OT: Help with greek language and grammar!

[Tellos Athenaios]

Well, often ships (especially military vessels) are denoted by their 'class', e.g. "pentekonteros". Would you believe "pentekonteros" is feminine?

Spoiler Alert, click show to read: 

You see, it is. As it 'triere', 'pentere' etc. etc.

EDIT: And incidentally I never quite as often saw the word 'to ploion'. Come to mention it; I only saw it when the author had to conform to a metrum, or in the very, very basic introduction texts; those which aren't authentic sources but constructs to teach the language without running into all the quirks, inconsistencies etc. etc. immediately. 'He naus' is typical of such quirks; indeed it's so irregular (there existing quite a few duplicate forms for various declensions) it attests its frequent use.