An interesting publication which dates back exactly a century. PDF available on
An interesting article discussing how ships could emit less green house gas
https://www.sciencedirect.com/science/article/pii/S2666822X22000065?via%3Dihub#fig0002
I was reading through this and there are some interesting takeaways
- ship route should be planned to take advantage of higher wind
- ship speed is low ~12 knots, which makes wind propulsion a better idea compared to twice that speed for eg. Ever Given [22.8 knots, source: ChatGPT]
- when taking emissions of building the ship into account, slower speed does not indicate less emissions, as more ships need to be built
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In a second article search I was looking at the capabilities of a traditional cargo hull to act as a sailboat without an added keel. I am assuming that adding a keel is not practical due to depth limitations. I found these articles:
https://doi.org/10.1016/j.oceaneng.2016.03.029
https://doi.org/10.1016/j.apor.2021.102689
There are also open source OpenFOAM simulations available, eg. at
And some more at
https://www.sintef.no/en/sintef-research-areas/maritimetransport/sintef-ocean-bulk-carrier/
https://www.sintef.no/en/latest-news/2024/sintef-shares-ship-design-data/
The general capability of leeward force to hull drag seems to be reported in the range 4 - 10. These numbers could be seen as the “glide ratio of the hull”. Typical leeward drift angles are up to 20 degrees. The data sets don’t seem very directed to sailing though. I would have liked many more numbers, it seems strange to me such are not produced given that every CFD calculation is done within 48 hrs according to one of these papers.
To state my wishlist for such a paper: Provide the hull leeward force vs drag force of such hulls, relative to direction of travel (as opposed to the direction the hull is pointing), at different side slip angles \beta. Provide these for beta angles eg. \beta = 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40 degrees… Also provide for standard hulls as used for motorized propulsion as well as hull shapes optimized for sailing and cargo transport.
The way these papers state their data, it may be understood that drag is constant with any leeward sail force. But this is not the case if the data of the hull is presented as would have been for a wing foil. Eg. the speed of the flow over the hull is changing if you keep “forward speed” constant and then vary the leeward slip speed.
What I conclude is that such a ship does have some capability to withstand the high sideways force of “WASP” (wind assisted sail power?). But, adding wind power to such a vessel would also increase leeward slip and furthermore increase the total drag of the vessel that is now going faster (increasing hydrodynamic drag due to v^2 dependency of such drag) and also making the ship face the flow slightly sideways instead of head on. Adding to all of this, a sail powered ship is drifting off course, which needs to be compensated to arrive at the correct destination.
I feel some of these problems are being ignored or glossed over in these papers. But maybe that is to be expected as each set of authors have different focus and competencies.
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300.000+ hours of operation…
A reference publication I often cite. The pdf is available and more complete, including the equation (3) page 29:
Fig. 13 indicates that the power consumption scales with the cube of
the cylinder tangential velocity. Using the analytical formula proposed by (Subramanya, 2005):
Power = Cf . ρ . Utan³ / 2 . Areas (3)
A close agreement with the experimental results is found by setting Cf = 0.007, that can be considered a reasonable value for the friction coefficient. It should be noticed, however, that the actual power consumption of a Flettner rotor is arguably also affected by the functioning of its mechanical systems.
Page 20 (Nomenclature):
Areas Cylinder surface area, π D H
Now if we take a look of the page 4/4 of Norsepower pdf (first link above), we can see (for example for the first rotor) a diameter of 4 m and a height of 18 m, and 225 rpm, leading to a tangential speed (Utan) of about 47 m/s.
With the formula: 0.007 x 1.2 x 47³ / 2 x 72 x π = 98.58 kW, so about 100 kW.
With a wind speed of 10 m/s, 47 m/s tangential speed allow a very good spin (TSR) ratio of 4.7, and a high lift coefficient (Cl) of 8.5 (Fig. 4).
Now, if we take a crosswind Magnus balloon like Wind Fisher , the apparent wind speed becomes the reference wind speed to determine the spin ratio (TSR). If wind speed is 10 m/s, the apparent wind is about 26 m/s (for a reasonable L/D ratio of 2.6). If we want keep the same very good spin ratio (TSR) of 4.7 to keep the high Cl of 8.5, the tangential speed (Utan) becomes about 122 m/s.
And the power consumption increases by the cube of the tangential speed (122³), becoming about 17 times the power consumption with a tangential speed of 47 m/s (47³). In the meantime, the wind speed remained the same, i.e. 10 m/s. The improvement in Cl (the L/D ratio practically does not change) will therefore not be able to compensate for the electricity consumption which will become too higher.
And for an inflatable balloon the power consumption becomes far higher (perhaps 2 or 2.5 times more (by equalizing the parameters), as we can deduce from the curve of power consumption. But with a vertical trajectory with a wind speed of 10 m/s, and with a TSR (spin ratio) of about 1.2, this remains workable.
For AWE use, perhaps systems like turbovoile at a high lift coefficient would be less demanding at high wind speeds, or quite simply crosswind kites with high lift coefficient.
Spoke to someone in the know. Flettner rotors are installed in great numbers now and are saving a lot of fuel.
