Purple wrote:
Very easy. Although it'd be much easier to just board them and scuttle them.
No. Not going to work that way. All the bombs used for JDAM guidance, and indeed all the bombs the US military buys short of MOP all have casings that would probably break up trying to penetrate a steel deck that thick, that 4,500lb penetrator might have a shot but it certainly isn't designed for the job.
This would also not even remotely matter when the ship was hit ten or twenty times with 2,000lb GP bombs with non penetrating fuses and simply had the crap blown out of it. It'd breakup from the gross structural damage and flooding, forget about fighting the fires. Yamato is poorly protected against this kind of attack even as the final generation battleships go too, as she had only one armor deck and minimal deck armor on the ends. A single hit from the 30,000lb MOP would also work, said weapon might have trouble penetrating the deck too but it'd be more then powerful enough to rupture it by direct explosive blast. It might not sink from one, but it'd be utterly wrecked as a fighting unit.
Also modern precision weapons could do things battleships where predicated against not being possible, such as dropping the bomb in the water right alongside the hull on a long delay fuse, causing it to explode below keel level with results similar to a torpedo. People tried such attacks in WW2, but the very low accuracy made it undesirable (long delay fuse on GP bomb meant if you then got direct hits they'd breakup) and so it wasn't a big deal for the warships. Today we can be certain this will work.
Modern warheads designed to penetrate thick concrete will generally breakup against more then four or five inches of steel, because they have fairly high burster weights and long bodies to hold said burster. The nose is too blunt and uncapped, and the long body will flex and crack. But what do you want, they designed them for concrete which just doesn't get all that hard, but can be very deep,.
GP bombs aren't likely to penetrate armor decks thicker then 2in, but they can blow holes in thicker metal depending on how big they are and the actual angle of impact.
If anyone was really concerned about a ship like Yamato they'd just quickly pour some 1,000-2,000lb sized shaped charges and drop those or fit them to existing missiles, be much faster then making new armor penetrating bombs and nearly as effective. Battleships passive defenses aren't a joke, but it just doesn't mean so much once you get past five or six heavy bomb hits. In WW2 a 100 plane heavy bomber raid might not score such hits, today a flight of four fighters would expect to do better.
"This cult of special forces is as sensible as to form a Royal Corps of Tree Climbers and say that no soldier who does not wear its green hat with a bunch of oak leaves stuck in it should be expected to climb a tree"
— Field Marshal William Slim 1956