>>640713
the hull may be thin but the structural members inside HAVE to be much thicker.
It's the difference between monocoque and semi-monocoque.
with monocoque structures the body is the frame and has all the rigidity to support its own weight and all expected stresses. this is good for things where you need a thick skin and don't mind a lower production time to fabricate all the thick steel body sections. pretty much all armored fighting vehicles are built like this. because the armor is so thick there's really no point to having a support frame.
semi-monocoque is where there's an internal skeleton to support everything, and the skin is just there to separate the inside from the outside. pretty much every large vessel is made this way. with strong internal members providing form and structure with a thin hull keeping the water out. the advantage to this is it allows for a lighter overall structure for the same displacement, so more internal volume can be devoted to more or heavier stuff.
even back when battleships were the queens of the sea, cruisers and below rarely had armor. this is because cruisers, destroyers, and frigates are not ships of the line; their jobs all require speed and endurance.
frigates are good for patrol and day-to-day connectivity in peacetime. in wartime they almost exlusively act as pickets to warn of incoming enemy fleets or as sub hunters.
Destroyers came about as specialized frigates configured to counter specific threats, i.e. sub destroyers, torpedo destroyers "for anti ship work," anti-aircraft destroyers, etc. over time they just combined all these roles into one and now destroyers just do all of those jobs by default.
Cruisers were developed specifically for the cruising mission. hence the name. typically frigates and later destroyers wuld perform this job as they're the ones that had the speed to do it. the problem was that neither was particularly good at punching up, being mostly configured for threats that weren't exactly on the same horizontal plane. so the cruiser was developed with the speed of a destroyer, better range, and better weaponry to effectively deal with whatever it could catch or couldn't outrun. while this typically necessitates a larger hull, the convention that there is a hard size association with the classes with frigates being smallest and cruisers being biggest, and destroyers being in the middle is inaccurate. the ships are big enough to perform their missions, and typically a destroyer can be just as big as a cruiser, and a frigate can be just as big as a destroyer. what matters is their role in the fleet, and there is a shitload of overlap between the three.
i told you all that to tell you this:
the ship classes we use today have been around for at least a hundred years, in the case of frigates it's been around since the 1650s, and they have never been armored.
hull construction techniques have not actually changed much since WW1, with most of the advancements occurring in armament, hydrodynamics, powerplant, meterial sciences, and superstructure.
while it's not exactly impossible to put anodized aluminum over steel, it's kind of not the thing you want to weld together. being vastly dissimilar metals and all. further it is very much easier to patch a hole in steel than it is to patch a hole in aluminum.
bee-tee-dubs, the skin of a vessel isn't actually that thin. it's all made from plate, which has a minumum thickness of 6mm, or 1/4". it's very easy to use plate that could stop small arms fire, but they sheer mass of the vessel combined with the forces at play mean that any time it contacts something of large mass it's going to fail and make a hole, and it's a lot easier to repair a hole in plate than it is to bang out a dent. it's actually the real reason why the USS Missouri still has that dent from when it got hit by a kamikaze. easier to leave it alone than to cut the whole section out and replace it.
remember, the USS Cole got a hole blown in the side of her, and she was back in service in about a year, which is pretty normal for a large vessel.