"Levels for the big ships are spiraling downward," brokerage Fearnleys wrote in its weekly report.

"Every rally is followed by a correction, something that is currently taking place following one of the strongest rallies in recent memory," commented Breakwave Advisors, creator of the BreakWave Dry Bulk Shipping ETF.

Rates for Capesize bulkers (ships with capacity for around 180,000 deadweight tons or DWT) were down to $17,300 per day on Friday, according to Clarksons Platou Securities. That's about half the recent peak of $33,800 per day on July 6 and down 39% year-on-year. Listed owners of Capesizes include Star Bulk, Golden Ocean, Genco Shipping & Trading and Seanergy.

Rates for Panamaxes (65,000-90,000 DWT), the vessel type used to transport bulk U.S. farm exports, are down to $11,400 per day, 40% below where they were a year ago.