A faulty controllable-pitch system caused a loss of propulsion in a vessel traveling in the Pacific Ocean two years ago, a National Transportation Safety Board investigation has found.
Maunalei, a U.S.-flagged, 681-foot-long containership, was en route to Portland, Ore., in August 2022 for drydock repairs when the crew intentionally shut down the main engine due to a controllable pitch propeller (CPP) system problem, resulting in a loss of propulsion about 245 miles from the entrance to the Columbia River. While no injuries were reported, the vessel lost about 1,632 gallons of hydraulic oil. Damages were estimated at just over $3 million.
A study of U.S. Coast Guard data conducted after the collapse of the Francis Scott Key Bridge in the Port of Baltimore in March found more than 6,000 incidents of power loss, loss of steering or loss of propulsion on large ships in the previous 22 years. The Maritime Injury Center stated mechanical failures and power losses are on the rise, with at least 900 of the recorded incidents occurring near bridges.
In the case of Maunalei, the NTSB blamed high-cycle fatigue for the loss of propulsion, caused by a crack in the blade base that progressed into a fracture. Analysis found the blade did not meet the manufacturer’s design specifications.
CPP systems are custom-designed for individual ships. The blades are fastened to the hub in a way that allows them to rotate and change pitch, rather than being installed in a fixed position. The blade pitch determines both the vessel’s speed and direction through the water. To increase speed, the blades are set at a higher pitch, and to slow the vessel, the pitch is lowered, decreasing the distance traveled per revolution. To change direction, the blades are rotated from positive to negative pitch.
Maunalei was built in 2006 by Aker Philadelphia Shipyard in Philadelphia, Penn. Its CPP system was installed in 2020 at COSCO Nantong Shipyard in Nantong, China. The vessel routinely transported cargo to destinations including Southeast Asia, Guam, Hawaii and the West Coast of the U.S. A single inline, seven-cylinder, slow-speed, two-stroke diesel engine provided propulsion.
On Aug. 4, 2022, while heading to Anchorage, Ak., an engineering alarm sounded, and the engine crew found the CPP system hub head tank – the reservoir for hydraulic oil for the hub lubricating system – was empty. The crew suspected a blade seal failure, and in response, they reduced propeller pitch and replenished the hydraulic oil and monitored the CPP system. Over the next three days, while the vessel was operating at full speed, the CPP system lost about a liter of hydraulic oil per hour. The crew reported the condition to the U.S. Coast Guard. The Coast Guard and Transport Canada gave approval for Maunalei to continue to Anchorage to offload cargo and make repairs.
Technicians inspected the vessel in Anchorage, determining it should be drydocked in Oregon for repairs. Maunalei departed Anchorage for Vigor Shipyards in Portland, remaining 200 miles from land, because it continued to lose hydraulic fluid. On the way to Oregon, the hydraulic oil leak worsened, and the crew supplemented it with fresh water. The system continued to lose oil and water, leading to contamination of the stern tube lubricating system. On Aug. 11, the crew shut down the main engine to avoid further damage to the propulsion system, and the vessel was towed to the shipyard.
A diver performed an underwater survey, finding two of the blades had fractures at the base. The vessel was drydocked. On Aug. 29, the vessel was refloated and tests were conducted at the pier.
Maunalei returned to service, making a round trip to Anchorage with reduced main engine power. The reduction of speed and operational limitations led the company to deem the vessel as not viable for its commercial purpose. It returned to Portland for installation of new blades.
An analysis by a third-party company found cracks and fractures at the base of the No. 4 blade of the five-bladed propeller were visually consistent with progressive cracking caused by high-cycle fatigue that initiated at the bolt hole counterbore edges. It added that a cross-section of one of the bolt hole counterbores measured a radius of about 0.62 millimeters, which did not meet the original manufacturer’s designed machining radius of 0.8 millimeters. It did not meet materials specifications for impact toughness, tensile and yield strength or percent elongation, and the silicon content exceeded the specified minimum.
New blades were installed, and repairs were considered final on Nov. 22.
The manufacturer revised design standards following the incident, but analysis suggested the failure was an isolated case, as an inspection of similar vessels did not identify any other instances of cracks. In response to the incident, the manufacturer enlarged the internal radius requirement for all seven-bolt counterbores to improve fatigue fracture resistance.