NTSB says maintenance error caused $2.2M marine accident with Canadian-flagged ship

NTSB investigators traced the problem back to a small set screw that was installed in a piece of the controllable pitch propeller machinery that controlled pitch.

The incorrect installation of a single set screw led to the loss of propulsion control on the Canadian-flagged, 736-foot-long Atlantic Huron, causing the ship to strike a pier at 6.8 knots, the National Transportation Safety Board said.

Marine Accident Brief 21/10 is the NTSB report on the July 5, 2020, contact between the self-unloading bulk carrier and a pier associated with the Soo Locks, in Sault Sainte Marie, Michigan, resulting in $2.2 million in damage. There were no injuries.

While on approach to the locks and attempting to slow, there was a propulsion problem involving the vessel’s controllable pitch propeller system that resulted in the ship moving forward with increasing speed instead of slowing or moving astern, as ordered by the captain. In a controllable pitch propeller, the blades are not fixed in position but are fastened to the hub in a way that allows them to rotate and thereby change pitch. The blade pitch determines both the vessel’s speed and its direction.

NTSB investigators traced the problem back to a small set screw that was installed in a piece of the controllable pitch propeller machinery that controlled pitch. Vessel maintenance records show the set screw was last removed and reinstalled during a shipyard period more than four years before the accident. When the set screw was examined post-accident, technicians found no evidence that any manufacturer-required thread-locking fluid had been applied. As a result, the set screw was able to back out, beginning a sequence of mechanical failures that resulted in the ship moving ahead when it was supposed to be doing the opposite, while still indicating an astern pitch on the ship’s bridge.