The National Transportation Safety Board (NTSB) has released an investigation report on an incident where the containership Maunalei was transiting the North Pacific Ocean, when the crew intentionally shut down the main engine due to problems in the controllable pitch propeller system, resulting in a loss of propulsion, on August 11, 2022.
Analysis
On August 4, while the Maunalei was underway en route to Anchorage, the engine crew discovered the vessel’s CPP hub lubricating system was leaking
hydraulic oil. The engine crew attempted to mitigate the loss of hydraulic oil and its effect on the propulsion system by reducing the pitch of the propeller as needed and replenishing the hydraulic oil in the CPP hub head tank. The engine crew believed the CPP system had experienced a blade seal failure, but, because the vessel was at sea, they could not attempt repairs, and the vessel continued to Anchorage.
Three days later, the vessel arrived in Anchorage, where technicians boarded the vessel, examined and tested the CPP system, and determined the vessel should
be drydocked to further inspect and repair the system. After the port engineer and captain informed the Coast Guard of the situation, the vessel headed toward a shipyard in Oregon for an emergency drydocking. On the voyage, despite their efforts, the loss of hydraulic oil continued to worsen—so much so that they began using fresh water to supplement the hydraulic oil in the CPP system. The system continued to lose the combined water and hydraulic oil, and the crew noticed the hydraulic oil in the stern tube lubricating system was being contaminated with water.
Because the stern tube lubricating system was compromised, the continued use of fresh water as a substitute for hydraulic oil to lubricate and seal the system could have rendered the propulsion system inoperable, risking a full seizure of operation and the potential for the ingress of seawater into the machinery space. As a result, the chief engineer and master decided—in consultation with the owner/operating company—to shut down the main engine due to concerns that water in the stern tube system would cause additional damage to the propulsion system.
A day after the propulsion loss, a tug arrived and towed the vessel to the shipyard for repairs. At the shipyard, a diver conducted an underwater survey and found fractures and cracks on two of the propeller blades (nos. 2 and 4). The free surface cracks found on the no. 2 blade did not extend to the base of the hub and therefore would not have allowed hydraulic oil to leak. The fracture on the no. 4 blade was larger, extending from the hub near the leading edge, through the bolts, and approaching the trailing edge. Additionally, postcasualty examination and testing found that the potable water used to supplement the hydraulic oil during the casualty transit drained from the no. 4 blade, but not the no. 2 blade. Therefore, the fracture in the no. 4 blade base of the CPP system allowed hub hydraulic oil to exit the CPP system, diminishing the fluid reservoir to a level that the crew felt was unsafe to continue to operate the system.

Postcasualty testing completed by a third-party company found no significant corrosion, wear, or impact damage to the propeller blade that might have caused the fractures. Instead, the company found that the cracks and fractures on the no. 4 blade initiated at the bolt hole counterbore radius and were consistent with progressive cracking due to high cycle fatigue. The company also found that the bolt hole counterbore radius did not meet manufacturer machining requirements (the radius was about 0.2 millimeters smaller than the required 0.8 millimeters).
Additionally, the no. 4 blade did not meet other manufacturer design specifications, such as material specifications for Charpy impact toughness, tensile strength, yield strength, or percent elongation. Lastly, the chemical composition of the blade did not meet compositional requirements (the silicon content exceeded the specified minimum). Propeller blades require adherence to specified engineering design, material selection, and manufacturing requirements to maintain optimum performance and extend fatigue life. Because the no. 4 blade did not meet manufacturer design specifications, it was more susceptible to high cycle fatigue, which resulted in the development of cracks and fractures in the blade base. As a result of the casualty, the CPP blade manufacturer revised the internal radius requirement—enlarging it—for all seven bolt hole counterbores to improve fatigue fracture resistance.
Conclusions
Probable cause: The National Transportation Safety Board determines that the probable cause of the loss of propulsion on the containership Maunalei was a crack developing in a controllable pitch propeller blade base and progressing into a fracture due to high cycle fatigue as a result of the blade not meeting manufacturer design specifications.