Late in August 2022, the Royal Navy’s newest aircraft carrier, HMS Prince of Wales, sailed from Portsmouth, UK bound for training exercises in the United States. She didn’t get very far. Two days after departing, the carrier broke down off the Isle of Wight. The external shaft coupling on the starboard propeller shaft failed, resulting in serious damage to the shaft and the propeller, and lesser damage to the rudder. After proceeding to an anchorage for inspection, the ship was escorted by tugs to a dry dock at Rosyth, Scotland, for necessary repairs.

The repairs that were originally estimated to cost £3 million and were to be completed by spring have now soared to an estimated £20 million and the ship deployment is now projected to be in the autumn of 2023. 

What happened on the HMS Prince of Wales? The failure of a propeller shaft coupling is extremely rare. The First Sea Lord described the failure as “unprecedented.” Indeed, few marine engineers can remember an instance of this happening.

Immediately after the breakdown, several media sources attributed the failure to the shaft “not being greased properly,” a suggestion described as “contemptible nonsense” by RN engineers.

Another source suggested that the ship had touched bottom on her way from Portsmouth, damaging the shaft and coupling, a more plausible theory but not one supported by any evidence.

It now appears that the coupling failure was due to poor shaft alignment. A misaligned propeller shaft can cause bearing wear or failure as well as increased vibration and could crack a shaft coupling if severe enough.

Disturbingly, new reports indicate that the starboard shaft alignment problem has been a known issue since at least 2019 when the ship went on sea trials. The Royal Navy has responded that during tests performed on the HMS Prince of Wales before delivery in 2019, both shafts and the vibration measurements had “remained within the established limits.”

Now, the port propeller shaft is also being overhauled over concerns that it too may be misaligned. 

Recently, ex-Royal Navy commander Tom Sharpe said: ‘Someone in the trials process accepted the risk [surrounding the propeller] that this would pose to the running of HMS Prince of Wales.

‘The Royal Navy must make doubly sure similar shortcuts are not taken in the many other new-build warships. 

Shaft alignment should not be done badly these days. Someone in the build process made a big mistake.’

Defence chiefs signed for HMS Prince of Wales despite Royal Navy officers being aware of the propeller issues. This decision has exposed MoD to the costs of recovering and repairing the carrier.

A source close to the investigation said: ‘Someone knew about the problems and didn’t flag them or amplify them as we would have liked. Lessons must be learned as this has proved a very expensive mistake.’

Thanks to Alaric Bond, David Rye, and Roger Eastman for contributing to this post.