Like most of you I have my own convention. I use a 2 letter prefix with a number based on the number of slots , but I don't take in account the material. I think it would be to hard to keep track.
PT Patrol Boat 1
GB Gunboat 2
SL Sloop of War 3
CT Corvette 4
FP Patrol Frigate 5
FF Frigate 6
FA Heavy Frigate 7
DE Destroyer Escort 8
DD Destroyer 9
CL Light Cruiser 10
CC Cruiser 11
CA Heavy Cruiser 12
CB Battlecruiser 13
BB Battleship 14
DB Dreadnought 15
For non-combat I use CT with the number.
I also put CV in from of the 2 letter if it has a hanger. For Example CVBB-14 {Ship Name here} represents a 14 slot battleship that carries fighters. CV can also be put with the CT tag. When cloaking and stealth are introduced I will add a "-S" to the end.
I think we all agree that some type of classification needs to used.