Radar-only contact in fog with conflicting AIS data
At 2240 you are OOW of a laden 225 m bulk carrier steering 265° T at 13.5 kn in thick fog, with a radar range of visibility of about 1.2 nm and an actual visibility of about 0.5 nm. The vessel is not in a traffic separation scheme. The ARPA on the S-band radar shows a small contact bearing 006° relative, which over the past 14 minutes has closed from 8.5 nm to 5.2 nm with a steady bearing of 005–006°, a CPA of 0.3 nm and a TCPA of 20 minutes. The ARPA-derived course and speed of the contact are 090° T at 9 kn. The contact's AIS, received intermittently, shows 350° T at 6 kn. A fog signal is heard on the port bow and does not appear on either radar. Own ship's whistle is on automatic, sounding one prolonged blast at intervals of not more than two minutes, and gyro error is 0.0°. The master is asleep and the engine room is on UMS.
1. Assess the risk of collision and the reliability of the information you have, and state what you will do in the next few minutes.
Read the worked answer
Risk of collision exists. Under Rule 7 a risk is deemed to exist if the compass bearing of an approaching vessel does not appreciably change. Here the bearing is steady at 005–006° while the range halves, with a computed CPA of 0.3 nm - that is a risk, and at 13.5 kn it is an unacceptable one.
Reliability of the data
- The ARPA plot is the more defensible source: it is derived from measured ranges and bearings, is continuous, and can be checked against the raw video. Its 090°/9 kn output is consistent with a target crossing from port to starboard.
- AIS is not a collision-avoidance sensor. It can be misconfigured, stale, delayed, or deliberately falsified, and it may not even be the same target. The two data sets disagree fundamentally, so use the ARPA and assume the worst case.
- A small craft may have no radar reflector, no AIS and no lookout. The unanswered fog signal on the port bow is a second, unplotted hazard, and 13.5 kn is not a safe speed for 0.5 nm visibility under Rule 6.
Immediate actions
- Call the master; put the engine on standby; switch to hand steering; post an extra lookout forward and a dedicated radar observer.
- Reduce speed substantially - to a speed at which the ship can be stopped within half the visibility distance - and note the time.
- Plot the contact manually as a cross-check on the ARPA, and keep the fog signal on the port bow under observation.
- Make the next avoiding manoeuvre early, in ample time and be prepared to take all way off (Rule 19(e)) if the situation deteriorates.
2. What avoiding action will you take under Rule 19, and why is the direction of the alteration constrained?
Read the worked answer
The rule. In restricted visibility the steering and sailing rules (Rules 11–18) do not apply; Rule 19 governs. Rule 19(d) provides that, so far as possible, an alteration of course to avoid a vessel forward of the beam shall not be to port, unless the other vessel is being overtaken, and an alteration towards a vessel abeam or abaft the beam is to be avoided.
Application here
- The contact is on a relative bearing of 006° - fine on the starboard bow and clearly forward of the beam. A turn to port is therefore prohibited in practice; it would turn the ship into the crossing target and is unpredictable to the other vessel.
- Alter course boldly to starboard, at least 30–40°, so the change is unmistakable on the other vessel's radar (Rule 8(b) - a succession of small alterations must be avoided).
- Combine the turn with a reduction in speed. With an S-band plot showing a 9 kn target crossing at 90°, a 40° starboard turn plus a reduction to about 8 kn opens the CPA quickly and also buys stopping distance for the unplotted craft on the port bow.
- If a risk of immediate danger develops - for example the target's bearing starts to draw aft or the fog signal on the port bow closes - Rule 19(e) applies: reduce speed to the minimum at which the ship can be kept on course, or take all way off, and navigate with caution until the danger has passed.
- Keep the manoeuvre under continuous review; do not resume speed or original course until the contact is well clear and the position of the unplotted craft is resolved.
3. Should you call the other vessel on VHF to agree an avoiding manoeuvre? Give reasons, and state what you will record.
Read the worked answer
No, not as a means of agreeing a manoeuvre.
- The Collision Regulations contain no provision for negotiating a passing arrangement by radiotelephone. If a VHF agreement conflicts with the Rules, the Rules prevail, and a vessel that has agreed one thing by VHF may still manoeuvre in accordance with the Rules.
- The identity problem is serious here: AIS and ARPA are giving different data and the AIS may belong to a different unit. A call to 'the vessel on my starboard bow' may be answered by the wrong ship, or by none.
- In this case you have a clear, rule-based action available: a bold starboard alteration and a reduction of speed. That is predictable and does not depend on the other vessel's cooperation.
- A short, factual safety call is acceptable if it does not replace the manoeuvre - for example, to warn an unidentified small craft of your position and intentions, or to confirm that a vessel which has not answered your fog signal has heard you. Say clearly that you are altering to starboard and reducing speed; never offer to 'hold your course' or ask the other vessel to do so.
Records
- Time of first detection and every plot (bearing, range, CPA, TCPA).
- The time, nature and extent of every course and speed alteration made, and the reason.
- Fog signals heard, their bearing and interval, and the VHF channel used if any call is made, with the content of the exchange.
- Notification of the master and his arrival on the bridge.
These entries are what will be examined if there is a subsequent near-miss report or inquiry.