Arran has been without its second summer ferry since last week after a technical fault put the MV Isle of Arran off the main Brodick to Ardrossan route.
The vessel, which only started full-time on the route at the end of April, has an issue with a propeller control system which first became apparent on Saturday.
A spokesman for Caledonian MacBrayne said: ‘The manufacturer attended the vessel on Tuesday to make adjustments to the system which improved it slightly but not enough to allow the vessel to return to service. More checks and adjustments were carried out through the night and the vessel undertook sea trials on Wednesday morning. The outcome of these trials will determine whether the vessel goes back into service or has to go into dry dock for a permanent repair.