 kn, boat 8 kn → θ = arcsin(2/8) = 0.25 → θ ≈ 14.5°. Set north pushes you north of track, so head south of east: CTS ≈ 090 + 15 = 105°T.
3. SOA ≈ √(8² − 2²) = √60 ≈ 7.7 kn.

Current problems are where Master candidates separate from rote memorizers. Draw the triangle every time, label vectors, and match the stem’s exact ask—set/drift or CTS—before you bubble an answer.