Oil tanker earnings tumbled as more ships are willing to enter the Strait of Hormuz, capping a dramatic week of swings in costs to hire the world’s biggest crude carriers.

Hiring a supertanker to haul 2 million barrels of crude oil from Saudi Arabia to China cost about $287,000 on Friday, down 44% from more than $514,000 on Tuesday, according to data from the Baltic Exchange in London. While the latest level is still massively profitable for shipowners, the drop seen in recent days is four times larger than last year’s average daily earnings.