Magnus effect for ship propulsion

Discussion in 'Propulsion' started by 1J1, Mar 6, 2017.

  1. philSweet
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    philSweet Senior Member

    But they aren't, and that is the fundamental physical problem with these things. They don't actually create the sort of vortex field that we image they do based on analogy to the flow field decomposition studied in wing lift. There is no far field approximation, and there isn't any dipole either. The lack of a farfield effect is precisely why these things don't analyze or scale "properly" with respect to Reynolds numbers or other typical dimensionless aero coefficients. I'll dredge up some supporting info and add it when I can.
     
  2. rwatson
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    rwatson Senior Member

    Not quite.

    None of the commercial ships using the Rotors had keels, because the Lift is very perpendicular to the air flow, to the extent that on a reach, the ship heels INTO the wind, not away from it.

    On any Reach, ( the most efficient sail state) the propulsion vector is almost totally along the keel line.

    This is a key difference to conventional sails.
     
  3. daiquiri
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    daiquiri Engineering and Design

    The effect on the flow lines vanishes in the far field, but the bound circulation doesn't. It remains constant at whatever finite distance it is measured. And the bound circulation creates lift.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Actually, that is just a difference between flettner rotor sailboats and flettner-assisted motor boats.

    Regardless of the boat type, a propulsion (or assistance) by wind requires two fluids flowing at an angle relative to each other. Like wind and water. A sailboat has sails (or a flettner rotor) and keel, each working in their own fluid flow, at an angle to its counterpart fluid flow. A motorboat has a propeller, and a sail (or a flettner rotor). In each case the rudder negotiates the direction.

    Only in very particular points of sailing the underwater counterpart is not necessary, like for example the downwind sailing of the classic sailboat.
     
  5. rwatson
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    rwatson Senior Member

    A Rotor Ship can sail directly East if the wind is blowing from the North, with NO keel. There will be some 'slip' to the South from the Drag Vector, but not a lot.
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    It's a very academic discussion about an invention that never worked. There are many inventions of this kind; the Darrieus rotor, the 6 strokes engine, the quasi turbine and the list is extremely long.
    The idea by Flettner has almost 100 years, and the trials begun almost 90 years ago, with none result good enough to have applications, even in a very restraint domain. So it's useless.

    Let's compare for example with the Diesel engine; brevet in 1892, first engine 20 HP working well in 1897, first commercial engine 1900, first submarine with diesel engine 1902, generators and industrial applications from 1901, after WW1 the engine was used in all kind of boats and ships, trucks and cars in the 1920-30 and so on. It can work with a large variety of combustibles, it can be scaled from 2 HP to 110000 HP. It can be made cheaply in iron with little means, or very sophisticated. It's a successful and versatile invention...
     
  7. rwatson
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    rwatson Senior Member

  8. Ilan Voyager
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    Ilan Voyager Senior Member

    I followed the affair. They got 5% of savings on the prototype, so it's not economically feasible. The report of the trials is just an advertisement.
    With money poured by the EU to Norsepower and Bore they made the second on the M/V Straden in November 2015.
    After total silence from the two companies for almost a year. The savings for the Straden were 6.1%, desperately too close to the results of the first boat, not enough to be economically interesting for a normal company. To have a return in 4 years the saving must be of 20% with conditions very hard to obtain.
    So let's say that a solid 12% and 8 years for the return may be interesting if the EU gives a big help. For a company self funding or taking a loan from a bank it's very risky bet.
    Norsepower got 2.6 millions euros in August 2016 from the EU and Norsepower announces the 25/01/2017 that the M/S Viking Grace will be equipped. It's a ferry between Turku and Stockholm where there are big concerns of pollution of the Baltic Sea, and where the ferries being vital are heavily subsidized by Sweden and Finland.
    In clear words Viking does not spend a cent, and the operation is done by Norsepower with EU money, ie taxpayer money.
    We are in a very extreme case. And for the while very few people of the shipping companies are enthusiastic about these rotors.
    The new IMO rules about CO2 monitoring which will apply on 2019 will put pressure on the shipping companies. The days of the cheap bunker are counted, ships are very big polluters, the motorists are working very hard. There is now a place for the new technologies.
    A competitor is Skysails, their solution is far simpler and cheaper than Norsepower. But also there are severe constraints.
    With the fares of shipping going down, plus a big Korean shipping company in bankruptcy, no company felt the urgency of tickling with rotors or kites. Maesk and others slowed down the speed of the ships for saving fuel and are waiting for better days.
    We'll see what happens. Trump is another unknown nobody needed in the global trade. A recession caused by a dispute between China and the USA and hundreds of boats have no more use...
     
  9. daiquiri
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    daiquiri Engineering and Design

    Well... A classic sailboat can run downwind with no keel.

    Does this fact prove that a sailboat doesn't need a keel? ;)
     
  10. rwatson
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    rwatson Senior Member

    But a 'classic' sailboat has a much more angled drive vector.

    If a standard sail boat put its sails at right angles to the wind, but still maintained the curve in the sails, it could also proceed directly at right angles to the beam wind, using the 'lift' effect alone.

    But, a standard sail is usually at around 45 degrees to the beam wind to
    a) Keep the shape in the sails
    b) take advantage of the 'drag' component, like the blades of a windmill. The lift component over the front of the sail is a much smaller component of the overall drive.

    As a result, standard yachts benefit greatly from a keel to resist the 'sideways' movement induced by the drag pressure, to translate it into forward movement.
     
  11. daiquiri
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    daiquiri Engineering and Design

    So what does a flettner-rotor boat do when the apparent wind comes from, say, 45 degrees abaft?
     
  12. rwatson
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    rwatson Senior Member

    Its all in the figures posted throughout the old thread.

    The wind at 45 off dead astern produces a Lift Vector at 45 degrees off dead ahead of course. This is where a keel could help recover some power towards the directed course, but in practice on the cargo style hulls, the lift from a keel at under 8 knots would be marginal at best, and just create more drag.
     

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  13. daiquiri
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    daiquiri Engineering and Design

    So how does a ship with a lift vector tilted 45° to the course keep the desired direction?

    Which leads to the second question: let's say that this time true wind comes from 90°. The apparent wind will come from, say, 60-70°. Can the system work without the underwater counterpart of the rotor?
     
  14. 1J1
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    1J1 Senior Member

    Need to say that the cargo rotor ship E-SHIP 1 has 3 rudders:


    (look from 3:23)
     

  15. daiquiri
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    daiquiri Engineering and Design

    The point which I really need to stress out is that any wind-driven vehicle requires two media in relative motion to each other, if it is to be controllable.

    The two fluids, in our case, are air (wind) and water. Without the second fluid, in motion relative to the first one, it would be like a feather pushed by the wind - would be just dragged along by the wind.

    The second fluid allows to generate an equilibrating force and moment around the CoG. A sailboat has a keel and a rudder moving through water (which is at a different angle and speed relative to the wind) for that purpose. A motor boat has a rudder and the propeller thrust.

    Hence, the flettner rotor alone is not sufficient - except for some very particular combination of wind, heading and speed when forces and moments of the rotor and the bare hull are in equilibrium. An underwater balancing counterpart is required - be it a keel/rudder or a prop/rudder.
     
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