I've been looking over IN ships from BFG and RT and having some odd thoughts about the way they are classified. By RT, a vessel's class is determined by its hull. Regardless of how that hull is outfitted, the resulting ship is still considered to be of whatever class that hull is named. This strikes me as somewhat off, particularly after reading the sidebar in Battlefleet Koronus on representing various IN cruiser types.

It seems odd to me that an Imperial Cruiser with an Armoured Prow, Torpedoes, Macrocannon Broadsides, and Lance Batteries is NOT considered a Lunar-class just because the Hull component selected is labeled "Tyrant-class" (because someone needs the extra 2 space for the cost of 1 SP). Likewise, if a Lunar-class hull is constructed (or refitted, since evidence in WH40K suggests ships can change class with refits) with Jovian-pattern Landing Bays to replace its Lance Batteries, doesn't it effectively become a Dictator-class despite having slightly different stats than if it was built with a 'true' Dictator-class hull?

I'd like to know what others think. Should a vessel's class be determined by how the vessel is outfitted or by the hull it's built upon?

Without stopping to fact-check my incredibly hazy sense of military nomenclature, I'm pretty sure that it's based on the hull or main chassis in the real world, too. A Churchhill tank with a flamethrower is still a Churchhill.

The Boston class cruiser was basically just a Baltimore class cruiser with it's aft gun turret replaced with a missile system and the necessary electronics for that system. This is pretty much the type of thing I'm talking about. By RT rules, it would still be considered a Balimore.

In BFG a number of the cruisers are basically the same Lunar class frame with different weapons, (Dictator and Gothic?) so you are right that sometimes it is the weapon armament that is important. Equally a Lunar class cruiser with macro batteries and dorsal torpedoes and no armoured prow is closer to being a Murder Class as anyhting else.

However the actual class of the ship (Lunar/Tyrant) doesn't have too much effect rules wise it is simply a way of differentiating between hull types (Ship Points, Hull Integrity etc). In background terms if your equipping of the ship makes it more suited to being classified as a different type of cruiser then I shouldn't imagine there would be any problem in simply saying it is that class.

A couple of points to add to this:

Firstly, because the players are assumed to be Rogue Traders (or associates), it may be the case that it is assumed that the hulls have had their weapons stripped by the navy before the hull was sold on. Basically, Rogue Traders be doin' their own thang.

Secondly, unlike in the real world, where classes of ships/tanks may be melted down, decommissioned or scrapped, ships in 40k are precious and ancient, and will have been refitted countless times to replace damaged weaponry, destroyed components, and whatever parts cannot be produced anymore.

Also, while I like and agree with the above Churchill tank analogy, 40k breaks the rules with the countless weapon configurations applied to the Baneblade hull (Shadowsword, Banehammer, Stormsword, etc).