The cost of hiring a ship to haul iron ore more than doubled in the last month amid speculation that China is adding to stockpiles while the global vessel fleet shrinks. Rates for Capesizes, the vessels that dominate the ore trade, were at $18,250 a day as of Friday, compared with $8,369 a month ago, according to data from the Baltic Exchange in London. The cost advanced to $19,499 on Aug. 5, the highest in eight months.