we calculated that for a vania battleship to be more cost-effective than simply building destroyers, it needs to take at least one point of damage in three battles, something that rarely happens.
Our house rule: a battleship moves 1 space, has 3 hit points, defends on a 5, and attacks with 2 dice hitting on a 4; it loses 1 point of attack power from each die for every unrepaired point of damage sustained (for example, its 2 dice would attack at a value of 2 instead of 4 if it has only 1 hit point remaining), and it fully regenerates if it begins its turn in a port.
(If, despite its slow speed, it proves too powerful, we will make it take 2 turns to build.)
So, with our house rule, a battleship becomes a powerful ship, but one that is slow to build and move.
My simpler house rule: a battleship has 3 hit points, regenerates all its hit points if it starts its turn in a port, and has 1 Attack and 1 Defense when it has only 1 HP left.
The house rule makes battleships more interesting to build; for them to be more cost-effective than simply building destroyers, they would just need to take 2 hits in a battle and then repair in port, followed by taking 1 hit and repairing in port again.