Not sail freight but in a similar vein, the flettner rotor is pretty neat: https...

leoedin · on July 16, 2020

That's fascinating - and kind of what I was hoping this article was about.

That lead to the E-Ship 1 (https://en.m.wikipedia.org/wiki/E-Ship_1) - a wind turbine servicing ship with 4 Flettner Rotors.

There's a detailed breakdown of that ship here: https://www.stg-online.org/onTEAM/shipefficiency/programm/06...

That paper gives some interesting numbers - in one voyage spinning the rotors gave a speed boost of 2.4 knots with a constant power going to the propellers, which they calculated as a 1.7MW power boost (engine was running at 2.8MW). Overall the rotors give - with favourable wind and sailing direction - a maximum of about a 50% fuel reduction.

Obviously actual performance was much worse (you often want to go where the wind isn't blowing) - for the test voyage they used around 85% of the fuel they would have without the rotors.