Set and Drift Calculator

Calculate set and drift from DR and fixes, solve course to steer with a known current, or predict course made good using standard marine navigation vector math.

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Mode

Calculation Direction

Find Set and Drift

When to use: Use this when you steered a known course at a known speed for a period of time, but your actual fix does not match the DR position.

Given: Given start position, CTW, STW, elapsed time, and actual fix, the calculator derives the DR position and plots the current vector from DR to fix.

Asked: Asked: set, drift, drift angle, course made good, speed over ground, and computed DR position.

Inputs

Find Set and Drift

DR is computed from start position, CTW, STW, and elapsed time. Set is measured as the true bearing from computed DR to the actual fix.

Ready to calculate

Enter the navigation data and press Calculate to see the result and current-triangle plot.

Plot

Current Triangle Plot

N/E vectors
NEWater/DR: 10.47 at 45 degGround/Fix: 11.34 at 56.9 degCurrent: 2.42 at 119.7 deg

The plot shows the current triangle: water vector plus current vector equals ground vector.

How to Use

  1. Select Find Set and Drift when a fix does not match the DR position expected from CTW, STW, and elapsed time.
  2. In Find mode, enter the start position, CTW, STW, elapsed time, and actual fix. The calculator derives the DR position internally.
  3. Select Course to Steer when set and drift are known and you need a CTW that makes good the desired track.
  4. Select Course Made Good when you want to predict the CMG and SOG produced by a planned CTW/STW and known current.
  5. Enter all courses and sets as true bearings from 0 to less than 360 degrees.
  6. Press Calculate. CSV and PDF export become available after a result is produced.

Formula Reference

Reverse solve

DR position = start advanced by CTW x STW x timeCurrent vector = actual fix - DR positionDrift = current vector length / elapsed time

Course to steer

Water vector + current vector = ground vectorCourse to steer = bearing of the required water vector

Course made good

Ground vector = steered water vector + current vectorCMG = bearing of ground vector; SOG = ground vector length

Terms

Set is the true direction toward which the current flows.

Drift is the speed of the current, usually expressed in knots.

Course to steer is the true course ordered or steered through the water so the vessel makes good the intended track.

CMG is course made good, the actual track over the ground.

SOG is speed over ground, the actual ground distance per hour.

DR is dead reckoning, a position estimated from course, speed, and time.

Worked Example

A vessel starts at 00 degrees 00.0 minutes N, 000 degrees 00.0 minutes E. It steers 090 degrees true at 10 knots for 2 hours.

The DR position is about 00 degrees 00.0 minutes N, 000 degrees 20.0 minutes E. The actual fix is 00 degrees 12.0 minutes N, 000 degrees 23.0 minutes E.

The current vector from DR to fix is therefore 12 NM north and 3 NM east. Its bearing is 014.0 degrees true, and its length is 12.37 NM.

The drift is 12.37 NM divided by 2 hours, so the drift is 6.18 knots. The drift angle is about 27.6 degrees left of the steered course.

Manual set-and-drift estimates are a planning and training aid. Currents vary with tide state, eddies, wind, and time, so practical navigation still depends on continuous fixes and prudent cross-checks.

What Set and Drift Mean

Set and drift describe the current acting on a vessel. Set is the direction toward which the current flows, expressed as a true bearing. Drift is the rate of that current, usually expressed in knots.

In international wording, set and drift may also be called rate and direction. Direction is the set; rate is the drift.

Worked Example: Finding Set and Drift

A vessel starts at 00 degrees 00.0 minutes N, 000 degrees 00.0 minutes E. It steers 090 degrees true at 10 knots for 2 hours.

  1. The DR position is about 00 degrees 00.0 minutes N, 000 degrees 20.0 minutes E.
  2. The actual fix is 00 degrees 12.0 minutes N, 000 degrees 23.0 minutes E.
  3. The current vector from DR to fix is 12 NM north and 3 NM east.
  4. The current vector has a bearing of 014.0 degrees true and a length of 12.37 NM.
  5. The drift is 12.37 NM divided by 2 hours, which equals 6.18 knots.

The final answer is set 014.0 degrees true and drift 6.18 knots. The drift angle is about 27.6 degrees left of the steered course.

Course to Steer and Course Made Good

Course to steer is the course through the water that should be steered to make good a desired track after current is applied. Course made good is the actual track over the ground after the vessel's motion through the water and the current vector combine.

Why Manual Set-and-Drift Calculation Still Matters

Manual set-and-drift calculation remains useful for license exams, fallback navigation, and sanity-checking GPS, ECDIS, or chart plotter output. It is especially useful when a navigator wants to understand why a vessel is not making the expected track.

Manual estimates should not be treated as a substitute for continuous fixes. Real currents change with tide state, eddies, depth, wind, and time.

FAQ

How do you calculate set and drift?

Advance the start position by CTW, STW, and elapsed time to derive DR. Draw the vector from computed DR to the actual fix. The vector bearing is set, and the vector distance divided by elapsed time is drift.

What is the difference between set and drift?

Set is direction. Drift is speed. A current might set toward 060 degrees true with a drift of 2 knots.

What is course to steer?

Course to steer is the true course steered through the water so that, after the known current acts on the vessel, the desired ground track is made good.

What is course made good?

Course made good is the actual track over the ground after the course steered, vessel speed through water, and current vector combine.

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