“MANAGEMENT and build errors” meant the two overdue ferries at Ferguson Marine ended up costing an extra £90 million, the chief executive of the shipyard has said.

This included the decision under the yard’s previous management to build a “largely empty ship” on the slipway rather than assembling modules under cover.

The current cost of the two vessels in Port Glasgow: the Glen Sannox and the unnamed hull 802; is estimated to be £277m, though this could rise further in the new year.

David Tydeman, who runs the nationalised shipyard, said contract issues also arose after it went into administration in 2019.

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In a letter to Holyrood’s Public Audit Committee, Tydeman said the sum of about £90m “is a function of mistakes and other factors, eg. recovery from administration through the pandemic etc, over the whole period since the contracts were placed in 2015”.

He set out five main issues, including the decision to start building the hull and superstructure before the design was complete.

Building a “largely empty ship” on the slipway may have doubled direct labour and subcontractor man-hours, he said, as it meant materials had to be lifted onto the ship while it was afloat.

A high turnover of staff during the pandemic also caused problems, Tydeman said.

The committee had asked him for further clarity on how costs for the ferries had increased from the original £97m price tag.

A further £50m is attributed to overheads.

By the time hull 802 is completed – currently scheduled for March 2024 – the two ferries will have carried the costs of the shipyard for nine years.

Tydeman also wrote to Holyrood’s Transport Committee in a separate letter published on Thursday laying out the timetable for delivery.

He said the Glen Sannox is still expected to be finished by the end of May 2023, but there are still technical problems with its LNG (liquified natural gas) system.

The ferries were originally designed to be dual fuel, running on both diesel and LNG.

The Public Audit Committee also published a letter it had received from Keith Brown.

Brown had been accused of providing “unsatisfactory and evasive” answers to the committee’s earlier questions.

In his latest correspondence,Brown produced an email from August 2015 which showed he was “content to approve the award” of the shipbuilding contract to Ferguson Marine.

He said the issue of the lack of a builder’s refund guarantee was not raised with ministers until later.

Between 2014 and 2019, the shipyard was owned by Jim McColl’s Clyde Blowers Capital.