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  #91  
Old 09-02-2010, 05:45 AM
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rwatson rwatson is offline
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Originally Posted by EuroCanal View Post
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.
I dont understand where you get this info from. I invite you to go back and review the details in the is thread, and you will find that the "tacking angle" is extremely good. Figure 53 in the attachment "performance02.jpg" shows that downwind performance is *not* the same as upwind performance. This is based on actual results from the original Flettner Ships.

yes, for optimum performance, you may "tack" downwind, and this is what 80% of high speed yachts (particularly multihulls) do when racing.
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  #92  
Old 09-02-2010, 05:49 AM
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rwatson rwatson is offline
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Originally Posted by DougCim View Post
....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.

~
again - Flettner himself found rotating rotors were not efficient. Why do you persist in berating poor windmill results when eveyone agrees they are not viable ?

Consider - the number of commercially operating rotor ships over the years far exceeds the number of commercially operating wing sail ships.
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  #93  
Old 09-02-2010, 07:05 AM
DougCim DougCim is offline
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Originally Posted by rwatson View Post
again - Flettner himself found rotating rotors were not efficient. Why do you persist in berating poor windmill results when eveyone agrees they are not viable ?
And aircraft, too. Don't forget the aircraft.

Wikipedia says that the Buckau was only sailing with the rotors for about a year, but this portion of the article does not contain any reference source.

I keep coming back to the question of why is Mecaro using spiral fins, when everyone else is still using smooth cylinders.

Quote:
Consider - the number of commercially operating rotor ships over the years far exceeds the number of commercially operating wing sail ships.
I did not know there was any official count of either.
I would guess that most of the wing sailers are race boats.

To that end it could be pointed out that although constantly-controllable-pitch props are much more efficient than fixed props, most small boats are still fitted with fixed props.

Perhaps the reluctance to try any kind of sails is due to the loading sensitivity of large cargo ships? The sails could add extra instability during storms, which would be the worst possible time.
~
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  #94  
Old 09-02-2010, 07:49 AM
ancient kayaker ancient kayaker is offline
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I can think of several reasons why rotors are not common on large cargo ships. There are the thorny questions of effectiveness and ability to ride out a storm. Together with any mast-mounted sail or wing sail, they would constitute an obstruction to loading and unloading, and also would be subject to damage during cargo-handling.

Ship owners and handlers have long since become accustomed to ships that go where they are pointed without need for tacking. So, if engines are going to be needed anyway for all those occasions when thrust from the rotors is unavalable, they constitute an additonal source of cost, failure, and need for specialised crew expertise.
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  #95  
Old 09-02-2010, 07:56 AM
zenex1980 zenex1980 is offline
 
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The basic design of the Flettner rotor has been around for almost 90 years. Flettner applied for his patent on the design in 1922. The Flettner rotor ship Buckau set sail in 1925, first crossing the North Sea and then the Atlantic. While technically successful, the low cost of fuel and the limitations on bearing design made conventional ships more cost effective.

Regards
boatmo
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  #96  
Old 09-02-2010, 10:07 AM
apex1
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Originally Posted by DougCim View Post
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I have understood them all along,

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.
Well, that was far too bold a statement! You clearly DO NOT understand the principle!

Quote:
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.
The answer is yes! Proven already 90 years ago!
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.

~
Well, that: , confused was the right statement!
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  #97  
Old 09-02-2010, 10:25 AM
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philSweet philSweet is online now
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The strangest thing to me is that after five hours of looking, I couldn't find 1 single data point on a Fletner Rotor test that provided the minimum data needed to make an engineering decision. As a minimum one needs-
Rotor geometry
Rotor speed
Wind speed
Magnus force
Drag force
Applied power
Any comparisons with other forms of propulsion need to be done in terms of work (or power), not forces. This has led to a lot of confusion so far with respect to sailboat comparisons. You cannot use lift-to-drag ratio in any meaningful way with respect to Fletner rotors. These are forces. You must use energy. A Fletner rotor is not a sail for the same reason an aircraft engine is not a wing. The sail and wing do not consume power and can be analyzed on a force vector diagram. The rotor and the jet engine cannot be analysed as force vectors because the input power cannot be represented. A better analogy would be to compare the rotor to a heat-pump or air conditioner that transports more heat energy than it consumes and whose efficiency depends on the environmental conditions. The rotor transports more wind energy to the ship than it consumes. The ratio of energy transport to energy consumption depends on environmental conditions.

