More Fantasy Naval Warfare, Armor

The first serious improvement over wooden sailing warships, was the addition of steam engines and iron armor.  In fact, the vessels were called ironclads.  The first combat test, Hampton Roads in the US Civil War, had ironclad Merrimac engage and sink two traditional broadside wooden Federal warships.  Merrimac's armor made her impervious to Union guns, the cannon balls just bounced off her.  Yankee sailors on the two doomed wooden vessels stood to their guns and kept firing broadsides at Merrimac until their ships sank beneath their feet. 
   Following this success, all future warships carried as much armor as they could float.  As a rule of thumb, it takes as much armor thickness as the gun has bore to keep the shot out.  For instance a 4 inch gun can pierce 4 inches of armor.  US Civil War monitors carried 15 inch guns, which made everyone try for 15 inch armor, right up thru WWII.  Although both guns and armor improved a lot since the 1860's the ratio of armor thickness to gun bore stayed about the same.   Unfortunately, it was impossible to put 15 inches of armor all over a ship, the weight was just too great, the ship could not float that much armor.  So the armor was concentrated over the vitals, engines, guns, and ,magazines, and the rest of the ship was left to absorb hits as best it could.  Aircraft made the problem worse, against ships guns, all you needed was an armor belt along the sides, the decks remained un armored.  To provide deck armor thick enough to keep out aircraft bombs was just never doable.  Which is why battleships were mostly retired after WWII, they were just too vulnerable to bombing. 
   The one exception to the armored ship was the brainchild of Admiral Jackie Fisher, RN.  Fisher wanted a scout vessel, fast enough to locate the German battle line and strong enough to survive the contact.  His solution was a big ship (big ships are faster than small ships) with a battleship class battery of guns, but no armor to save weight and keep the speed up.  They called them battlecruisers.  Trouble is, the captains of battlecruisers had a vessel that looked like a battleship, was as big as a battleship, and the skippers got battleship ideas.  When the four British battlecruisers found the German High Seas fleet, instead of turning around and running, and radioing the enemy position back to Grand Fleet, they formed a battle line and opened fire.  The Germans fired back and sank three out of four battlecruisers in just a few minutes of action.
That diminished interest the the battlecruiser permanently.  The last battlecruiser, HMS Hood, launched after Jutland, lasted until she engaged Bismarck in WWII. A single hit from Bismarck and Hood blew up.