Tidal Vector Calculator
The tidal vector calculator adds your boat's water track vector (course steered and speed through water) to a tidal current vector (set and drift) to find your actual course and speed over the ground. This is essential for accurate passage planning and position estimation when a significant current is running. Enter your boat's course (degrees true) and speed in knots, and the tidal current's set (direction the current flows toward, degrees true) and drift in knots. The calculator returns the resultant course over ground and speed over ground.
Tidal vector addition formula
Boat North = Vboat * cos(Course), Boat East = Vboat * sin(Course)
Tide North = Drift * cos(Set), Tide East = Drift * sin(Set)
Total North = Boat North + Tide North
Total East = Boat East + Tide East
SOG = sqrt(Total North^2 + Total East^2)
COG = atan2(Total East, Total North) converted to 0-360 deg
All courses are true. North and East components are in knots (equivalent to nm per hour).
Using tidal vectors in navigation
- NOAA current predictions give set and drift for tidal stations referenced to the time of high water.
- For a one-hour plot, draw the boat vector and the tidal vector tip-to-tail to find the resultant ground track.
- Building a vector diagram on the chart helps visualize the current's effect and check calculator results.
- Tidal streams in restricted channels can be several knots; a 2-knot tide can dramatically alter a sailboat's track.
Tidal vector: frequently asked questions
What is a tidal vector?
A tidal vector describes the movement of water due to the tide: it has a direction (set) and a speed (drift). When you combine your boat's water track vector with the tidal vector, the result is your actual movement over the seabed, called the course and speed over ground.
How are two vectors combined?
Vectors are combined by adding their North and East components separately. For each vector, the North component is speed * cos(course) and the East component is speed * sin(course). The resultant vector's magnitude and direction are found from the combined components using Pythagoras and atan2.
When would I use the tidal vector calculator?
Use it when planning a passage to determine your actual speed and course over ground, or when building an EP (estimated position) using known tidal stream data from tide tables or pilot charts. It differs from course-to-steer in that it gives the resultant track rather than solving for the required heading.
Where can I find tidal stream data for US waters?
NOAA publishes tidal current predictions at tidesandcurrents.noaa.gov for hundreds of US stations. The data includes maximum flood and ebb speeds and times referenced to nearby tide stations. NOAA also publishes tidal current charts for major harbors.
Is tidal stream the same as current?
Tidal stream refers specifically to periodic horizontal water movement caused by the tides. Ocean currents (like the Gulf Stream) are driven by temperature, salinity, and wind and are more persistent. The tidal vector calculator applies to any current, not just tidal streams.
Official sources
- NOAA Tides and Currents: NOAA Current Predictions.
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.