It is the skipper's responsibility to ensure a proper look-out is maintained at all times.

This may require positioning crew where a good all round view can be found. On a sailing boat the areas of particular concern are:

Behind the foresail, especially if it is a large genoa.

Astern, especially at night.

In bad weather, in to the wind, as most people will not keep an adequate look-out in to heavy spray or rain.

There should also be enough crew to maintain a proper look-out at all times. An impossible feat for the long distance single hander!

In reduced visibility, the best way to improve the look-out and listening watch may be to place a crew member in the bow of the vessel. This has several benefits:

They are away from the engine noise.

They tend to me more focused on the job in hand than people in the cockpit.

They will probably detect another vessel sooner than if positioned at the aft end of the vessel.

If you do position a crew member in the bows, change them round at regular intervals, after 30 minutes they will probably start to imagine they can see things!

To keep a lookout by all available means appropriate in the circumstances, implies that if you have operational radar, you should be using it, and have sufficient experience or training to do so effectively.