Minecraft wiki entry for Drowned_farming, associated text: The shaft doesn't have to be tall, but if you want to kill the drowned with one hit from your sword, ideally the drowned should fall 21 blocks before landing on the hopper in your killing platform. Make it too tall, and the drowned die from the fall, and you don't get any experience or tridents or other drops. If you make the shaft shorter, you just have to hit the drowned more times to kill them — up to four hits with a stone sword, which is also required from the flooded dungeon farm described above. One hit with a stone sword is sufficient to kill a mob that fell 21 blocks.