Why not “north-north-west” and “was not visible from the docks of Vitylo”? As for the rest, which can be improved, why not “An expanse of six to seven miles of sea parted it from the land.

You can translate the compass direction from French to English, and some language permit that word by word (for cardinal direction).

You can refered to that beautiful Wikipedia article or watch this beautiful picture below :

so nord-nord-ouest would be north-north-west

Your sentence could be translated as :

The ship, which raised closer against the pretty cool breeze from north-north-west, which could not be seen from the dock of Vitylo. This was separated of a distance of six to seven miles away.

1. This isn’t really a question for French SE since the answer mainly concerns how to represent French in another language.

2. None of the translations proposed so far are great English. Here’s what I would suggest:

The ship, which was hauling as close as possible into a brisk north-north-west breeze, could not be seen from the docks of Vitylo. A distance of six or seven miles separated them still.

Analyzing the possible sources of confusion:

Le bâtiment, qui s’élevait au plus près contre une assez fraîche brise nord-nord-ouest, ne pouvait être visible des quais de Vitylo. Une distance de six à sept milles l’en séparait encore.

• s’élever : A ship can be said to “rise”, but more normal in nautical terms is “heave“.

• au plus près : I don’t believe this has the comparative sense (“closer”) but the superlative (“closest, as close as possible, as close as ever”).

• Edit after Papa Poule’s and jlliagre’s comments: The two terms seem also to work together to mean hauling (instead of heaving) directly into the wind; cf. close-hauling. Without more context it’s hard to know which one is being described, but this makes more sense to me.

• une assez fraîche brise : The qualifier doesn’t seem necessary in English because it’s hard to find a word that doesn’t de-intensify it (“fairly, rather, pretty”).

• nord-nord-ouest : A north-north-west breeze (or “nor’-nor’-west” if imitating sailor speech) is one that comes from the north-north-west in both English and French, so no need to change the direction in translation. “North-north-west” means two thirds of the way between the western compass point and the northern compass point.

• ne pouvait être visible : Literally “could not be visible”, which sounds like a conclusion rather than an observation, but without context I suspect the French is referring to an observation.

• l’en séparait : Literally “separated it from them (the docks)”, but this short, natural phrase in French (one syllable, l’en) is awkward in English (“it from them”), hence my rewording.