Watchkeeping
CPA and TCPA
From two radar plots of a target (true bearing and range, a few minutes apart) and your own course and speed: the CPA, the time to it, and the target's true course and speed.
Use true bearings: add own heading to relative bearings first.
Fill in the fields to see the result.
How it works
This is the relative-motion plot done by numbers. The two plots give the target's movement relative to your ship (the O–A line). Extended, the closest it passes to the centre is the CPA; the time to get there at the relative speed is the TCPA.
Adding your own ship's motion over the same interval to the relative motion gives the target's true motion: its course and speed.
Plot at least three times to see whether the target is changing course or speed; this tool assumes it holds both between and after the two plots. ARPA and COLREGs decisions stay with the officer of the watch.
For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.
More tools
All tools- Port to port distances
- Compass error
- Great circle and rhumb line
- Speed, distance, time and ETA
- Current triangle
- Compass error by the sun
- Sunrise, sunset and twilight
- Dead reckoning position
- Distance off
- Sextant altitude and noon latitude
- Required speed for an ETA
- True wind
- Horizon and light range
- MLC hours of rest
- Ship’s clocks and time zones
- IMO and MMSI number check
- Dock water allowance and loading
- Height of tide
- Mean draught and trim
- GM from rolling period
- Squat and under-keel clearance
- Anchoring
- Search patterns (IAMSAR)
- Fuel consumption and endurance
- Speed and consumption
- Propeller slip
- Indicated power
- Bunker quantity
- Fuel heating temperature
- Cylinder oil feed rate
- Electrical power
- Pay and contract dates
- Unit and position converter
- Signal flags and phonetic alphabet
- COLREGs lights and shapes
- Beaufort and sea state scales
- Marine VHF channels