Navigation and information
A ship cannot be steered safely without good information. A navigator gathers it before the voyage and keeps updating it:
- Charts and sailing directions (books that describe coasts and ports).
- Tide tables and current information.
- Weather and sea forecasts.
- Notices to Mariners: warnings about new wrecks, moved buoys, broken lights or works at sea. The chart is corrected with them.
Old information is dangerous. A chart that is not corrected may show a buoy where there is none.
Charts: the map of the sea
A nautical chart is a map made for ships. It shows the coastline, depths (soundings) in metres, depth lines, rocks, wrecks, shallow banks, lights and buoys. Latitude is drawn on the sides and longitude along the top and bottom, so a navigator can read positions and measure distances (1 minute of latitude = 1 nautical mile).
Depths are measured from chart datum, a very low water level. So the real depth is almost always equal to or more than the number on the chart: real depth = charted depth + height of tide. Colours help: shallow water is shown pale and deep water is white or darker blue. Always read the chart scale and the year of the latest correction.
Navigational aids: lighthouses, buoys and beacons
Navigational aids are fixed or floating signs that help a ship find its way.
- Lighthouses have a bright light seen from many miles. Each has its own pattern (for example one flash every 10 seconds), so the navigator knows which one it is. At daytime they stand out as tall towers.
- Buoys float and are anchored to the bottom. Lateral buoys mark the sides of a channel: in IALA region A (Europe, India, Africa, Australia) red is on your left (port) when entering harbour; in region B (Japan, USA, Korea, Philippines) red is on your right. The chart and local rules tell you which region you are in.
- Cardinal buoys show where the safe water is: north, east, south or west of a danger.
- Beacons and daymarks are fixed posts on rocks or shores. Some aids also have fog signals, radar reflectors or AIS signals.
Overview of currents
An ocean current is a steady flow of sea water over a long distance. Winds, the spin of the Earth and differences in temperature and salt make it. Famous examples are the warm Kuroshio near Japan and the Gulf Stream in the Atlantic. In the north Indian Ocean the currents even change direction with the monsoon.
Sailors give a current two numbers: its set (the direction it flows towards) and its drift (its speed in knots). A 2-knot current acting for 3 hours moves the ship 6 nautical miles. The navigator allows for it by steering a little against it.
Overview of tides
Tides are the rise and fall of the sea level, mainly caused by the pull of the Moon and also the Sun. In most places there are two high waters and two low waters in a day (about 12 hours 25 minutes apart from one high to the next). The difference between high and low water is the tidal range.
- Spring tides: biggest range, near new moon and full moon (Sun, Moon and Earth in a line).
- Neap tides: smallest range, at the half moons.
The horizontal movement of water caused by the tide is called a tidal stream. Tide tables give the times and heights of high and low water for each port. Depth over a shoal = charted depth + tide height. The ship also needs a safety gap under the keel, the under-keel clearance.
Try it
At home: fill a basin and push a paper boat with a straw. Then blow gently from the side: the boat is pushed sideways like a current pushes a ship.
In 3D: in the last step, set the tide to 0.5 m and the current to 4 knots. Is the route safe? Now raise the tide to 3 m. What changed?
Key formulas and definitions
- Real depth = charted depth + height of tide
- Under-keel clearance = real depth − ship draft
- Drift distance = current speed × time
- Interval between two high tides ≈ 12 h 25 min
- 1 minute of latitude = 1 nautical mile
Worked examples
1. A shoal is charted at 2.0 m. The tide table says the tide is 1.5 m high. Find the real depth.
Real depth = 2.0 + 1.5 = 3.5 m.
2. A ship has a draft of 3 m. The real depth is 3.5 m. What is the under-keel clearance?
Clearance = 3.5 − 3 = 0.5 m.
3. A current sets east at 2 knots. How far does it push a ship in 3 hours?
Drift = 2 × 3 = 6 nautical miles to the east.
4. High tide is at 06:10. When is the next high tide (about)?
Add 12 h 25 min: 06:10 + 12:25 = 18:35.
5. A ship with a draft of 6.0 m must cross a bar charted at 4.2 m. It needs 0.5 m clearance. What tide height is needed?
Needed depth = 6.0 + 0.5 = 6.5 m. Tide needed = 6.5 − 4.2 = 2.3 m or more.
6. A ship steers north at 10 knots for 2 hours. A current sets east at 3 knots. How far from the start is it?
North = 10 × 2 = 20 nm. East = 3 × 2 = 6 nm. Distance = √(20² + 6²) = √436 ≈ 20.9 nm.
Common mistakes
- Reading the chart depth as the real depth. You must add the tide height.
- Thinking a current is the same as a tide. A current is a flow of water; the tide is the rise and fall of the level (tidal streams are the flow caused by tides).
- Mixing up red and green buoys. Check the buoyage region: the sides are reversed between region A and region B.
- Using an old chart without correcting it from Notices to Mariners.