# Everything Old is new again - Flettner Rotor Ship is launched

Discussion in 'Boat Design' started by rwatson, Sep 1, 2008.

1. ### apex1Guest

Doug,

the lift of the rotor is provided by wind, not engine power. You need only a few hp to rotate the rotor but gain several hundred hp of lift. (depending on size of course)

There is no backward conversion. These rotors are sails.

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### ancient kayakeraka Terry Haines

I don't understand the "overbalanced wheel" analogy.

It works the same as spin on a tennis or golf ball. The air hitting the rotating surface is partially deflected to one side by the spin. As the air moves round both sides of the curved surface it forms a partial vacuum on each side. Without the spin, the airflow is divided equally on both sides and the lift cancels out, but with the spin the airflow is displaced more to one side providing lift.

The amount of lift can be an appreciable fraction of the weight of the ball, but very little power is needed to keep the rotor spinning. A golf ball hit with a driver or a tennis ball hit for a backspin drop shot will hang in the air for a surprisingly long time without its rotation failing.

Drag is at a right angle to the lift, like a sail or wing. Both force vectors pass close to the center of rotation, so neither forms enough torque to effect rotation significantly. That is why very little power is needed to maintain power, and is also the reason why any useful energy extracted from the system comes from the wind not from the rotational drive.

In the case of the tennis or golf ball the energy comes from the forward motion of the ball. in the case of a tennis ball, the slowing of the ball - compared to a ball hit without spin - "a flat ball" - is very noticeable.

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### rwatsonSenior Member

Some comments remind me of the "what is an elephant like" story - it depends on what part of the elephant you observe.

Its true that the flettner rotor *must* have at least the "spin power" to get the rotor going. Whether its a "sailing boat" or not, is like asking if the super yachts that have diesel powered canting keels are sailing yachts or not, or whether motor cruisers are yachts or not. The classification is of academic interest only. Dont forget, for reasonably large sailing boats, the effort of raising, trimming, furling, controlling and stowing sails also requires quite a lot of "power" (be it man or machine)

However, if you wanted to create a purist, 'self sufficient vessel', you wouldnt only use solar power as the "spin source", as the suns input isnt co-dependant on the boats propulsive method. You might have a sunny day to produce lots of spin, but no wind - or you might have a cloudy day, with lots of wind but no power to spin the rotors from sunlight.

What you would do is to generate "spin power" from a propeller driven generator after the boat started moving - the faster the boat moved, the more rotor spinning power you would want, and would then have available. The amount of power to make the rotors spin, would be less than the power to move the vessel, especially as the wind increases. If a bit of excess power is reserved in a battery to re-start the rotors -you have still achieved 'self sufficiency'.

The really important question, is whether the 'cost' (power requirements) to spin the rotors is worth the increase in the boats performance.

The answer is easy - if the examples of vessels from reliable experiments ( eg the Buckeau, the Tracker) derive savings in fuel, then the answer is yes! If the total fuel usage (to drive primary and rotor motors) results in moving a boat faster with less fuel with the rotors, then the case is clear.

Then comes the economics :-
If a diesel powered freighter fits sails, and finds it uses 200 litres less fuel than it usually does in a comparable voyage - then its only the cost of fitting and manning the sails versus 200 litres of fuel that needs to be calculated.

If a boat gets better improved performance with rotors, then its only the cost of fitting and controlling the rotors versus savings in fuel that is to be considered. The Tracker story indicates the potential very well.

To my mind, the only challenge is whether there are enough boats that travel in conditions that would save sufficient fuel (money) or benefit sufficiently from ease of handling to warrant the further development of "rotor sails".

30 knot wave piercinng catamarans are probably not a likely candidate for a rotor, but heavy cargo carrying inter-island craft in favourable wind patterns may deliver more cargo, faster and cheaper with a rotor or two.

An elderly motor-sailor who wishes to save money, and be able to "furl" sail in seconds with the movement of a hand, may also be a candidate for a rotor ship. Consider also the convenience of having "sails" that never need changing in varying conditions, never need setting or stowiing, and are fully controlled from a sheltered navigation position.

My opinion is that although rotors are not a widely understood or comprehensively documented methodology, there is plenty of science and evidence to indicate real potential to add considerable value to a wide variety of marine craft (yachts or not)

4. ### dskiraPrevious Member

It is the windage which will concern me.
North Atlantic in winter, force nine is not unusual, how the ship or yacht, mostly yacht, will behave with all the area up in the air, without possibility of reefing.
it is the reason why, I agree with Richard, the kite assisted is a better solution.
Some me members have been kind to explain me why the kite system work (I was in denial), and I am impress.
Daniel

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### TackwiseMember

The amount of lift of a rotor depends on the prevailing wind speed and the rotational speed of the rotor!

