https://en.wikipedia.org/wiki/Slow_steaming
Using sails to increase fuel economy seems like a no brainer, but now I’m wondering if cargo ships were ever using sails to increase their maximum safe cruising speed. Does anyone know?
Sail died off pretty quickly once steam came in. [EDIT: Apparently in some applications much faster than others.] In the case of modern examples, I'm pretty sure it's been exclusively for fuel economy. And, until recently, the overhead associated with using sails was generally more trouble than they were worth.
The most famous clipper (fast sail ship) Cutty Sark (later Ferreira) operated between 1870 - 1922 (17.5 kn max speed).
It was the size. Sail ships had practical upper limit for their size and weight. Size of the mast and sails could not just grow.
Square–cube law: (drag) surface area is proportional to the square while the volume is proportional to the cube. Bigger the ship, more efficient it becomes.
Where combined steam and sail ships really benefited wasn’t fuel, it was consistent power. Early steam engines where unreliable as was sail, but they rarely failed at the same time. However with more reliable steam it was just a question of economics.
There's a Liberty Ship that sails out of Baltimore at times [1] and if you go down into the engine room you can meet (or could 3 years ago) a greaser who sailed with these ships during WW II. They were the last mass-produced steam ships in the world [2].