Okay, so recently I've been working, Battlefleet Gothic rulebook in one hand, Rogue Trader in the other, to try and turn the warships of BFG into hulls and component configurations for Rogue Trader. I know others have done this, but I wanted to take my own crack at it and see how it works out. I was interested in the idea that really, a Lunar class hull should be able to represent all the different Imperial cruisers, since their only real difference (save for an extra turret on the Dictator) is the weapons loadout.
Previously, a lot of this was working out really well. For example, if you give the Lunar class two Mars pattern macrocannon broadsides, one on Port and one on Starboard, and then a Sunsear laser battery on each side, it works out exactly like a Tyrant class. In BFG, the Tyrant has batteries of Strength 6 at 30cm, compare this to Strength 6 at Range 6 in BFG, and Strength 4 at 45cm, compared to Strength 4 at Range 9 in Rogue Trader. Perfect!
It works for the Dominator class too. Two Mars pattern macrocannon broadsides, giving Strength 12 at Range 6 (or 30cm in BFG.) This would also work for the Gothic class, except the Gothic class will probably need either a special plasma drive or a special lance component, since the power requirements for the equivalent strength of lances is too high.
BUT, here's the problem. It occured to me that while this works out well for cruisers, when you compare it to escorts it all breaks down. The Sword-class frigate, for example, has a Strength 4 dorsal weapons battery in BFG. In Rogue Trader, the Sword class has two dorsal mounts, and is described, as it is in BFG, to have laser based batteries. But if you give the Sword two Sunsear laser batteries, as you would only naturally do, it ends up giving the Rogue Trader Sword an effective Strength of 8, at a BFG equivalent of 45cm instead of 30cm.
****, I was so close!
I realize this is really kind of pedantic, but I'd like BFG and Rogue Trader to have at least some symmetry, especially since the cruisers had been worked out so well. But think about it, since a Sword-class frigate's dorsal mounts can both fire port OR starboard, that means that a Sword class frigate's 2 mounts could theoretically do as much damage as the broadside of a Lunar class hull if that Lunar class were armed with two Sunsear laser batteries on one side. THAT can't be right.