The Isle of Man Ship Registry has suggested the problems which have kept the £78m Manxman out of service may be down to a design flaw in the vessel rather than conditions in Douglas Harbour.

The assessment, contained in an email released by the Department of Infrastructure following a Freedom of Information request, questions suggestions that rocks and stones in the harbour are responsible for wearing out the ferry's propulsion shaft bearings.

The FOI request asked the Department for copies of reports, assessments and inspections showing the suspected or identified cause of damage or abnormal wear to the Manxman's propulsion system.

In its response, the Department provided an email containing technical observations from Isle of Man Ship Registry principal surveyor Chris Martin, who was asked for a 'high level view' of the issue.

The Ship Registry is a government body which sits within the Department for Enterprise (DfE) responsible for registering ships and overseeing maritime safety standards.

Mr Martin explained that the vessel was originally intended to have oil-lubricated stern tubes, but the design changed early in the project to use water-lubricated bearings.

He said the Registry was not party to the technical analysis of the worn shaft and bearing material, but questioned whether the problem could be attributed specifically to Douglas Harbour.

In the email, dated August, he wrote: ‘Bearing in mind all of the above we would consider that the issue is more of a design flaw in the vessel rather than an issue of what is in the harbour.’

Mr Martin said a new stern tube would have a relatively small clearance between the shaft and liner, which would not allow stones to enter. Only much smaller particles would fit into the original gap.

However, he said that once wear had increased to the extent found on the Manxman, larger particles such as small stones could enter and potentially increase the wear dramatically.

He added: ‘Use of the well proven and traditional oil sealed stern tube bearing would be a better option than using a water lubricated bearing in the environment that the vessel operates in.’

He referred to the Ben-my-Chree, which has a traditional oil-lubricated system, saying the Registry had not encountered similar problems during its 25 years of operation in Douglas Harbour.

Mr Martin acknowledged that water-lubricated bearings were becoming more common, but said they might not be suited to certain types of vessels and trades.

The comments contrast with the explanation previously given by Steam Packet Company managing director Brian Thomson, who told election candidate Dr Rachel Glover, now DEFA Minister, that rocks and stones in Douglas Harbour were likely to be responsible for the damage.

In a blog talking about her meeting with Brian Thomson during the election, she wrote: ‘In his opening remarks, Brian explained that Manxman's problems come down to the bearings wearing out.

‘He believes Douglas Harbour is the likely cause, since other operators using Heysham haven't seen the same issues.

‘The damaging material in Douglas Harbour isn't so much silt as rocks and stones, and it needs a deeper form of dredging than is currently carried out. He had a technical term for it that I didn't quite catch.’

Documents released under FOI legislation have also previously revealed concerns among the Steam Packet board that sediment in the harbour may have contributed to the degradation of the shaft bearings and liners.

However, no definitive cause has been established. Mr Thomson has said an independent report has been commissioned to investigate the wear.

The Manxman remains out of service, with the Steam Packet previously estimating the total cost of repairs and dry-docking at around £5m. The company has said the bill is expected to be met from its own reserves.

That figure is higher than the £2.5m threshold previously notified to Treasury. The company clarified that this was the point at which shareholder consent was required, rather than its final estimate of the repair bill.

Mr Thomson has said the company is seeking to recover costs through insurance and warranty where appropriate.

He said prudent financial planning was a cornerstone of how the business operated and that it maintained contingency funds to cover unforeseen events.

The comments from Mr Martin was contained in an appendix attached to the FoI response which has since been removed from the FoI website.

Isle of Man Today has approached the Steam Packet Company and the Department of Infrastructure for comment.