A total of £35m has been spent on redesigns of a beleaguered ferry that is already late and over-budget, the head of Ferguson Marine has said.

The Glen Sannox – and its sister vessel the Glen Rosa – have been beset by issues which have seen multi-year delays and cost overruns which have put the final price at £360m, compared with the initial £97m price tag when the contract was signed.

In an appearance before the Net Zero, Energy and Transport Committee at Holyrood on Tuesday, Ferguson Marine chief executive David Tydeman said almost 10% of the final estimated price tag has been spent redesigning one of the ferries.

“I estimate that 50% – about £35m – of the increases over the last 18 months on Glen Sannox have come directly from re-working the design, sometimes regrettably involving two or three iterations,” he said.

Asked what design issues had been found on the ferry, the chief executive said work involved “re-routing cables, re-routed pipework” and gave the example of hydraulic pipework used to open the clamshell doors at the front of the vessel was installed with mild steel as opposed to stainless steel.

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