There’s been some confusion as to how stacking bonuses works in Battle Pirates, so let’s clear that up.
First, there are two types of bonus systems: additive and multiplicative. Most (though not all!) bonuses in Battle Pirates are multiplicative. Here's how a multiplicative system works:
Imagine that I have a Battle Barge, which has 2 slots for special items.
Let’s say my Battle Barge fights an enemy with an explosive attack that deals 100 damage per hit. With no resistances, my ship would take 100 damage for every successful attack. Not awesome.
To make my ship tougher I put Ablative Armor I on my Battle Barge, which provides 33% Explosive Defense. Now every successful attack from this explosive weapon only deals 67 damage. This is because 33% of the 100 damage is mitigated by the Ablative Armor I. Awesome.
I decide to put Zynthonite Armor DI-X on my Battle Barge for another 10% Explosive Defense, in order to make it even more resistant to explosive weapons. I expect that each attack from this explosive weapon is now going to only deal a measly 57 damage, but instead each successful attack is dealing 60.3 damage!
Remember what I said about bonuses using a multiplicative system? The bonuses aren’t applied all at once. Instead, imagine them being applied one at a time.
First the attack deals 100 damage…
BUT WAIT! There’s an Ablative Armor I here. Remove 33% of that damage.
Ok, then the attack deals 67 damage….
BUT WAIT! There’s a Zynthonite Armor DI-X here. Remove 10% of THAT damage.
Ok, then the attack deals 60.3 damage.
Fine, 60.3 damage it is.
Hope that helps - you'll have to experiment to find out which systems are additive and which are multiplicative.