The 8,700-gt Loch Seaforth (built 2014) made its first sailing between Stornoway and Ullapool on 13 February.
Now the £42m ($64m), 700-passenger ship's technical fault means its operator has turned to its predecessor to keep sailings running.
CalMac said: "Loch Seaforth has a technical fault and Isle of Lewis is travelling from Stornoway to take over tonight's sailing."
The company later clarified that an engine room ventilation fan had malfunctioned, the BBC said.
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