‘You see ahead of you a single white light. What could it be? What are your actions?’
This is an extremely common question, particularly at OOW level. With the former part of this question they are assessing your general knowledge of lights, with the latter they are testing your reasoning skills. The straightforward answer, and I strongly recommend memorising this list, is:
A vessel under 50m in length at anchor (Rule 30 b)
A sternlight (Rule 22)
A sailing vessel under 7m in length (Rule 25 d i)
A vessel under oars (Rule 25 d ii)
A power driven vessel under 7m in length, capable of doing less than 7 knots (Rule 23 d ii)
A power driven vessel under 50m in length under way, at such a distance that her sidelights are not visible (Rule 22)
A vessel less than 12m in length aground (Rule 30 f)
If you’ve gone through that list and the examiner asks you what else it could be, bear in mind that if you’re close to the shore this could be something like the porch light of a house. On a clear night, it could be a low star or planet. These answers, while not ‘textbook’, show that you’re capable of thinking outside of the box.
As to your actions, naturally you would try to ascertain which of these you are seeing. Depending on the situation, you may want to reduce speed to give yourself adequate time to make a full appraisal of the situation (Rule 8 e). In making your decision, you would think about:
Distance. If your RADAR tells you that the light is 0.8 miles away, you can safely rule out #6.
Is the light under way? To check if the white light is a vessel aground or at anchor, put a RADAR in ground stabilised mode. If the target doesn’t have a trail, it is stopped over the ground and likely aground or at anchor. This could let you rule out #1 and #7.
Consult the chart. If the depth of water is 100m the light is unlikely to represent a 12m long vessel aground. If you’re in the middle of the Pacific (which you should know without consulting the chart!) it’s unlikely to be a vessel under oars (but not impossible).
Additionally, if pushed by the examiner:
If the target has an AIS signal, interrogate this using your AIS unit or your ECDIS. This will give you its self-reported status. This is not to be relied upon for collision avoidance; to do so would be in breach of Rule 7 c (“assumptions made on the basis of scanty information”). It may however be used to improve situational awareness, provided its limitations are understood*.
Be wary of mentioning VHF radio in COLREGs questions, but understand that it is a tool we have available to use (as per Rule 5, ‘all available means’). You’re not using it for collision avoidance (and you never should) if the light is five miles away and your TCPA with the target is an hour and a half. In that case, calling up to ask what the status of their vessel is is one option open to you. I’d avoid mentioning this option unless pushed by the examiner, and even then mention it only with the above caveats.*
*Have a read of MGN 324, regarding use of AIS and VHF as elements of safe watchkeeping. The takeaway from this is that AIS information is scanty information, VHF assisted collisions are still common and that COLREG decisions should be made based on systematic visual and/or RADAR observations.
It’s a good idea to finish your answer with:
‘In any case, I would navigate with caution and, if in doubt, I would keep well clear.’