Minister had signed off on the building of two ferries for Islay at the Cemre Marin Endustri shipyard in Turkey, costing £105m, with a further £17m in required pier upgrades.
However, after a series of ferry experts queried the cost, the Scottish Conservatives have demanded answers.
Shadow transport minister Graham Simpson said: “We need SNP ministers to show much greater flexibility when it comes to ferry procurement and at least examine the options of using catamarans.
“If they are not going to use them, then they must explain why they have been ruled out.
“There are concerns that the ferries that are being built are not suitable. Islanders who have been continually let down just want vessels that will take them between islands and to and from the mainland, rather than hearing arguments over designs.”
The critical study was produced by the Mull and Iona Ferry Committee. The document claimed the state-controlled ferry owners and procurers Caledonian Maritime Assets Limited (CMAL) had approved a catamaran design that was "designed to fail".
It claimed they could have got three catamarans which would could carry 1,161 passengers instead of 700 with the preferred ships and 294 cars instead of 214, for just £60m.
This would also mean they would not need the £17m pier upgrade.
The study argued catamarans are cheaper, more reliable, efficient and environmentally friendly catamarans than the conventional monohull vessels.
Dr Alf Baird, former professor of marine business at Edinburgh Napier University, said: “You have to wonder who’s marking CMAL’s homework, if they can get away with this sort of thing.
“This blatantly biased and error-riddled so-called ‘consultation’ is a scandal. If this is the calibre of work and level of oversight that goes into decisions worth hundreds of millions of pounds, goodness help us.
"Unfortunately I am not surprised, but the blatant squandering of enormous sums of public money in return for ever worsening ferry service provision is totally unacceptable."
Joe Reade, chairman of the ferry committee, said: "CMAL have a long history of being anti-catamaran. But why? It appears to be a mixture of dogma and pride.
"Decisions around the spending of tens of millions of pounds of taxpayers money don’t seem to be founded on objective analysis, but on subjective preference given a veneer of impartiality through a superficial and flawed options appraisal."
CMAL dismissed the study and accused those commenting of not being experts in the field.
A CMAL spokesman said: “We do not recognise the individuals quoted as ferry experts and they don’t have all the information necessary to make robust assessments and comparisons.
“Our analysis and consultation for the two Islay vessels was extensive, and the Islay Ferry Committee has stated its satisfaction with the level of engagement and the two boats on order.
“CMAL is not anti-catamaran. In fact, we are considering a catamaran for the Dunoon-Kilcreggan-Gourock route. We deal in facts, not fiction, and will only ever order the vessels best suited to the routes and communities they’re intended to serve.”