A relief ferry is being sent north to cover for the Sound of Harris route.
The MV Loch Portain has been withdrawn from service following a "technical issue" on Thursday.
The vessel was forced to cancel her last sailing for the day as a result.
Cal Mac said the MV Loch Portain's early morning runs on Friday morning will also be put off.
Later services may be cancelled or disrupted.
Cal Mac said it aims to resume a service from approximately lunch time using a relief vessel.
However, forecast morning fog or swell conditions may affect the relief vessel on passage.
Relief ferry to cover for broken down vessel
1 October 2015