You won't be able to rip it apart, but it is certainly not "unsinkable". Having loaded NSM4, I managed to sink a Yamato with 5 Torpedo hits.
Not knowing what/if any mods you have loaded, I'd try to get two hits on any gun turrets and the other two hits in the aft section to damage engines and/or propellers.
It won't sink it right away, but should slow it down and the gun explosions look heart warming.
