COLREGS encounter explorer
Drag ships on a chart and see, as you go, which COLREGS situation they are in, who gives way and who stands on, when and how close they pass, and where the rules turn ambiguous. Ship domains, encounter stages and velocity obstacles are drawn from the research literature.
B is on A's starboard bow and they are closing: A gives way and keeps out of the way, B stands on (Rules 15 to 17).
- own ship (ringed) and target
- ship domain, dashed at the CPA
- velocity obstacle of the own ship (red edge: she is on it)
- closest point of approach
- CR, CS, ID points and the last hard-over turn
Rule 15
Crossing
A has B on her starboard side, 43° off the bow, so she is the give-way vessel and B stands on.
Keep out of the way: take early and substantial action, and avoid crossing ahead of the other vessel.
Rules 15, 16
Keep course and speed. Act alone only once it is clear the give-way vessel is not keeping out of the way.
Rule 17
head-on (±5.7°)other on her starboard sideon her port sidemore than 22.5° abaft the beam
- CPA
- 0.18 NM
- TCPA
- 20:37
- Range now
- 5.84 NM
- Too close: the CPA of 0.18 NM is inside the 0.50 NM safe passing distance.
- If nobody acts, the passing turns unsafe in 18:58 (neither domain).
Domain check if nobody manoeuvres (Szlapczynski and Szlapczynska 2017)
- a) B stays out of A's domainin 19:05
- b) A stays out of B's domainin 19:11
- c) neither enters the other's domainin 19:05
- d) the two domains do not overlapin 17:13
Stages of the encounter
He et al. (2017): risk of collision (CR), close quarters (CS) once a hard-over turn can no longer keep the passing safe, and immediate danger (ID) once it can no longer prevent contact.
Stage 2: Risk of collision. Take early and substantial action to keep well clear (Rules 8 and 16).
Last steering point (FTCS) in 17:16, hard to starboard. A turn to port would buy 0:20 more, but the rules ask for starboard.
Stage 2: Risk of collision. Keep course and speed (Rule 17(a)(i)).
Last steering point (FTCS) in 17:28, hard to starboard.
No CR: no risk of collision yetCR: risk of collisionCS: close-quarters situationID: immediate danger
Ambiguities and gaps
No ambiguity: for every encounter with risk of collision the rules give one clear answer.
Vessels
On the chart: drag a ship to move her, drag the end of her arrow to set course and speed (hold Shift to snap). Focus a ship and use the arrow keys for fine steps.
All encounters
| Pair | Situation | Duties | CPA | Stage now |
|---|---|---|---|---|
| Crossing | A: Give wayB: Stand on | 0.18 NM in 20:37 | CR / CR |
Measurements
| A sees B | B sees A | |
|---|---|---|
| Range | 5.84 NM | |
| True bearing | 043° | 223° |
| Relative bearing Q, Q₁ | 043°, starboard bow | 313°, port bow |
| Aspect | Red 47 | Green 43 |
| Lights by night | Masthead lights (2)Red sidelightin sight from A | Masthead lights (2)Green sidelightin sight from B |
| Relative motion | 17.0 kn towards 225° (of B, relative to A) | |
| DCPA, TCPA | 0.18 NM in 20:37 | |
| Bow crossing | B crosses 0.25 NM ahead in 20:00 | A crosses 0.25 NM astern in 21:15 |
| Domain | By situation (crossing): 4.95L ahead, 3.05L astern, 4.33L to starboard, 3.67L to port | By situation (crossing): 4.95L ahead, 3.05L astern, 4.33L to starboard, 3.67L to port |
| Turning | advance 547 m, tactical diameter 579 m | advance 669 m, tactical diameter 708 m |
| Speed, length | 12.0 kn, 180 m | 12.0 kn, 220 m |
Aspect is how the other vessel presents herself: Green 30 means you are 30° on her starboard bow and see her green sidelight. Light ranges are the minimums of Rule 22; turning is from Wang's (2010) regressions on speed.
Model and display
Neither vessel enters the other's domain: the only reading that is both safe and true to the classic definitions.
A CPA closer than this also counts as unsafe, and it sets the width of the velocity obstacle cones.
Converters
1 NM = 1852 m exactly, and a knot is 1 NM per hour, so 1 kn = 0.5144 m/s. A cable is a tenth of a mile, 185.2 m. At 12 kn a ship covers 1 NM in 5 minutes.
