Upgradable ships

I've had this idea for a while, and my laptop is littered with failed attempts. What I wanted to make was a ship that would fit into an RP type game, where the player could buy upgrades as they progressed. What was most difficult was making there some sort of visible progression from one to the next, and making each step up more powerful or more useful. So, with that in mind, here they are. I've included a person and a 50m line for scale.

Frigate

Heavy Frigate

Cruiser

Carrier

Battleship

Dreadnought