Learn French: Quelque silencieusement que tourne l'hélice, le calmar s'approche du hublot. - However silently the propeller turns, the squid approaches the porthole.

In this sentence, quelque modifies the adverb silencieusement. When the quelque... que structure frames an adjective or an adverb to mean 'however [adjective/adverb]', quelque acts as an invariable adverb itself. This means it never takes an 's'. It only takes an 's' when it modifies a plural noun.

Is tourne in the subjunctive mood here? It looks just like the present indicative.

Yes, it is the subjunctive! The structure quelque [adverb/adjective] que always triggers the subjunctive mood. Because tourner is a regular -er verb, its present subjunctive happens to look identical to the present indicative for the il/elle/on form.

Why is the word order tourne l'hélice instead of l'hélice tourne?

In concessive clauses like this one, it is highly customary to invert the verb and the noun subject. While que l'hélice tourne is grammatically correct, que tourne l'hélice flows better and is the standard stylistic choice in well-written French.

The English translation says 'approaches the porthole', so why do we need du before hublot?