I stumbled on a Yamato parked by the Truk harbor, along with some other BB. I was in a S-boat with a full load of Mk10's.
I figured both of the ships would take off soon as I attacked, but they didn't, they just sat there while I reloaded and fired again, and again.
It took the full load of mk10s to down the Yamato. When I was leaving there were 5 destroyers headed to the scene of the crime.
I visited Truk harbor again later and sunk another BB, and then saw another Yamato sitting there, so I hurried back to base to rearm and then back to Truk. It was still sitting there, but after I sank that one, it didn't count it for me
The one time it doesn't count a ship, it's a Yamato!
Running RFB + RSRD