Watch: Quest for the cause of major Scottish ferry fault that could push repair bill to £8.5m

The unexpected extra repairs to the 1,000-passenger Caledonian Isles will extend its lay-up to a total of between 11 and 14 months, which is unprecedented in the CalMac fleet.

It is also likely to increase the cost of the work from around £6 million to between £7m and £8.5m.

The ferry, which was due to return as the main vessel on the principal Arran route - CalMac’s busiest - in two weeks’ time - after being off since January is now undergoing “significant steel replacement” in a dry dock in Greenock.

It follows areas of the “frame” or skeleton of the 31-year-old ship being found to have “deformed” by some 2cm due to stresses on its hull, which CalMac showed The Scotsman on Wednesday in an exclusive behind-the-scenes tour.

The fault has also affected other key parts of the ship - the engine gearbox and the stern tube bearings, where the propellers meet the hull.

Scottish Government-owned CalMac likened it to squeezing a pot of yogurt which distorted the shape of the contents.

It was spotted hours before Caledonian Isles was due to return to service in August when debris was found in the gearbox during a routine oil change.

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Engineers at Dales Marine in Greenock are working round the clock on the 52 square metres affected but the ferry will not be ready to return to passenger service on the Ardrossan-Brodick route until late December at the earliest.

CalMac fleet management director Craig Ramsay told The Scotsman: “We are trying to understand where the defect came from - we need to know how it came about.”

Mr Ramsay said “all the indications” were that the damage happened after the ferry left the Merseyside yard where the previous work was done to replace steel that had become too thin.

He said: “It looks like the deformation appears to have been sustained or worsened between that departure and the gearbox issue [being spotted] in Ardrossan.

“What’s absolutely crucial is that we understand the source of the stresses that led to this defamation.”

Mr Ramsay said the ferry being out of the water for much of this year could be a “strong link” to the problem.

But he did not think the ship would be unrepairable. He said: “That’s not really a legitimate possibility. We are confident we can achieve a final fix.”

He said CalMac “heard and felt” Arran islanders’ frustration at the prolonged repairs. He said: “We are doing everything we possibly can.”

Transport Secretary Fiona Hyslop told MSPs on Wednesday: “It’s deeply frustrating and concerning that the community of Arran is facing a further delay to the return of Caledonian Isles.”

West Scotland Scottish Conservatives MSP Jamie Greene said: “The palpable anger on places like Arran is very real.”

Scottish Labour transport spokesperson Claire Baker said: “The residents and businesses of Arran are facing a winter of disruption as they limp on with an ageing fleet following years of underinvestment by the Scottish Government.”