The Crimean War, or immediately, might be a good time for this. Armor was sufficiently thick on most ships that was able to prevent contemporary naval or coastal guns from having much effect,
iirc. However the early ironclads were essentially floating batteries, and practically worthless in making maneuvers. A heavy forward armor scheme with a strong ram might be good defense against such a type of battleship.
For reference, this is the
Lave, one of France's first ironclads, which was absolutely devastating against the Russians during the Battle of Kinburn.
Even well after the American Civil War several ironclads still followed the floating battery scheme; below is the USS
Cairo & CSS
Palmetto State;
Hitting such a ship at or below the water line with a piercing iron or steel ram would all but guarantee to sink the ship in question.