PS Waverley steam engine.

PS Waverley is the last seagoing passenger-carrying paddle steamer in the world. Built in 1946, she sailed from Craigendoran on the Firth of Clyde to Arrochar on Loch Long until 1973. Bought by the Paddle Steamer Preservation Society (PSPS), she has been restored to her 1947 appearance and now operates passenger excursions around the British coast.
Since 2003, Waverley has been listed in the National Historic Fleet by National Historic Ships UK as "a vessel of pre-eminent national importance".
Waverley is powered by a three-crank diagonal triple-expansion marine steam engine built by Rankin & Blackmore, Engineers, Eagle Foundry, Greenock, Scotland. It is rated at 2,100 IHP and achieved a trial speed of 18.37 knots (34.02 km/h; 21.14 mph) at 57.8 rpm. Passengers can watch this engine from passageways on either side of the engine room.
The main crank is solidly attached to both paddle wheels so they cannot turn independently. The Waverley therefore has a much larger turning circle than modern ferries.

[@IainSherriff]

As a kid I had many a trip around the Kyles of Bute on the Jeanie Deans (sister ship to Waverley I believe) . I can almost smell the engines watching this video