A poor forecast has forced NorthLink to introduce an early departure for its southbound sailing.
The Hjaltland was due to leave Lerwick and sail to Kirkwall and Aberdeen at half past five tonight.
But the operator says the service will now leave Holmsgarth at 4pm instead.
Both stop-offs, in Orkney and in Aberdeen, are still planned.
The estimated arrival time in Kirkwall has been put at 9pm, with a planned departure from Orkney of 10pm.
Meanwhile, NorthLink is warning the arrival time of its northbound sailing of the Hrossey may be delayed by up to two hours
On the freight side, the Helliar was due to leave Aberdeen and sail to Kirkwall at 3pm.
But due to the bad forecast, this sailing is now cancelled.
The Helliar is next due to leave Aberdeen, sailing for Lerwick, at 6pm tomorrow.