Re: using spaceships for battletech-style tactical mecha combat
 

WWI battleship armour was generally face-hardened, aside from deck protection, which might or might not have been, depending on thickness, navy, and date. I'd be comfortable with calling that armour RHA for simplicity's sake.

However, post 1920 (or thereabouts) face-hardened heavy plate was definitely better than RHA (especially in the case of British and German armour). By how much is open to debate, but even if you assume that US face-hardened plate was RHA equivalent (making Japanese armour a little worse), that makes British and German plate somewhere between 5% and 25% (depending on source) better, and most likely 10-15% better (the thicker the plate, the greater the advantage). So even if you count US hardened plate as RHA (DR70/inch), which would make US non-face hardened plate worse than RHA in multi-inch thick layers (which seems odd seeing as it's pretty much the definition of 'RHA'), the best WWII hardened plate would be DR80/inch or thereabouts).

Re: using spaceships for battletech-style tactical mecha combat
 

General purpose bombs of WWII were about 50% filler, with later high-capacity versions making 55-60%. Modern GP bombs, with more emphasis on streamlining run at 40-45% filler (the smaller the bomb, the lower the proportion that's filler).

The WWII 'cookie' or 'blockbuster' bombs used by the RAF in their strategic bombing campaigns were pure blast weapons, made with shells as light as possible. This and their large size and weight (4-12,000 pounds) meant they managed 75% filler. They were (and are) very much the exception.

Re: using spaceships for battletech-style tactical mecha combat
 

That hit is also gnerally assumed to have been to a large magazine of shells. That would make Hood's designers (or Admiralty backers) less than competent which is a thing I am certainly willing to assume.

However, it makes discussing the physics, material science and engineering much more difficult. Perhaps Hood's shortcomings could have been easily fixed at relatively little cost or perhaps enough armor for the coverage she needed would have seriously changed her performance.

Re: using spaceships for battletech-style tactical mecha combat
 

There another important difference - HE shells have a linked explosion, while SAP (and APEX, etc.) has a follow-up explosion.

Re: using spaceships for battletech-style tactical mecha combat
 

That's mostly because GURPS rules for explosives are all kinds of messed up, starting with the square root rule being wrong (if you actually look up things like explosives safety, they scale pretty much everything by the 1/3 power of the quantity, not the 1/2).

Re: using spaceships for battletech-style tactical mecha combat
 

To the secondary magazine, which is thought to have gone up, breaking into and setting off the 'X' turret's magazine.

It couldn't have been, or it would have been - the Admiralty was never really happy with Hood's deck armour, and if there'd been time and money she'd have had a major deck armour re-work during the major refit she should've gotten between the wars.

For what it's worth, the hit that did the damage might well have only succeeded because the shell's fuse took longer to set it off than it was designed to do. Alternatively, it had an extremely lucky trajectory (and one that doesn't match the eye witness reports). Also, Bismarck was just about as vulnerable to such hits at the ranges the battle was fought - it could've been a simple coin toss that Hood lost.

To make matters more interesting, it turns out that a lot of treaty and post treaty battleships had deck armour that was less protective than their designers thought (the exception being the RN battleships) because insufficient allowance was made for hits like the one that probably sank Hood - attacks to the deck were assumed to be fairly vertical, so there wasn't a lot of 'overhang' fore and aft of the magazines, so hits in the range where the magazine armour was supposedly 'immune' and the machinery armour (thinner to save weight) wasn't could actually reach the magazines.

Battleship armouring was an exercise in trying to supply adequate protection on inadequate size, weight, and monetary budgets, and it was never as good as the designers would've liked.

Re: using spaceships for battletech-style tactical mecha combat
 

Which is one of the reasons for the abysmal performance of battleships throughout the 20th century. They are probably one of the worst military investments in history (though aircraft carriers post-WWII come close). If a nation wanted to build up to a contemporary naval superpower, the best naval investments are probably drone carriers, guided missile destroyers, anti-aircraft frigates, submarine tenders, and diesel-electric submarines. A properly designed drone carrier could pack as much punch as a supercarrier while being the size of a cruiser and a submarine tender with a wetdock at the bottom could refuel and resupply diesel-electric submarines without anyone being the wiser.

Re: using spaceships for battletech-style tactical mecha combat
 

Until about 1942, give or take, if someone else showed up with a battleship and you didn't have one in your fleet, you had to run away. So maybe they were a bad investment, but a fleet that didn't have them was a worse one if you wanted to actually be able to contest anything.

Re: using spaceships for battletech-style tactical mecha combat
 

I don't know if you thought Graf Spee was a "real" battleship but the Captain of the Spee obviously didn't.

For that matter, Bismarck was running too. If the purpose of a battleship is to engage and destroy enemy combatants some people seem to have thought the day of the battleship was already over. Commerce Raiders of Unusual Size don't seem to have been such a hot idea either.

Re: using spaceships for battletech-style tactical mecha combat
 

Graf Spee was a largish (for her build time) a slow cruiser with over-sized guns. She messed up Exeter, a small 8" cruiser, but did about as well as you'd expect overall against three small cruisers (i.e. badly).

It's that last part that made Bismarck's actual use questionable. That and the German's unwillingness to actually risk them meant that sometimes it didn't even take having an old battleship trailing a convoy to scare the German battleships off. However, that made the old Revenge-class battleships the RN used for escorting convoys an amazingly good investment.