Calculator Inputs
Example Data Table
| Case | Course | Water Speed | Time | Observed East | Observed North | Use |
|---|---|---|---|---|---|---|
| Coastal practice | 090° | 8 knots | 2 hours | 13.2 NM | 4.0 NM | Find tidal set and drift |
| River passage | 045° | 6 knots | 1.5 hours | 7.0 NM | 8.5 NM | Estimate river push |
| Training fix | 180° | 5 knots | 1 hour | -1.2 NM | -4.4 NM | Check cross-current |
Formula Used
Elapsed time: time = hours + minutes ÷ 60.
Dead reckoning distance: DR distance = speed through water × time.
DR east: DR east = DR distance × sin water course.
DR north: DR north = DR distance × cos water course.
Current vector: current east = observed east − DR east. Current north = observed north − DR north.
Set: set = atan2 current east, current north. The result is normalized from 0° to 360° true.
Drift: drift = current vector distance ÷ elapsed time.
Course made good: CMG = atan2 observed east, observed north.
Speed over ground: SOG = observed ground distance ÷ elapsed time.
Required heading: required water vector = desired ground vector − current velocity vector.
How to Use This Calculator
- Enter the course steered in true degrees.
- Enter the speed through water in knots.
- Add the elapsed hours and minutes between fixes.
- Enter observed east and north movement from the starting point.
- Add leeway if you want to adjust the water track.
- Enter magnetic variation if magnetic output is needed.
- Add a target course and ground speed for correction guidance.
- Press calculate and review the result below the header.
- Download the CSV or PDF file for records.
What Is Set and Drift?
Set and drift describe the real movement of water across a vessel track. Set is the direction the current flows. Drift is the current speed. A boat may point along one course, yet the ground track can slide away. This calculator compares the expected dead reckoning run with an observed fix. It then returns the current vector, ground speed, course made good, and practical steering guidance. The tool is useful for coastal work, sailing lessons, small craft planning, and chart practice.
Why Accurate Current Work Matters
Navigation depends on knowing the difference between motion through water and motion over ground. Engine speed or sail speed shows progress through water. A GPS fix shows progress over the earth. The gap between those two vectors often comes from tide, river flow, wind leeway, or measurement error. When the gap is large, arrival time and clearance from hazards can change quickly. A clear set and drift estimate helps you decide whether to alter heading, slow down, wait for tide, or choose another route.
Planning With the Result
Begin with a reliable course steered, speed through water, and elapsed time. Then enter the observed north and east movement from the starting point. The calculator builds a dead reckoning vector from course, speed, and time. It subtracts that vector from the observed ground vector. The remaining vector is the current. The result gives set as a true bearing and drift as knots. It also shows the course made good and speed over ground.
Advanced Notes for Better Logs
Use consistent units. Nautical miles and knots make time based work simple. Record the exact start and finish times. Use true bearings unless your chart work needs magnetic values. If you add a target course, the calculator estimates a heading to steer that offsets the current. Treat this as a planning guide. Real seas change. Repeat the calculation often, especially near headlands, channels, and tidal gates. Save the CSV or PDF result for your logbook, training record, or passage plan. Good records make later checks easier and improve future decisions. Compare each saved run with weather notes before changing route. This habit shows patterns quick mental estimates can miss during navigation aboard.
FAQs
What does set mean?
Set is the direction toward which the current moves the vessel. It is shown as a true bearing unless you use the magnetic variation output.
What does drift mean?
Drift is the speed of the current. This calculator reports drift in knots by dividing current distance by elapsed time.
What is course made good?
Course made good is the actual direction traveled over ground between two fixes. It can differ from the steered course because of current and leeway.
What is speed over ground?
Speed over ground is the observed distance over ground divided by elapsed time. It shows actual progress across the chart.
How should I enter west or south movement?
Use negative values. West movement is negative east. South movement is negative north. Keep all movements in nautical miles.
Can I use magnetic bearings?
The main input expects true bearings. You can enter magnetic variation to display magnetic set and magnetic course made good.
What does leeway allowance do?
Leeway changes the water track used for dead reckoning. Positive values rotate the steered course clockwise before the current is estimated.
Is the required heading exact?
It is a vector estimate based on the current found from your fix data. Recheck often because current, wind, and steering errors can change.