The MV Seaforth which operates between Ullapool and Stornoway developed major engine problems last week.
Another vessel was redeployed from the main Arran route as a temporary replacement, reducing that busy link to a single vessel.
The problems come as traffic is expected to increase with the re-opening of hotels and other visitor accommodation across Scotland on Monday with the lifting of Covid restrictions.
CalMac said: “Specialist engineers working on the MV Loch Seaforth have advised they expect her to back in service by May 17 at the earliest.
"This date is subject to change, and we will continue to keep our customers informed throughout.
“A team is carrying out repairs to the engines on the vessel at the James Watt dock in Greenock.
“CalMac staff are working extremely hard to ensure all customer demands are being accommodated across the network.
"However, passenger bookings will have to be carefully managed until the MV Loch Seaforth is back in service and the summer timetable resumes.”
The operator said it had increased sailings to double capacity on the secondary Arran between Claonaig on the Kintyre peninsula and Lochranza while the main link between Ardrossan and Brodick has been reduced to one vessel.
CalMac managing director Robbie Drummond said: “Clearly, the loss of the MV Loch Seaforth and how this has affected other services has caused a lot of disruption to customers, and we are so sorry about this.
"We are working closely with local businesses to make sure all demand for travel is being met, and I am most grateful to them for their support, patience and understanding as we deal with this situation.
“We are confident that the MV Loch Seaforth will be back in service in a matter of weeks and the normal summer timetable will be back up and running as planned.”
CalMac said its Wemyss Bay to Rothesay route was due to return to normal after the MV Argyle ferry returned to service today following repairs.
New ferries to replace CalMac’s ageing fleet are planned by its Scottish Government-controlled vessels’ owner Caledonian Maritime Assets.