Nortlink Ferries (Hamnavoe) sail from Thurso (Scrabster) to Stromness on Orkney Mainland and advertise and manage to do a 11/2 hr crossing. Pentland Ferries (Claymore) sail from Gills Bay (midway between Thurso and John O Groats) to St Margarets Hope on South Ronaldsway. They advertise a 1 hr crossing but it is usually nearer 11/2 hrs.
The Hamnavoe is a large, comfortable RoRo ferry with bars, lounges and a resteraunt, wheras the Claymore is smaller with fewer facilities and is not RoRo - you have to turn around on the ship as it loads and disembarks at the stern. However, Pentland Ferries are much cheaper for non-residents.
HTH