Boat portal mob farm

A full perimeter mob farm by MethodZz, tutorialised by KelpMC over 38 minutes. Mobs spawn across a cleared 16x16 chunk perimeter, get flushed into a grid of 441 boats sitting in front of 441 dispensers, and are pushed through nether portals so they die on the other side. Sending them to the nether is the whole trick: it clears the overworld mob cap the instant they cross, so spawning never stops. Around 23,000 blocks in total, and it was originally built on SciCraft as a creeper farm.

• Replace the sea pickles with slabs. Gr1mPeaper says it works identically and KelpMC confirms it, which takes 1,766 sea pickles off the shopping list.
• The baby zombie despawn system is not optional. Without it they fill the mob cap and the farm stops spawning anything, which KelpMC repeats to everyone asking about conversions.
• If you already built KelpMC's perimeter creeper farm this is the same thing. Remove the trapdoors, add the baby zombie despawn system and rework the storage, and HydraBomb3.0 points out that farm's free world download covers it.
• The waterlogged leaves only exist to line the boats up. zephryus2001 says the farm runs fine without them on older versions, you just place each boat from a dispenser by hand.
• Slimes spawning inside eat the mob cap and slow the whole thing down, which firezinnftn1 and Dario-i8p both hit.
• Mobs wearing Feather Falling boots survive the drop. MariusRightHere raises it and nobody answers.