Probably not a lot. Blowing up a ship in the middle of the Panama or Suez canal might do it, especially if you wreck it badly enough to block the canal for mont

Probably not a lot. Blowing up a ship in the middle of the Panama or Suez canal might do it, especially if you wreck it badly enough to block the canal for months. Even easier if you target a big oil tanker. I don't think this is linear though. It's easy to target a weak point to inflict a small amount of damage, but hitting say 10% of GDP would mean targeting multiple sectors of the economy and putting millions of people out of productive work.