Why ships need traffic laws
A big ship needs a long distance to stop (sometimes more than a kilometre). So two ships must agree in advance on what each will do. The world agrees on one set of rules: the International Regulations for Preventing Collisions at Sea (COLREGs, 1972). Every country writes them into its own law. In Japan this is the Act on Preventing Collisions at Sea; in India it is the Merchant Shipping Act rules based on COLREGs. The rules are almost the same everywhere.
Act on Preventing Collisions at Sea: the basic rules
- Look-out: keep a proper watch by eyes, ears and radar at all times.
- Safe speed: go slow enough to stop in time in fog, traffic or at night.
- Risk of collision: if the compass bearing of another ship does not change while it gets closer, there is a risk of collision.
- Act early and clearly: change course or speed by a large amount, in good time, so the other ship can see it.
Head-on
Two power-driven ships meeting nearly end-on: each alters course to starboard and they pass port to port.
Crossing
The ship that has the other on her starboard side is the give-way vessel. She turns (preferably to starboard) or slows and avoids crossing in front. The other is the stand-on vessel: she keeps course and speed, but must act if the give-way ship does not.
Overtaking
A ship that comes up from behind (more than 22.5° behind the other ship's beam) is overtaking and must keep out of the way, even if the other changes course later.
Who gives way to whom
In general a power-driven ship gives way to a sailing vessel, a fishing vessel working with nets, a ship not under command and a ship with restricted ability to manoeuvre.
Lights and sound signals
At night ships show a green light on the starboard side, a red light on the port side and white lights ahead and astern. In sight of each other: 1 short blast = "I am turning to starboard", 2 short = "to port", 3 short = "engines going astern", 5 short = "I do not understand your action: danger".
Maritime Traffic Safety Act: busy waters
Some sea areas are very crowded, for example big bays and narrow straits. Special laws, such as Japan's Maritime Traffic Safety Act (it covers Tokyo Bay, Ise Bay and the Seto Inland Sea), add stricter rules on top of the basic collision rules:
- Traffic routes (lanes): ships follow a lane in the marked direction, like a one-way road. This is similar to a traffic separation scheme in other countries.
- Speed limits in narrow or crowded routes.
- Special care for very big ships, ships towing, ships carrying dangerous goods, and ships working at sea, such as fishing boats.
- Reporting and traffic information: a coastal traffic centre watches ships by radar and sends warnings by radio.
In a narrow channel ships keep to the starboard side of the channel and do not hinder a ship that can only move inside the channel.
Port Regulations Act: rules in ports
Inside a port, ships and small boats are close to each other, to jetties and to people. Port laws, such as Japan's Port Regulations Act, set local rules:
- Enter and leave through the fairway (the main channel). Do not anchor in the fairway.
- Anchor only in the designated anchorage and moor only at the proper berth.
- Keep a slow speed so that your wash does not damage other boats and quays.
- Ships that carry dangerous goods need permission and must follow special rules.
- Report entry and departure to the port authority (harbour master) and follow his orders.
- Do not throw rubbish, oil or ballast water in the port.
Try it
At home: two friends walk towards each other along a corridor. Practise the head-on rule: both step to your right. Then try a crossing at a corner. Who waits?
In 3D: go to the last step, choose "Crossing", turn the rules off and slide the time bar. See the red ring. Turn the rules on and look at the closest gap.
Key formulas and definitions
- Head-on: both alter to starboard (pass port to port)
- Crossing: the ship with the other on her starboard side gives way
- Overtaking: the overtaking ship keeps clear
- Risk of collision: compass bearing constant + range decreasing
- Sound signals: 1 short = starboard, 2 short = port, 3 short = astern, 5 short = doubt/danger
- Closing time = distance apart ÷ closing speed
Worked examples
1. Two power-driven ships meet head-on. What should each do, and on which sides do they pass?
Each alters course to starboard (right). They pass port side to port side (left to left).
2. You are in ship A. You see ship B on your starboard bow. Its compass bearing stays constant and it comes closer. Who gives way?
There is a risk of collision. A has B on her starboard side, so A is the give-way vessel and B stands on.
3. A power-driven ship meets a fishing boat with nets in the water. Who gives way?
The power-driven ship gives way to the fishing vessel at work.
4. Two ships are 11 nm apart, head-on, at 12 knots and 10 knots. In how long do they meet if neither changes?
Closing speed = 12 + 10 = 22 knots. Time = 11 ÷ 22 = 0.5 h = 30 minutes.
5. Ship A (14 knots) is 2 nm behind ship B (10 knots) on the same course. How long until A is alongside B?
Closing speed = 14 − 10 = 4 knots. Time = 2 ÷ 4 = 0.5 h = 30 minutes. A must keep clear when it passes.
6. You alter course to port. Which sound signal do you give when ships are in sight of each other?
Two short blasts.
Common mistakes
- Turning left (port) in a head-on meeting. The rule is: both turn to starboard.
- The stand-on ship also turning at the last moment without a reason. She should keep course and speed, and act only if the give-way ship does not.
- Making a small change of course that the other ship cannot see. Action must be large, early and clear.
- Thinking the larger ship always has the right of way. The rules depend on the situation and the type of ship, not just size.