I'm getting this done to me all the time, and can't figure out how they do it. I, and they, have maxxed out Artillery (6,464 damage) & Barrage (14,400 damage), and with 3 Engine Rooms, each with 20,000 hp. So it takes 3 x Artillery & 3 x Barrage to take out the 3 Engine Rooms. But - that takes 63 gunboat energy to do that (10+16+22 barrage and 3+5+7 artillery), and a maxxed-out gunboat has 52 energy. Now, taking out 2 engine rooms will give (I think) 6 energy (3 each) (or maybe it's more? 5 each), leaving me still 5 energy short, meaning I have take out at least a couple other fortifications to have the available energy to take out all 3 Engine Rooms.
But with my attackers, they attack with ONLY Artillery & Barrage, destroying no other buildings and not using any infantry. How are they doing that? None of the other attack modifiers in Boom Beach apply to Warships, do they?