Indeed, the number of ships equipped with Flettner rotors seems to be seriously increasing.
And this is understandable: considerable is the space saving by using cylinders with a lift coefficient Cl of 8.5 for a tangential velocity of 47 m/s and a wind speed of 10 m/s, all this for a reasonable power consumption. With higher winds the efficiency remains considerable even taking into account of lower Cl. And with lower winds we will be at the maximum Cl possible with a lower energy rotation expense. These rotors are solidly mounted, which can limit the power consumption.
We could find the space saving qualities within AWE field by the ease of stacking trajectories of units flying slowly and not having to half turn.
I think that vertical trajectories such like investigated in both chapter 12 (of which Garrett Smith (@WindFisher) is a co-author) and chapter 13 allow to benefit from a higher Cl with a current wind speed of 10 m/s (thanks to a relatively slower motion: “the cycle of Fig. 1.18 with a maximum of va = 14.26 m/s in the production phase” (Chapter 12, 1.5.1; and table 1.2: wind speed 10 m/s)), knowing that inflatable balloons seem to undergo a higher power consumption and that Modeling and control of a Magnus effect-based airborne wind energy system in crosswind maneuvers would lead to a too high power consumption, because of a too high linear motion leading to a too high tangential speed which is cubed.
Such a vertical trajectory would be possible with exactly the same material used by Wind Fisher which is a progress in reeling Magnus balloon field.
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A cargo ship can have propulsion as large as 80 MW [according to ChatGPT]. So I would say that even for these «smaller» installations compared to onshore electricity, AWE still needs to scale up seriously. Wind Fisher also, I suppose.
Helium based magnus is interesting though, because handling of lighter than air equipment is perhaps easier to implement on a ship. At least as a stepping stone.
Applied Energy
Volume 113, January 2014, Pages 362-372
Propulsive power contribution of a kite and a Flettner rotor on selected shipping routes
Michael Traut, Paul Gilbert, Conor Walsh, Alice Bows, Antonio Filippone, Peter Stansby, Ruth Wood
Abstract
Wind is a renewable energy source that is freely available on the world’s oceans. As shipping faces the challenge of reducing its dependence on fossil fuels and cutting its carbon emissions this paper seeks to explore the potential for harnessing wind power for shipping. Numerical models of two wind power technologies, a Flettner rotor and a towing kite, are linked with wind data along a set of five trade routes. Wind-generated thrust and propulsive power are computed as a function of local wind and ship velocity. The average wind power contribution on a given route ranges between 193 kW and 373 kW for a single Flettner rotor and between 127 kW and 461 kW for the towing kite. The variability of the power output from the Flettner rotor is shown to be smaller than that from the towing kite while, due to the different dependencies on wind speed and direction, the average power contribution from a Flettner rotor is higher than that from the kite on some routes and lower on others. While for most forms of international cargo shipping wind may not be suitable as the sole source of propulsive energy, a comparison of average output to main engine power requirements of typical vessels serving the routes indicates that it could deliver a significant share. For instance, installing three Flettner rotors on a 5500 dwt general cargo carrier could, on average, provide more than half of the power required by the main engine under typical slow steaming conditions. Uncertainties and simplifying assumptions underlying the model analysis are discussed and implications of the results are considered in light of the urgent need for decarbonisation. This paper demonstrates the significant opportunities for step jump emissions reductions that wind technologies have to offer. It outlines next steps towards realising the potential, highlighting a demand for more detailed studies on socio-economic and technical barriers to implementation, and providing a basis for research into step-change emissions reductions in the shipping sector.
Would it be possible to install both Flettner rotors and towing kites on a single vessel? Would there be interaction between the two systems?
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I’d like to hope these flettner rotors will be successful. It’s always nice when a sexy gadget can save fuel and money. However, I do remain skeptical. I’ve also read of an increase of installations of compressors injecting bubbles below the hull to reduce friction, also said to reduced fuel use by ~10%. Maybe flettner rotors AND bubble injection together could reduce fuel use by 20%! I still perceive a hidden mixed bag of results from the flettner rotors. By now, we’ve seen enough promising articles and predictions for clean energy schemes that end up in the trash can, or at least “going nowhere”. Flettner rotors add weight, possible danger in storms, problems getting under bridges, loading and unloading cargo, and require operational attention and maintenance, not to mention the power used. And when not in use, the extra weight and aero drag would INCREASE fuel use. Kind of like kite-reeling - using power when not producing power. Overall, I’d say even the numbers are overly optimistic, possibly representing certain favorable routes, at best, in an attempt to promote sales.
Greenwashing is a big thing, and may persist for some time, and these days, credits can be “earned” for just the appearance of an attempt at reducing fuel use. And I recently read an article about people being arrested for fraud in a major carbon credit scheme. Is it possible that greed could lead to fraudulent numbers being produced in order to qualify for “green” credits? That shipping companies could be “going through the motions” to sidestep unwarranted scrutiny from critics? Regardless, in the end, if it’s not worth bothering with, it will fade into oblivion, as the flettner rotor concept always has, for over 100 years. Why not just use some more normal sails? Why aren’t wind turbines or airplanes using flettner rotors for their blades or wings? Hmmm… 
