(CollinsCobuild)You can use not in front of a word referring to a distance, length of time, or other amount to say that the actual distance, time, or amount is less than the one mentioned.
The tug crossed our stern not fifty yards away.
As far as I understand it approximately means: The tug (a boat) sailed past the stern (of our boat) less than fifty yards away.
50 yards = 45m
I don't understand why do they use the verb 'cross' (something) if the distance between objects (boats) is 40-45 m ?
Or do I not understand the sentence?(That's most likely)