Glen Sannox is due to be completed between March and May 2023 – five years late – as part of a two-ferry contract that could cost £240 million, two-and-a-half times over budget.
The vessel is expected to enter service on the main Arran route in autumn or winter 2023 after several months of sea trials, but CalMac will not gain initial access to the ship until at least September this year.
Among major changes to the ferry made after Port Glasgow shipyard Ferguson Marine was rescued from collapse by the Scottish Government in 2019 were cables being replaced because they were too short to meet the necessary equipment.
CalMac managing director Robbie Drummond told The Scotsman: "A vessel that's had its build life extended over five-six years – you'll have concerns [like] ‘are all those parts resilient and working as you'd expect them to?’.
"Any vessel that's [taken] that long to build you'd have concerns over.”
Glen Sannox is due to be followed by unnamed sister ferry, currently known as hull 802, being completed between October and December 2023 for the Skye-Harris-North Uist triangle.
Drummond said: "Our concern is that when the vessels come out will they operate in a resilient fashion, and that's the major challenge.”
The nationalised yard, Ferguson Marine (Port Glasgow), said Glen Sannox had been taken to a dry dock at Dale’s Marine in Greenock “where finishing touches will be made to internal systems including its propulsion, and for a fresh coat of paint to its hull”.
The three-week work, which also includes on the ferry’s seals and bow doors, will involve some 130 Ferguson Marine staff and others from Dale’s.
Ferguson Marine said the transfer to a dry dock was a “standard and routine part of the shipbuilding process” and unrelated to the project’s delays.
There is no dry dock at the Port Glasgow yard.
Ferguson Marine chief executive David Tydeman said: “The work being undertaken in collaboration with Dale’s is another important milestone in the delivery of MV Glen Sannox.
“The sub-waterline and aesthetic works being undertaken show that the ferry is coming along well, and we look forward to welcoming her back to Port Glasgow in August for a continuation of the commissioning programme.
"Our next major milestone is the testing of main engines and generators before October, which will bring the ship to life.”
Caledonian Maritime Assets Limited, the Scottish Government firm which ordered the ferries for CalMac, revealed last month that its confidence in the yard had grown since Tydeman took over in February.
It has concurred with the ferries' delivery dates for the first time since the ill-fated contract was placed in 2015.
Chief executive Kevin Hobbs said: "There is not much now which is standing in the way of both of them being delivered.
"There are a lot of detractors out there saying rather spurious things about them, but we’ve always had a view that both would be finished.
"The dates which we’ve got at the moment, give or take a month, are absolutely achievable, barring another pandemic or whatever.”