Originally posted by TheDS:
Ship-type designations are not as simple a matter as they should be. There is a long and complicated history of what things like "cruiser" and "frigate" and "destroyer" mean, and the terms change based not on anything logical, like mission, but on political needs. For instance, a treaty to limit the number of cruisers will perhaps result in a class of ships that SHOULD be called cruisers being labels as destroyers instead. The treaty says you can have only so many cruiser, but nothing about destroyers, so destroyers may wind up being more powerful. Indeed, an AEGIS destroyer is almost identicle to an AEGIS cruiser!
In the modern US navy, frigates are taking on roles that were the realm of destroyers, which are taking on cruiser roles, which are taking on battleship roles (that is, they are going away).
The "modern traditional" role (if anything can be called such) for ships is as follows:
Frigates are the smallest ship which can operate independently, and it is generally used as a screen against enemy ships. Frigates are NOT very fast or capable, but they are generalist ships; they can do anything, just not very well.
Destroyers are specialist ships. They are focused for a particular mission, at the cost of being able to do anything else. An anti-sub DD can't defend well against air attacks, but it's excellent against subs. An anti-air sub sucks when enemy ships come close for a gunnery duel, but they are good at shooting down aircraft and missiles. A DD loaded to the hilt with SSMs will annihilate an enemy surface group, but is a sitting duck against subs and air threats.
Crusiers are intended to be local command ships. (CC means "Command Crusier".) They are generalist ships, so that they can do anything, and since they are bigger than destroyers, they can do that multitude of things almost as well as the destroyer does its one thing. (Destroyers are specialized to give them SOME way to outperform a cruiser.) Crusiers head up small SAGs (Surface Action Groups), or can operate independently. A group of cruisers is good at cheaply denying an area to the enemy. A cruiser should be able to outfight any enemy ship, other than its battlewagons.
Battleships are designed for one primary purpose: to destroy enemy battleships. They are not supposed to go after enemy shipping. They are not supposed to help land troops. They are not supposed to patrol. (But they do these missions pretty well, eh?) When the enemy has warships where you don't want them, you send your battleships to clear them out. The enemy responds by sending his own battleships to stop them.
On the water, things can get really blurry. For instance, in WW2, we were worried about the ability of enemy destroyers to rush the guns and sink battleships. A group of DDs as few as 3 ships COULD have survived long enough against the 5-inchers that they could have caused some serious harm to the BBs. DDs were also good at torpedoing enemy ships, and even BBs feared torpedoes.
Submarines are DD-sized, and capable of sneak attacks. A small group is quite capable of destroying a fleet that is not leary of them.
In space (in the Imperium), you have the opportunity to do what Earth has never done: define things.
Define a cruiser as a ship in the range of 10,000-50,000 Dtons, multi-purpose (has a big gun and lots of little ones), 4G (meaning it's faster than most battleships of 2-3G), and whatever else.
Define a destroyer as being 1000-5000 Dtons, specialized for a single mission.
Define a battleship as 100,000-500,000 Dtons, devoting the majority of its freespace as a single weapon (like a giant PAWS or Meson gun), few secondary weapons, heavy armor, and usually no more than 2G.
This doesn't stop you from making variations either, like you could define a battlecruiser as a ship 50,000-100,000 Dtons, 4G, a heavy gun, moderate armor, and several secondary weapons. There will be plenty of sub-classes of DD, too.
This is along the lines of what I did in my game, and it makes it so much easier to see what's what. I take the "patrol cruiser" as simply being a class name rather than a type-name. (I type it as a frigate.)