Collision Avoidance in Fog with a Radar-Only Contact
You are the OOW on a 190 m general cargo ship in fog, visibility about 0.4 nm, in a busy traffic area. Speed is 12 kn, radar operating on 12 nm range. The master has been called and is on the bridge. A radar contact is detected bearing 020° relative, range 5.5 nm, closing. Over the next 12 minutes the contact's bearing is steady at 020° relative and range decreases to 2.8 nm. The contact is not showing AIS. There is another weaker contact at 300° relative, 4 nm, apparently opening. The vessel is in a narrow traffic lane with a landmass 1.5 nm to starboard.
1. What does the steady bearing and decreasing range tell you, and what are your obligations?
Read the worked answer
A steady relative bearing with decreasing range is the classic radar indication of risk of collision - the contact is on a converging course and, if nothing changes, will pass close or collide. The fact that the bearing is steady over 12 minutes and the range has halved makes this a firm conclusion, not a tentative one.
Obligations under the Rules:
- Rule 19 (restricted visibility): I must proceed at a safe speed, have engines ready for immediate manoeuvre, and take avoiding action in ample time based on radar information. A steady bearing contact means I must act.
- Rule 19 also requires that, so far as possible, avoiding action avoids: (a) an alteration to port for a vessel forward of the beam, (b) an alteration toward a vessel abeam or abaft the beam. So a starboard alteration is the default here, but the landmass 1.5 nm to starboard constrains it.
- Rule 8: the alteration must be large enough to be readily apparent on radar - a small alteration is dangerous. A change of at least 30–40° or a substantial speed reduction, or both, is appropriate.
- Rule 5 and 7: maintain a proper lookout and use all available means; the fog signal should be sounded.
I would also reduce speed - 12 kn in 0.4 nm visibility is fast, and the ability to stop within the visible distance is doubtful.
2. The landmass to starboard limits your starboard alteration. What manoeuvre do you choose and why?
Read the worked answer
This is the crux: the textbook answer (alter to starboard) conflicts with the geographic constraint. My reasoning:
- A starboard alteration of the size needed (30–40°) would put me toward the land 1.5 nm away, which is unacceptable in fog with a closing contact.
- A port alteration is prohibited by Rule 19 for a vessel forward of the beam, and would anyway be unpredictable to the other vessel.
- The safest effective action is a substantial reduction in speed - ideally to bare steerage way or stopped - combined with a modest starboard alteration if the geometry allows it without approaching the land. Stopping or nearly stopping removes the closing risk and gives time to resolve the situation, and it is a manoeuvre the other vessel can detect (radar and, if close, sound signals).
I would also consider: sounding the fog signal (one prolonged blast at intervals of not more than 2 minutes for a power-driven vessel making way), switching radar to a shorter range to get better resolution, and using the second contact at 300° as a check that it is genuinely opening. I would keep the master informed and record the times, ranges and bearings. If the contact continues to close despite my action, I would take further action - possibly turning away from the land while keeping clear - and be prepared for the close-quarters situation, including the possibility of the other vessel not detecting me.
3. How do you use the weaker contact at 300° relative in your decision?
Read the worked answer
The 300° relative contact is a secondary but important factor:
- It appears to be opening, but in fog with a weak echo I cannot be certain of its course and speed. A weak echo may be a small vessel, a yacht, or a poorly reflecting target - exactly the kind of contact that may not be keeping a proper lookout or may not have radar.
- If I reduce speed and the 020° contact resolves, I must ensure my manoeuvre does not create a new risk with the 300° contact. A starboard alteration would open the bearing to the 300° contact only if it is genuinely abaft the beam; a port alteration would close it.
- I would plot it more carefully, use a longer range scale to see if it is part of a stream of traffic, and listen for its fog signal. If it is a small craft, it may be unable to manoeuvre effectively, so I should keep clear of it as well.
The principle is that in restricted visibility every contact must be treated as a potential risk until positively resolved. I would not let the primary contact distract me from monitoring the secondary one, and I would keep a written plot or ARPA record of both so the decision trail is clear.