I have no idea how helpful this may be for FTD, but this is how it was done historically, at least on late-WWII US battleships.
A torpedo bulges are technically a form of spaced armor. A torpedo bulge is having the hull far away from the armor belt (sometimes separated by like 5 meters of empty space or more) so that if the ship is torpedoed, the blast demolishes this relatively thin hull plate without taking out a chunk of the main armor belt. This commonly results in a bulge shape on the outsides of battleships or some other warships that needed to possibly resist torpedo strikes.
IRL, there is a thin hull plate on the outside, and a sloped armor belt placed inboard of that hull plate and sloping away from it. Look up the Iowa class's armor scheme as a diagram for a nice visual example. Generally you want a thin plate on the outside to detonate anything with a fuse before it reaches the armor belt, and a nice thick belt covering your ship's vitals underneath that. Similarly, in the US at least, deck armor was usually a rather thin plate serving as the physical deck...or sometimes no armor-grade steel at all...with a "splinter deck" underneath, which is composed of actual armor-grade steel and intended to catch fragments from exploding bombs or shells that had struck the top deck.