People living on the Small Isles of Eigg, Muck, Rum and Canna have enjoyed the use of the purpose-built vessel for more than 12 years as part of a £30m European-assisted programme.
However, anger is growing after ministers revealed the ferry Lochnevis has been earmarked for a move south to serve Colonsay seven days a week from next year.
Islanders say the ferry would barely have half the vehicle-carrying capacity they need during the summer when the numbers visiting are swelled by tourists.
The Small Isles Community Council said the overwhelming feeling following a meeting to discuss the proposal was that it was badly thought out.
They say the construction of the Lochnevis was partly funded by Europe, as were their new piers, to address their specific problems faced by one of the last island communities not to be served by a car ferry.
Local people are now concerned part of their service improvement will be based on the introduction of a daily fast RIB passenger service along with a minimum of two visits week from a vehicle-carrying ferry and freight provision. They see this as a significant diminution of their current four visits a week of the Lochnevis during the winter and five in the summer, with an inevitable reduction in passengers.
Eigg-based Camille Dressler, secretary of the community council, said: "We feel this is ill-conceived and fail to see how it will enhance the service. We made clear we wanted a roll-on- roll-off (vehicle carrying) vessel at least as often as we do just now with the Lochnevis. She can carry 190 passengers and we need that in the summer. We were asked our view on future provision but we never thought we would lose the Lochnevis before the end of her working life in something like 2025. But if there were to be any changes in the type of vessel, we certainly wouldn't be looking at a RIB. It is a preposterous suggestion."
Meanwhile, Mike Nicholl, secretary of Colonsay Community Council, said the island currently has three direct return sailings a week to Oban in the winter and five in the summer.
There is also one sailing a week from Islay to Colonsay then up to Oban and back again. He added: "We voted unanimously to reject the introduction of the Lochnevis and would prefer the status quo."
At present the island was served by either the ferry Lord of the Isles, which carries 54 cars, or the Isle of Mull, which can take 70. So the Lochnevis's capacity of 14 cars would be a real problem, he said.
The changes were unveiled in the Scottish Government's ferry plan last month. A spokesman for Transport Scotland said: "The recently published Ferries Plan makes clear that it is for the operator (CalMac) to reach final decisions about the most appropriate vessel for a particular route, and while all of our plans will be subject to final operational checks before implementation, we fully recognise it is essential vessels deployed on each route meet the needs of the community being served and are suitable for the waters in which they sail.
"While no decisions have been taken about the selection and deployment of vessels to replace the Lochnevis, in the medium term we plan to provide both a ro-ro vessel and a high-speed craft capable of carrying around 150 passengers, supplemented by an appropriate freight vessel."
He said this would mean all the Small Isles having a seven-day-a-week service for the first time.