Armor Guide
Deck:

Deck armour is for long-ranged shots (White damage numbers). The numbers (AP, HE) mean defense against those types of shells. Any added armour with no added number (preferably HE, as AP doesn't do much) is wasted money
Belt:

Deck armour is for short-ranged shots (Yellow damage numbers)
See above for details.
Bulge:

Bulge armour is for torpedoes. you may see two numbers when buying this kind of armour.The two numbers are as follows: Torpedo Defense/Bulge Health. The torpedo defense is the number that is minused from the damage you take from a torpedo (not 100% sure on this) and the bulge health is what it says. Apparently, the lower the bulge health, the less torpedo defense you will have, but if it runs out, naturally you won't have any defense. Also, I think bulge health is the bar thats under your DP bar in battle.
Bulkhead:

Bulkhead armour is all around armour, so it is like "General" armour, and it helps protect everything, although naturally you'll need more if you want it to actually do anything. Also, it protects your sailors from dying, and will protect your engine (for instance, if you are damaged badly, you slow down right? With bulkhead, you'll need to be damaged more to slow down).
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