What is navigation and why do we need it?
Navigation is the art and science of taking a ship safely from one place to another. A navigator must answer three questions: Where are we? Where do we want to go? How do we get there safely?
Fishing boats and cargo ships cannot stop to ask for directions. A small mistake can put a ship on rocks or waste fuel, so navigation is a core skill of every fishing or merchant crew.
A short history of navigation
Long ago sailors stayed in sight of land and used hills and rocks as signs. Then they learned to use the Sun and stars to know direction at night. The magnetic compass (used on ships for about 1,000 years) gave direction even on cloudy days. The sextant measured the height of the Sun or a star. The chronometer, an exact clock, helped find longitude. In the 20th century radar and radio arrived, and today satellite systems (GNSS/GPS) give the position in seconds.
Voyage and navigation methods
A voyage is the whole trip from the starting port to the last port. To find the ship's position we use one or more methods:
- Coastal (pilotage) navigation: use landmarks, lighthouses and a chart near the coast.
- Celestial navigation: measure the Sun, Moon or stars with a sextant.
- Radio navigation: use radio signals, radar or satellites.
- Dead reckoning: start from a known place and add the course and the distance run.
A good navigator uses more than one method so that one can check the other.
Direction, course and position
Direction at sea is measured in degrees from north (000°), going clockwise: east is 090°, south is 180°, west is 270°. The direction the ship is steering is its course. A position is given by latitude (north-south) and longitude (east-west).
Navigation calculations: nautical mile, knot and D = S × T
A nautical mile (nm) is 1,852 metres. It equals one minute of latitude, which is why it fits the chart so well. A knot is a speed of 1 nautical mile per hour.
Distance = Speed × Time, so Time = Distance ÷ Speed and Speed = Distance ÷ Time. Time in hours: 45 minutes is 0.75 h.
With dead reckoning you draw the course line on the chart, measure the distance run, and mark the estimated position. Current, wind and waves push the ship off the line, so the position is fixed again with landmarks or satellites.
Try it
At home: walk across a room at a steady pace and time yourself. Now predict the time for a distance twice as long, then check. You just used D = S × T.
In 3D: go to the last step, set speed 12 knots and 4 hours. Predict the distance first (48 miles), then check the readout.
Key formulas and definitions
- Distance = Speed × Time
- Time = Distance ÷ Speed
- Speed = Distance ÷ Time
- 1 nautical mile = 1,852 m = 1 minute of latitude
- 1 knot = 1 nautical mile per hour (about 1.85 km/h)
- Course is measured clockwise from north: N 000°, E 090°, S 180°, W 270°
Worked examples
1. A fishing boat runs at 8 knots for 5 hours. How far does it go?
Distance = 8 × 5 = 40 nautical miles.
2. A port is 90 nautical miles away. The ship steams at 12 knots. How long does the trip take?
Time = 90 ÷ 12 = 7.5 hours = 7 hours 30 minutes.
3. A ship leaves at 08:00 and must cover 120 nm at 16 knots. When does it arrive?
Time = 120 ÷ 16 = 7.5 h. 08:00 + 7 h 30 min = 15:30.
4. Change 5 nautical miles into metres and into kilometres.
5 × 1,852 = 9,260 m = 9.26 km.
5. A ship steers east at 10 knots for 3 hours. A current of 2 knots flows north. How far north is it pushed, and how far is it from its start?
Run through water = 10 × 3 = 30 nm east. Drift = 2 × 3 = 6 nm north. Distance from start = √(30² + 6²) = √936 ≈ 30.6 nm.
6. A ship covers 45 nm in 90 minutes. What is its speed in knots?
90 minutes = 1.5 h. Speed = 45 ÷ 1.5 = 30 knots.
Common mistakes
- Mixing kilometres and nautical miles. 1 nm is 1.852 km, not 1 km.
- Saying "knots per hour". A knot already means nautical miles per hour.
- Putting minutes into the formula without changing to hours (45 min = 0.75 h).
- Measuring course from the wrong place. Course is clockwise from north, not from the nearest road or coast.