More vessels are installing (Flettner) designed rotor sails. As commented in an upload several months ago, current roto sail technology is a redesign of a century old invention by Anton Flettner. At the time (1920's) the technology was not widely used due to rising popularity and low cost of diesel fuel. (Source www)
Based on the science and material available on the internet, todays roto sail technology has resulted in improved overall vessel efficiency.

Ian, I got those two names, but did not see the ships myself.

Sorry pieter I should have given you more info on these two the Bro Deliverer IMO 9313096 now Hammerhead with the Bro Designer IMO 9313101 now Nightcrawler.

Thanks again Ian, I have to look up which Bull tankers were in the Europoort recently, but I did not see the two Bro tankers. (which does not mean that they did not visit Rotterdam...)
So we may expect Maersk to put more of these things on their ships?

Yes the wind device owners say the trial was a success with a fuel saving of 8.2% and a reduction in c02 emissions, and Bull acquired both the Bro Deliverer and Bro Designer recently.

Thanks Ian, I have that logo here on some other ships here too. They seem to be quite active in the second hand market, just wondering whether they will remove the wind devices. Does anybody know whether the Maersk experiment was really financially succesfull?

The Indonesian company you are referring to pieter are Buana Lintas Lautan TBK PT or Bull Shipping, the company logo on her funnel is a clue.