Slow Chat III

[Windy_Skies]

Flettner Rotor For Ships - Uses, History And Problems

https://www.reddit.com/r/Futurology/comments/1f4t1p9/worlds_largest_sailing_cargo_ship_makes_first/lknqx4b/

“…for the Flettner rotors, the cylindrical rotors are a source of loss. Despite highly attuned designs to provide thrust in the right direction, the biggest drawback is that wind can blow from any direction, while the ship is only supposed to move at a certain heading.

For this reason, the direction of thrust varies erratically, while the propulsive power is also not constant. These are sources of inefficiency that make the Flettner system inconsistent in power generation.

Moreover, to generate sufficient power to move a large ship with the same speed as a conventional system, rotor sails exceeding 20 meters in height will need to be used. As this can also create hydrodynamic instability in the ship, it is not preferred and the height is restricted to below 15 meters.

Projections above a certain height can place excess stress on the bearings and joints that connect the rotor sails to the deck, leading to an increased chance of an accident.

So, by restricting the height of the rotors, the generated thrust is lesser than an equivalent marine diesel engine. Due to these 2 main reasons, Flettner systems are not used commonly for propulsion.”

The author of that article is to put it mildly, badly informed. He compares rotor sails to motoring. But it should be compared to traditional sailing*.* I have read a lot about this topic and feel qualified/informed enough to comment:

Sailing upwind: A rotor sail works very much like a traditional sail, building up high and low pressure regions, and redirecting the airflow to produce thrust. If the argument is that not every wind direction can be used for rotorsails (which is true - you can not sail directly up wind), it is still no argument against rotor sails because the same is true for traditional sails - except the no-sailing region (as in the angle to the wind) is much smaller for a rotor sail.

No constant thrust: this is the nature of sailing; the wind dictates how fast you can go somewhere. Also the direction of thrust does not vary as it is by definition forwards (+one can sail also backwards with rotorsails) with a varying sideways vector on the ship (drifting). All this is also true for traditional sailing, to a larger degree, because they offer a larger surface to the wind.

Stability: a rotor sail ship heels much less than a ship with traditional sails, which makes sense because for the same thrust a rotor sail only needs 1/8 the projected area against the wind. The rotor sail itself is heavier than a traditional setup, but compared to the ship it is a insignificant difference. Also the rotor(s) itself should act as a gyroscope and add further stability to the ship. Although I have to admit I don’t know any numbers on that effect off the top of my head.

Maximum size: Rotor sails (way) above 15 m are in use regulary. See for instance these ferrys between Germany and Denmark: https://www.scandlines-freight.com/media/ognpdjvq/dsc_4467-ferry-page.jpg

I think the rotor is over 30 m tall there and works just fine.

The inventor, Anton Flettner, tested the practicability of the rotor sail in the 1920s already with 2 larger ships - the Buckau and the Barbara (both having rotors above 15 m) and they were superior in every way compared to traditional sails.

On the Buckau, if I remember correctly, the motors to spin the rotors used around 20 kW or so of power - but they produced over 500 kW of thrust.