In the British Navy, ships were rated based on the number of guns and personnel. This is something authors can mention but screenwriters might find hard to work into a conversation without being obvious about it. In either case, if we have a character say, “That’s a first-rate ship,” people will assume that means “it’s a quality ship,” not that it means the ship has over a hundred guns and more than eight hundred crew and is therefore in the biggest class of ships. The same applies to “second-rate” and so on, with many audiences assuming the ship is inferior when this isn’t true. The ratings are not a judgement of quality, though in the “bigger is better” mentality, it is. Beware of this when writing about it.
Whether we emulate this or not, here is a basic rundown, according to Men-of-War, Life in Nelson’s Navy by Patrick O’Brian:
“First rate 100-112 guns, 841 men (including officers, seamen, boys, and servants)
Second rate 90-98 guns, 743 men
Third rate 64, 74 and 80 guns, 494, about 620, and 724 men
Fourth rate 50 guns, 345 men
Fifth rate 32, 36, 38, and 44 guns, 217-297 men
Sixth rate 20, 24, and 28 guns, 128, 158, and 198 men.”
Anything with fewer guns wasn’t rated and all the above had three masts. Not including the forecastle and quarterdeck, the first and second-rate ships had three full decks for just the guns. Third and fourth rate had two gun decks and the rest had one gun deck. With multiple gun decks, the guns on the lower decks were the biggest and this was where crew slept and ate. High decks had increasingly smaller guns and cannonballs for the obvious reason of not making the ship top heavy. Weaponry is discussed later in this chapter.
This large image of a third-class man-o-war gives a great view of the rigging and even interior of a vessel: http://www.artofworldbuilding.com/warship.