Playing as the Yor, I'm involved in a few wars. Invading a Korath planet, I stole Hardened Hulls (I have tech trading off). This increased the hit points of my attack ships from 23 to 26. I then attacked a Krynn planet. They have absolutely insane defenders, with attack around 120 and defense at 30. I destroyed the defender, but it did 23 points of damage on its shot. The next turn, my badly damaged ship was gone. I suppose it had something to do with the bonus HP not being applied consistently.