The following is from the COLREGS EXPLAINED website.

Rule 5 Look out.

Every vessel shall at all times maintain a proper look-out by sight
and hearing as well as by all available means appropriate in the
prevailing circumstances and conditions so as to make a full
appraisal of the situation and of the risk of collision.

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This rule has as consequence that all solo sailing for long trips is
against the COLREGS as there is no look-out when the skipper is
sleeping. Apparently the risk is tolerated, but it remains that most
authorities insist on a proper look-out, and check it was effectively
carried out after an incident. In spite of this the look-out duties
are more and more neglected on many ships. On large commercial
vessels the officer of the watch (OOW) is often busy with
administrative duties, and no extra look-out is posted by day
light. On other ships, such as the fishing boats, other works are
preventing any decent look-out.

An hearing look-out is seldom carried out on large ships where
unfortunately the bridge doors are kept closed with the eventual
look-out remaining inside.

'All available means' mainly designates a look-out, to which also
a poor attention is often paid on large vessels, some OOW trusting
excessively the 'guard rings' features of the radar. Keeping this
is mind is quite important for small crafts which can wrongly assume
that they are easily detected by the navigating on larger vessels.
The largest commercial vessels can have a total crew complement of
only 18 persons or less, on coasters they can be as few as 4 on board,
each of them with a lot of duties, which include a paperwork increasing
every year but also the GMDSS Radio communication equipment.

Modern wheelhouses are now fitted with one or several computers which
are inviting the OOW to keep himself busy with some typing work when
he assumes that an occasional look outside and on the radar PPI will be
enough to detect the eventual other crafts. This, added to the other
detection problems, can make that the OOW is totally unaware of the
presence of some small yachts on a collision course with his vessel.

This summary is written by
By Captain P. WOININ
Convention on the International Regulations for Preventing Collisions
at Sea, 1972 : Summary and comments mainly oriented to recreational sailing.