A botched project to build the Glen Sannox and a sister vessel will mean a “significant amount of pipework” being thrown away, according to Tim Hair, turnaround director of the nationalised yard.
The Scottish Parliament’s connectivity committee was told that more than four years after the contract for the ships was signed, they were still “significantly less than half-built”.
Mr Hair said construction had started in Port Glasgow before the design was completed, and it had still to be finalised.
He described the design engineering process as “badly flawed...it was a perfect ‘garbage in, garbage out’ situation.”
He said: “There is a significant amount of pipework that is going to have to be scrapped, because if you make a change to structure of the vessel, you make a change to the location of a pump, then the pipes no longer fit and you have throw them away and start again.”
Mr Hair said a coupling mechanism for joining pipes had also been used extensively without agreement.
As a result, “just about every piece of pipework in the engine room will have to be removed and put back”.
Elsewhere, Mr Hair said a 6ft section would have to be cut off the bow of the Glen Sannox and replaced because it did not need specifications.
He said almost all current work on the ferries was remedial.
Mr Hair also said parts for the ferries were being stored in warehouses that were "not in great conditions" and which had a potentially unreliable inventory.
He said: the ferries were "much further away from completion than they look from a distance".
Mr Hair said it was "highly unlikely" that anything "catastrophic" would be found, but that would not be confirmed until the vessels were taken out of the water into dry dock.
The revelations about the yard - now called Ferguson Marine Port Glasgow - come after finance secretary Derek Mackay announced last month that the cost of the work had doubled to nearly £200 million and the vessels would be completed until 2021-22 - three years late.
Mr Hair confirmed to MSPs it would cost an additional £110 million to finish the ships, on top of the original £97m fixed price contract.
Mr Hair, who was drafted in last August after the yard was taken over by the Scottish Government, said he did not see why the original contract could not have been completed, despite the complexity of the vessels, which are designed to run on liquefied natural gas as well as diesel.
He said the cost of simply scrapping the ships and starting again would not be "dramatically different" but said due to the time this would take Ferguson had "decided to pursue the option of completing the vessels".
Michelle Rennie, chair of the programme review board, from the Scottish Government's Transport Scotland agency, told the committee the contract had been "credible and deliverable".
However, Alex Logan, the yard’s GMB union convener, said the yard could not cope with building two such large vessels at the same time.
He said Caledonian Maritime Assets (Cmal), which owns the ferries and leases them to CalMac - “came to loggerheads” with the yard’s former owners.
He said: “They wouldn’t agree on the design so we couldn’t move forward, so it just came to a standstill.
“It was just a stand-off at the OK Corral, who was going to cave in first.
"We've still not go a proper design, it's still not complete the design after four years, it has never been finalised by Cmal or whoever."
Scottish Liberal Democrat transport spokesman and committee member Mike Rumbles said: “This has been a catastrophic failure.
“For £110m, we have got two half-finished hulks and a lot of red-faced officials.
“The Scottish Government need to come clean on what due diligence was done before a contract worth tens of millions was dished out.”
“Right now, there is no evidence any lessons have been learned or a nationalised shipyard will be run efficiently and effectively.
"This needs to be rapidly addressed.”