This device is designed for big farms such as bamboo farms.
It unload minecart hoppers to get items in a water stream for them to be easier to manage.
It's stackable (you can place multiple ones [​1/rail line]).
It has no moving parts.
Plus : 2 items per hopper delay



Note that the iron trapdoor can be replaced by other redstone powered blocks ( powered rails / hopper / doors / ... ) so as to make less noise or just use other ressources.

materials needed :
  1 redstone repeater
  1 redstone torch
  1 redstone comaprator
  1 iron trapdoor
  2 droppers
  2 hoppers
  4 observers
  some powered rails and rails
  1 minecart hopper
  + whatever you want to power the rails