5.5 Set, Drift & Course-to-Steer Problems

Set and Drift Defined

Current is described by two words that are easy to swap by mistake:

• Set — the direction the current flows toward, stated as a compass/true direction (a current with a set of 090°T pushes you east).
• Drift — the speed of the current, in knots (or the distance it moves you over a given time).

Wind adds leeway, a similar sideways push; on the exam you treat a stated current the same way whether it comes from tide or is lumped with leeway.

Finding Set and Drift From DR vs. Fix

Whenever you have both a DR position and a fix for the same time, the vector from the DR to the fix is the current's effect. Its direction is the set; its length ÷ elapsed time is the drift.

Worked example. At 1200 you take a fix and steer C 000°T at S 12. One hour later your 1300 DR lies 12.0 nm due north of the 1200 fix. But your 1300 fix (from two bearings) plots 1.5 nm due east of that DR.

• Set = direction from DR to fix = 090°T (due east).
• Drift = distance (1.5 nm) ÷ time (1 h) = 1.5 knots.

The current has been setting 090°T at 1.5 knots.

The reason this works is that both positions are for the same instant: the DR is where you would be with no current, the fix is where you actually are, so everything separating them is the current's doing. Always pair a DR and a fix of the same time — comparing positions from different clock times mixes in the distance you legitimately ran and corrupts the answer.

Course and Speed Made Good

The course made good (CMG) is the actual direction from your departure fix to your arrival fix; the speed made good (SMG) is the straight-line distance between them ÷ time.

Worked example (same numbers). From the 1200 fix to the 1300 fix you moved 12.0 nm north and 1.5 nm east.

• CMG ≈ 007°T — just east of due north (the eastward set nudged your track right of the 000°T you steered).
• SMG = straight-line distance ÷ time = √(12.0² + 1.5²) / 1 h = √146.25 ≈ 12.1 knots.

You steered 000° at 12 knots through the water, but made good 007° at 12.1 knots over the ground.

Course to Steer to Offset a Current

The reverse problem: you know the track you want to make good and your boat speed through the water, plus the current's set and drift — find the course to steer and the resulting speed of advance (SOA). Solve it as a current triangle:

1. From your start, draw a construction line in the direction of the course you want to make good.
2. From the start, lay off the current vector: in the direction of set, a length equal to drift (one hour's worth).
3. From the end of the current vector, open the dividers to your boat's speed (one hour's distance) and swing an arc to cut the desired-track line.
4. The line from the current-vector's end to that intersection is your course to steer; the distance from the start to the intersection is your speed of advance.

Worked example. You want to make good 270°T (due west). The current sets 180°T (due south) at 3 knots, and your boat makes 6 knots through the water. Because the current pushes you south of your desired westerly track, you must head north of west to counter it. Building the triangle:

• The offset angle satisfies sin θ = drift / boat speed = 3 / 6 = 0.5, so θ = 30°.
• Course to steer = 270° + 30° = 300°T (aimed north of due west, into the current).
• Speed of advance = √(6² − 3²) = √27 ≈ 5.2 knots made good along 270°T.

Steer 300°T and you crab westward at about 5.2 knots over the ground.

ETA When a Current Is Running

Once you know the speed of advance, ETA problems use 60 × D = S × T with the SOA, never the water speed.

Worked example. Continuing above, the waypoint on your desired 270°T track lies 10.4 nm ahead and your SOA is 5.2 knots. T = (60 × 10.4) / 5.2 = 120 minutes = 2 hours. If you departed at 0800, your ETA is 1000 — but if you had carelessly used the 6-knot water speed you would have predicted 104 minutes and arrived late, because the current stole speed made good.

Leeway

Leeway is the downwind sideways slip a strong wind gives the hull, distinct from current but treated the same way in a plot: it is a set (downwind direction) and a drift (its speed), added into the current triangle. On the exam a stated "current" already folds in whatever the problem wants you to offset.

Set/Drift Traps

• Swapping set and drift — set is a direction, drift is a speed.
• Reversing the vector — set runs from the DR to the fix, not fix to DR.
• Steering the wrong way — you head into the current, not with it.
• Using boat speed for ETA when a current is running — use the speed of advance.