The back-up passenger vessel itself has suffered “technical issues” in recent days, leading to cancellations on the Heysham route on Sunday and Tuesday.
The Manxman had been due to return to service on Wednesday but no revised date has yet been confirmed by the Steam Packet.
Managing director Brian Thomson said while it was recognised it would be “frustrating” for passengers, extending the ferry’s period in drydock was “necessary for safety and operational reliability”.
He added: “Completing these additional works now will ensure the vessel can continue to provide a reliable and dependable service in the years ahead.”
The government-owned ferry firm runs daily sailings between the island and Lancashire and Liverpool, along with regular sailings to Larne in Northern Ireland.
