Drift Estimator, or set and drift calculator, combines steered vessel course and speed with current set/drift and optional wind-drift vectors to estimate course over ground, speed over ground, and accumulated east/north offset. Use it for coastal pilotage checks, dead-reckoning updates, small-vessel passage planning, current drift checks, and explaining why course made good differs from the intended heading. The calculator assumes each vector remains steady over the selected interval, so it works best for short planning windows with reasonably stable current and leeway assumptions. Treat the output as situational-awareness support and verify it against GPS track, log speed, chartplotter history, tide/current data, and local observations.
Use this calculator as one part of a broader passage-planning workflow. The
Marine Navigation Calculators toolkit
explains how CPA/TCPA, UKC, tide windows, route distance, set and drift, fuel range, and manoeuvring checks fit together.
You can also browse the full
Marine & Navigation category.
Enter the vessel, route, tide, current, or clearance assumptions exactly as labelled, then verify the result against official data and onboard judgment.
How to use this tool
Enter the vessel course steered and vessel speed through the water.
Enter current set direction and drift speed, then add wind-drift direction and speed if you have a leeway estimate.
Set the duration for the planning interval and run the estimator.
Review course over ground, speed over ground, east/north offset, and total drift offset before comparing with charted track or GPS history.
Drift Inputs
Result
Course over ground: 96.06°
Speed over ground: 11.140 kn
Drift east-west: 4.311 nm
Drift north-south: -4.705 nm
Total drift offset: 6.382 nm
Formula or method
Headings are treated as degrees clockwise from north, with vector components resolved into east and north axes.
Set is the direction the current or drift moves toward; drift is the speed of that movement.
Course over ground and speed over ground are calculated from the combined vessel, current, and wind-drift vectors.
East/north offset is calculated from environmental vectors over the entered duration, not from the vessel's own through-water motion.
Wind drift is modeled as an already-estimated set vector, not derived from sail plan, hull shape, wind angle, or sea state.
Worked example
Checking expected set during a four-hour coastal leg
Vessel course/speed: 090 deg at 10 kn
Current set/drift: 140 deg at 1.2 kn
Wind drift: 130 deg at 0.4 kn
Duration: 4 h
Result: The estimator returns the combined course over ground, speed over ground, and accumulated drift offset from the environmental vectors.
Use the offset to brief the likely cross-track effect and decide whether the plan needs a correction, then compare it with observed GPS track and updated tide/current information.
How to interpret the result
Drift output explains the effect of steady environmental vectors; it is not a live navigation fix.
Course over ground is the direction the combined vessel/current/wind vector moves over the chart.
Speed over ground can increase or decrease depending on whether environmental vectors help or oppose the vessel track.
East/north offset isolates environmental set over the interval, which is useful for dead-reckoning and briefing corrections.
Cross-track drift can become operationally important near hazards, traffic separation schemes, narrow channels, or lee shores.
Common mistakes
Mixing "set toward" and "wind from" conventions without converting wind drift into a direction of motion.
Using one tide/current value across an interval where the current turns or changes strength.
Treating COG/SOG output as a commanded course rather than a prediction from the entered vectors.
Ignoring magnetic/true heading consistency when comparing with instruments or charts.
Offset over time: drift_offset = (V_current + V_wind_drift) * duration.
Assumptions
Assumes constant vectors for vessel, current, and wind drift during the interval.
No leeway nonlinearity, maneuvering transients, or sea-state dynamics modeled.
Review note and limitations
Method - vector addition of vessel, current, and wind-drift components with steady-offset projection.
Does not model changing tides, gusts, waves, leeway curves, steering error, shallow-water effects, or maneuvering transients.
Does not replace chart work, position fixing, route monitoring, or official tide/current sources.
Navigation planning support only. Verify with official charts, tide/current sources, instruments, visual fixes, and qualified navigation judgment.
FAQ
Is current set the direction water comes from or moves toward?
In this tool, set is the direction the current moves toward, expressed as degrees clockwise from north.
How should I enter wind drift?
Enter wind drift as the estimated direction and speed of vessel drift over the ground. Convert from wind-from direction or leeway assumptions before entering it.
Can this correct my course automatically?
No. It predicts the track from entered vectors. Course corrections still require chartwork, instrument checks, lookout, and navigation judgment.
Is this the same as course made good?
It estimates course over ground from the entered vectors. Course made good should be checked against observed positions over time.
Related tools and workflows
Drift estimates pair naturally with ETA compensation, tide-window, great-circle/rhumb-line, CPA/TCPA, and fuel-reserve tools.