What it does
- Head-on, crossing or overtaking, read from both ships' relative bearings, with the give-way and stand-on duties of Rules 13 to 17
- Closest point of approach and when it happens, bow crossing range, aspect and relative motion, with warnings when ships get too close
- Four ship domain models (He et al., Coldwell, Hansen et al., Wang's quaternion domain) and the four domain-based safety criteria
- The four stages of an encounter, from risk of collision to immediate danger, with the last moment a hard-over turn still works
- Velocity obstacle cones, and the masthead, side and stern light sectors with their Rule 22 ranges
- Ambiguities flagged as they appear, including the double crossing where no COLREGS situation holds
- Ready-made scenarios, playback, up to eight ships, shareable links, and nautical mile and knot converters
How to use it
Pick a scenario, or start dragging. On the chart, drag a ship to move her and drag the end of her arrow to set course and speed; the arrow shows where she will be in a few minutes. Click a ship to make her the target, and use Own ship in the vessel list to see the encounter from another bridge. Play runs the clock forward with every ship holding her course and speed, so you can watch the encounter move through its stages. Hold Ctrl (or ⌘) and scroll, or pinch, to zoom.
What it shows
- The situation. Each ship reads the encounter from two angles: where the other ship bears from her own bow (Q), and where she herself bears from the other's bow (Q₁). Following He et al. (2017), it is head-on when both bearings are within 5.7° of the bow, overtaking when one ship is more than 22.5° abaft the other's beam, and crossing otherwise. The two dials in the encounter card show both readings on the sectors that decide it.
- Who does what. The give-way vessel keeps out of the way (Rules 15 and 16, or Rule 13 when overtaking), the stand-on vessel keeps her course and speed (Rule 17), and in a head-on situation both alter to starboard (Rule 14).
- How close, and when. The closest point of approach (CPA) and the time until it (TCPA), drawn on the chart with both ships and their domains as they will be then.
- Ship domains. The water each ship wants kept clear. A passing counts as unsafe by one of four criteria from Szlapczynski and Szlapczynska (2017): the target stays out of the own ship's domain, the own ship stays out of the target's, neither enters the other's, or the two domains never overlap. A plain safe passing distance can be added on top.
- Stages. He et al. (2017) split an encounter into no risk of collision, risk of collision, close quarters (once a hard-over turn can no longer keep the passing safe: the last steering point) and immediate danger (once it can no longer prevent contact). The stand-on vessel's third stage, where she may act alone, starts halfway between the last two.
- Velocity obstacles. For the own ship, each cone holds the velocities that would bring a target closer than the safe passing distance. If the tip of her arrow is inside a cone, she is on a collision course; drag it out and she is not.
- Lights. Which of the other ship's lights you would see by night (Rule 21) and whether she is within their minimum range yet (Rule 22): six miles for a masthead light of a ship of 50 m or more, three for her sidelights and sternlight.
Where the rules run out
The rules are written for two ships and leave room for doubt. The explorer flags it:
- Double crossing. Each ship has the other on her port side, so under Rule 15 each is the stand-on vessel and nobody gives way, yet they will pass dangerously close. We found such encounters while generating test scenes for autonomous ships (ISSTA 2025): a large, slow ship and a small, fast one is the typical shape. Open the Double crossing scenario to see one.
- Both give way. The mirror image: each has the other on her starboard side.
- Head-on from one bridge only. One ship sees the other dead ahead, but not the other way round. Rule 14(c) says assume head-on when in doubt.
- Overtaking or crossing. Close to 22.5° abaft the beam the sidelight and sternlight sectors overlap, so by night a ship may not know which she is. Rule 13(c) says assume overtaking.
- Conflicting duties. With three or more ships, one may have to give way to one ship and stand on for another at the same time.
The models behind it
The turns behind the stage times are kinematic: a ship holds her course until the turn bites, then turns on a circle of half her tactical diameter, with advance and tactical diameter taken from Wang's (2010) regressions on speed. He et al. used a hydrodynamic (MMG) model of a bulk carrier instead, so the times here are an approximation of theirs; the He et al. (2017) scenario reproduces their eight-ship simulation for comparison. Ships hold course and speed otherwise, on a flat chart, in good visibility. This is a tool for understanding the rules and the research, not for navigation.
Sources
- Y. He, Y. Jin, L. Huang, Y. Xiong, P. Chen, J. Mou. 2017. Quantitative analysis of COLREG rules and seamanship for autonomous collision avoidance at open sea. Ocean Engineering 140, 281–291. doi:10.1016/j.oceaneng.2017.05.029
- R. Szlapczynski, J. Szlapczynska. 2017. Review of ship safety domains: models and applications. Ocean Engineering 145, 277–289. doi:10.1016/j.oceaneng.2017.09.020
- T. G. Coldwell. 1983. Marine traffic behaviour in restricted waters. Journal of Navigation 36, 431–444.
- M. G. Hansen et al. 2013. Empirical ship domain based on AIS data. Journal of Navigation 66, 931–940.
- N. Wang. 2010. An intelligent spatial collision risk based on the quaternion ship domain. Journal of Navigation 63 (4), 733–749.
- International Maritime Organization. 1972. Convention on the International Regulations for Preventing Collisions at Sea (COLREGS), as amended.