There is no limit imposed by the motors input power on the lift created other than that the power of the motor should be sufficient to rotate the rotor.

So next Q: How much power is needed to rotate the rotor?

Consider the rotor on a calm day (no wind). It is now possible to rotate the rotor without it creating any lift! The power required to rotate is now only dependant on the frictional resistance of the bearings. This is very small, and can be compared to the force needed to turn a merry go round on a childrens playground (same weight of course). As you will know little effort is required if the children are all seated in balance.

Once wind starts to blow a lift force is created on the rotor. This lift force is transmitted through the bearings onto the ship. This force on the bearings will increase the frictional force of the bearings. The same as with the merry go round with all children happily on one side it increases the effort required to turn!

Now I have not done the math on this, however I do not think the added friction of a loaded rotor will differ to much from that of an unloaded rotor.

I hope my analogy was good enough to get the general idea across?

Aha! Well I believe you can!
Under the boat you will need to install a small water turbine of some sort. The forward propulsion of the ship (created by the rotors lift) should then be able to generate ample electricity to power the electric motor turning the rotor....

Tackwise

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### ClarkeyJunior Member

The 'Buckau' successfully crossed the atlantic, weathering some gales on the way. The windage of the rotors was less than that of the original masts and rigging, the rotors were also lower.

The 'Barbara' was in service for some years and did not seem to have any particular problem.

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### EuroCanalJunior Member

Fill the tubes with foam, then at least it will be self-righting.

8. ### apex1Guest

There is less windage than on a bare mast and rigg Daniel. Although it looks like a severe mass it is not. And a cylinder has not much resistance.

Regards
Richard

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### srimesSenior Member

what's the advantage vs. a wing sail?

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### DougCimJunior Member

This question is the heart of the matter.

Flettner/Magnus rotors have a couple very odd characteristics, in that they take up a very small amount of deck space, and they don't require adjusting for the wind. In every other measure of efficiency, they do quite poorly compared to fabric or rigid-sing sails.

The Enercon ship was built mainly as a publicity stunt, it wasn't meant to bear serious scientific scrutiny. If Enercon had wanted to do something really impressive, they should have built a true motorsailer with rigid wing sails, and used the motors only when maneuvering in port--instead of using low-efficiency Magnus rotors to "augment" the constantly-run engines.
~

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### ClarkeyJunior Member

I take it you have managed to understand how they work then?

What measures of efficiency are you using to compare them to fabric or rigid-wing sails?

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### EuroCanalJunior Member

Another big disadvantage is the limited range of wind direction. You can't run at much of an angle into the wind, and you can't run downwind either. It only works when you're more or less abeam of the wind.

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### DougCimJunior Member

I have understood them all along, and most aircraft, ship and windmill engineers have understood them as well, which is why you don't see very many of them down at the marina. It is others who have not.

The easiest way to consider their true efficiency is the windmill example, because both the input and output power are expressed in the same units of measurement--watts. This is no less valid than comparing boat use, since both are trying to do the same thing--divert power from the passing wind into useful energy. And compared that way to airfoil blade windmills, the Magnus windmill does pretty poorly.

You can stick a Magnus rotor on a boat and say that "it helps save fuel" but the question most ordinary people would have is "does the Magnus rotor help more than any other type of sail would?" and the answer is, no.

I also am amused that if we were to discuss fabric or rigid-wing sails, it is very easy to present evidence that they work very well as aircraft lifting surfaces, as windmill rotors, and as boat propulsion.
(granted, windmill rotors aren't made from fabric much anymore, but there's ample historical evidence it can be done)
....Yet when the subject turns to Magnus rotors, people will admit that they don't work well as aircraft lifting surfaces, don't work well as windmill rotors--but for some odd reason they still think Magnus rotors will work really good as boat propulsion.

~

Last edited: Sep 1, 2010
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### ClarkeyJunior Member

I don't think that anyone has claimed that Flettner rotors offer raw performance advantages over wing or fabric sails, although it seems perfectly adequate. I believe that their advantages lie in ease of handling and automation, together with the fact that the rig can be physically small.

They produce more lift per unit projected area than either soft or wing sails and the l/d ratio (about 8:1 in ideal circumstances) is higher than any soft rig I can think of - hence the excellent windward performance noted in rotor sail equipped boats. Another useful property is the thrust tends to a constant value as windspeed increases (for a fixed rotational speed) rather than increasing in an exponential fashion like conventional rigs.

Against all of this is the need to supply a relatively small amount of power to spin the rotor - I don't think that sailing a rotor-equipped boat would be any fun either.

ETA - It is evidently true that rigid wing sails are more efficient than soft rigs - but you don't see many of them in marinas either. Many people obviously rate ease of handling higher than raw efficiency - maybe Enercon are the same?

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