ENGINE failures on board Wightlink's Wight Sky ferry have been blamed on assembly errors at Volvo Penta, an interim report on the investigation has said.

Two catastrophic engine failures happened during crossings on the Lymington to Yarmouth route — one on August 26, 2018, and another on December 14, 2018.

On August 26, a fireball engulfed the engine room as one of the ship's engines catastrophically failed as the vessel was preparing to enter the Lymington river.

The engine was nearly new, having only accumulated 2,241 running hours since its installation.

On December 14, another engine suffered a catastrophic failure as the vessel was closing on the berth at Lymington. This engine was also new, having only run 380 hours.

The technical investigation team from the Marine Accident Investigation Branch (MAIB) identified two other recent non-catastrophic engine failures that occurred on board Wight Sky’s sister vessel, Wight Light, and included them in the scope of the investigation.

The recently published interim report found that the December failure was due to assembly error during the build at the Volvo Penta factory.

The report also found that one of the non-catastrophic failures on Wight Light was due to assembly error during overhaul work conducted by the local Volvo Penta dealer.

The report said: "Although direct causes have not been established for all five engines, the investigation has identified several underlying factors that might have contributed to this unusually high incidence of failures.

"These include the way the vessels and their engines are driven during entry to and departure from port; the setup of the engines’ auxiliary systems; the management of maintenance on board and ashore; and quality control during engine assembly and overhaul."

Following the failure of Wight Sky in December, Wightlink withdrew W-class vessels from service.

After discussions between Wightlink, the MCA, Lloyd’s Register, and Volvo Penta, a mitigation plan was put in place to enable the vessels to return to service.

The initial mitigation measures included weather and engine load constraints, enhanced engine monitoring and shutdown procedures, and restrictions on personnel entering the machinery spaces when the main engines were running.

The report said: "Further precautionary steps have also been taken as the technical investigation has progressed, including shorter periods between routine fuel injector overhauls; modifications to the engine alarm system; replacement of flexible engine mounts; reduction of oil replacement and oil sampling hours; and testing of engine coolant.

"Following the discovery of the assembly error, Volvo Penta has strengthened its standard operation process to prevent this from occurring again.

"Volvo Penta confirmed the same error had not been repeated on engines manufactured around the same time."

Wightlink chief executive Keith Greenfield said: “We welcome the release of the interim report by the Marine Accident Investigation Branch (MAIB) into engine failures on Wightlink’s ships on the Lymington-Yarmouth route in 2017 and 2018.

“The investigation is complex and not yet complete however we have been able to ensure that the manufacturing and rebuild assembly errors are not present on any of our other Volvo Penta engines.

“We continue to work with the MAIB, Volvo Penta and other parties to establish root causes for all engine failures in 2017 and 2018.

“Wightlink has excellent engine reliability on all its other ships and routes and we are working hard to eliminate the problems we have had on our Lymington-Yarmouth route.

“Over the last three months, the Lymington-Yarmouth route has performed with 98.7% reliability, including cancellations caused by adverse weather.”