A minor point, but if we’re talking about the case of the Sjøbris then the '48 COLREGS would be the ones in play, and those lack some of the definitions provided in the '72 rules.
Some commentary on them includes this claim:
“The present Rules (1889) contain an Article (4) which deals with “A vessel which from any accident is not under command. . . .” The 1948 Rule is the same as the 1889 Article except the words, “from any accident,” which are deleted. It is clear that the scope of the 1948 Rule has been increased. A sailing vessel, in irons or in a calm, a steam vessel, whose steam pressure has dropped to the vanishing point, are now required to comply with this Rule (4) and show two red lights by night and two black balls or shapes, by day.”
In a WWI case, the cruiser HMS Drake lost her steering as the result of a torpedo hit, and had to make do with using the engines as in the example Jughead related. She was displaying NUC signals and collided with a merchant ship; the court ruled this was sufficient case for NUC. (The Elder Brethren advising the court were split on the matter.)
“If a vessel is in such a condition owing to an accident that she can only get out of the way of another after great and unusual delay, I think she must be considered as “not under command.” [Viscount Finlay]
A glance at Cockcroft reveals that the question of weather was considered at the '72 conference, with the opinion being that it could result in a “not under command” situation, but only in the most severe cases such as a vessel becalmed. Farwell’s also suggests similar. However, I’m reminded of a bit from a paper on the capabilities of sailing warships:
Even without the help of currents and tides, canvas or anchors, a ship could still move. Francis Liardet, a captain with thirty-three years experience in the Royal Navy and the author of the highly detailed Professional Recollections on Points of Seamanship ( 1849), was quite certain that the action of the swell and the “formation of a ships bottom” caused a ship to forge ahead even in a total calm - a fact so common, he claims, that “every seaman knows.” He goes on to recommend that ships dismasted in fleet action should keep their heads towards each other, thus preventing separation. White, in his Naval Researches (1830) agreed and goes as far as to claim that the only way of actually stopping a ship was to place another directly in its course.
My own take is that being becalmed without a working engine does count as NUC, even if smaller boats aren’t required to carry the appropriate lights or shapes to signal such. That a becalming may be more common than other events doesn’t alter the fact that one’s not able to move as might be required, and that the situation isn’t resolvable by choice. The case of applying the steering and sailing rules to a vessel that’s “stopped” is particularly problematic, but one should not assume it means one can’t be the give-way vessel, or otherwise required to keep clear.