AI combat priorities

Priority is attack/(defense + hp). Whether a ship cannot (meaning, it is very improbable) be hurt by the attacker is not evaluated.

So in that case, the indestructable ship would take all the hits and be fine?

It isn't really an exploitable situation anyways, since the small defended ships with high attack have to use the special weapons and hence cost a few hundred bc, but for optimization sake, defender's with defense too high to damage should be dropped to the bottom of the attack priority.

The system picks the ship with the highest attack to defense ratio.  That is, the ship that does the most damage that is most vulnerable.

But an invincible ship shouldn't be attacked. It serves no purpose to attack one if it would be possible to damage any other ship there.

Ship B would be attacked first probably. I've noticed that even though a ship has all the right defences, it still slowly loses hit points. Faster if the attacker has stronger attack ratings.

For example: I once made a mistake of loading up a "cargo" hull with all kinds of weaponry and defence measures. Statistically it was invicible relative to what others had at the time. BUT, the ship DIED 1 second after combat started. EVERY time. I only later noticed it had only 1 HP. So no matter what kind of defences I put on, a lousy colonial fighter (with a Laser 1 - maybe 2) destroyed it in a second...

Go figure.

But there's no such thing as an invincible ship. Even a ship with 1000 defense can be damaged by a ship with an attack of 5.

But an invincible ship shouldn't be attacked. It serves no purpose to attack one if it would be possible to damage any other ship there.

I don't understand your reasonning since Frogboy has stated:

The system picks the ship with the highest attack to defense ratio. That is, the ship that does the most damage that is most vulnerable.

Unless I am mistaken, invicible ship aren't ship with the highest attack to defense ratio, so they wouldn't be targetted.

As Brillig mentioned, there are no invincible ships. Defense gets a roll, just like weapons do. In the OP's example, ship B could be damaged by ship A. Ship A would receive an attack roll of 0-3. Ship B would receive a defense roll of 0-3 (1 for beam defense + sq. root of 1 (also 1) of missile defense + sq. root of mass driver defense = 3). So, if Ship A rolled a 3 and ship B rolled a 0, ship B would take 3 damage. If B rolled a 2, it would take 1 damage. Only if B rolled a 3 (25% chance here assuming A rolled a 3) would B not take any damage.

I was under the impression that defense and attack rolled from 1 to defense rating. That would make the ship with one of each defense have 3 defense every time. I must have been mistaken.

Umlaut - the manual's description of combat rolls is wrong. Nobody (outside Stardock) knows for sure, but the formula is assumed by Kryo to be 1-max for attack, 0-max for defense. I assume it to be 0-max for each, since that's more symmetric.

I think Kryo must be right, attacks are 1 to max. I've been fighting a lot of AI's lately in the early-early game with just 1 beam attack and me with no defence. I haven't seen them roll a zero in that situation yet. It means my 2-attack ships don't have as much of an advantage as I thought, as I average 1.5 damage to their 1, instead of 1 to their 0.5.