Last edited by philSweet : 09-02-2010 at 10:47 AM. Reason: inserted "find"
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  #98  
Old 09-02-2010, 10:41 AM
apex1
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Quote:
Originally Posted by philSweet View Post
Quote:
A Fletner rotor is not a sail for the same reason an aircraft engine is not a wing.

The Flettner Rotor IS a sail, simply put.


Quote:
The rotor transports more wind energy to the ship than it consumes. The ratio of energy transport to energy consumption depends on environmental conditions.
Due to the fact, that the energy consumption is nearly constant (you just have to balance a bit of additional friction on the bearings at higher wind forces), that statement does not bite. The system does NOT require substantially higher power at higher wind forces.

Regards
Richard
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  #99  
Old 09-02-2010, 01:05 PM
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philSweet philSweet is online now
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The main environmental factor that I had in mind was apparent wind angle. In reference to your example of wind speed- Power extracted may increase faster than power consumed (I'm holding rotor speed/wind speed constant) as wind speed increases, or it may not. If the vessel is primarily driven by the rotor then it will accelerate, and the vessel's speed increase will contribute to the power calculation. If, instead, the throttle to the props is reduced and constant speed is maintained at reduced fuel consumption, I don't know if power out increases faster than power in (percentagewise). I spent a good bit of time trying to figure this out, but the data doesn't seem to be out there. Magnus force increases with the square of the wind speed. Energy extracted will thus increase with the square of the wind speed if all else is constant. Rotor torque will increase with speed. The torque increase will be greater than linear but I haven't found a reference. I'm going to guess its about 1.7th power based on turbulent flow. The rotor power is torque times rotational speed. So drive power would be a function of the 2.7th power of rotor speed. Thus I think that drive power increases faster that extracted power unless you let the boat accelerate. None of this matters much if the power out is ten times power in at the design sweet-spot. Of course you can choose to apply any amount of torque you wish, but that complicates the formula for predicting Magnus force. The reason I'm spinning up the rotor as wind speed increases is that maximum lift to drag seems to occur at a fairly constant rotor to wind speed relationship. As I said previously, I think this is a flawed method of measurement and comparison, but it is how the tests I could find were structured.

Apex, your post implied a constant rotor speed. When all is said and done, this might turn out to be the best control system design. On or Off. Build it light an run it at its structual limit. The real power out always being more the the power consumed when switched on. Likely to have the shortest ROI this way.
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  #100  
Old 09-02-2010, 01:39 PM
mydauphin mydauphin is offline
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A cargo ships main purpose, use and what it optimize for, is maximum carrying capacity. Anything that prevents this is removed. It is also very important to arrive on schedule and as quickly as possible. Indeed some of these big ships move through the oceans at a very quick clip. A sail powered whether rotor or sail would fail on the first two counts. Indeed the clipper ships, the binnacle of sailing ships were replaced by steam ships. Why?

That said, I would love to stick a couple of Rotors on my Yachtfish but I would be concern about them in a storm. So I believe the KiteSail solution is easier to make work and has less down side once it fully developed.
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  #101  
Old 09-02-2010, 01:58 PM
Clarkey Clarkey is offline
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I think that studies are often focussed on l:d because that is what determines windward ability (in conjunction with the l:d of the hull). Of course, if not sailing upwind l:d becomes less important and you would want to maximise the total force imposed on the rotor by the wind.

It is often cited as a desirable quality that force on the rotor does not increase as the square of the wind speed if rotor revs/min are held constant. Presuming that sufficient power is already being extracted from the wind by the rotor this could well be an advantage in gusty conditions. I guess one valid approach would be to set the rotor surface speed at about 2 x wind speed going upwind and just leave it as you sail about.

Apparent wind angle will depend on the performance of the boat - we already know that the Magnus force will always be perpendicular to the wind direction and the drag force will always be directly downwind.

The NACA paper attached earlier in the thread has some good data but is missing the power required to turn the rotor.

Accepting that I have to put some power into the rotor to make the system work, what type of rotor boat would I like? I can envisage:

Electric river launch - twin rotor, low(ish) air draft, no need to trim sails in flukey conditions, soft gust response. Rotors turning when there is wind to be taken advantage of otherwise power goes to propellor. Twin rotors can provide interesting maneuvering options

Sailing canoe - boat can be run powered up without fear of being massively overpowered in gust. Low drag hull and efficient foils result in great windward performance. Sailor can concentrate on hiking and steering. Rotor might be human powered, should be buoyant to assist capsize recovery.

Trawler yacht - rotor provides some thrust in favourable conditions and functions as a steadying sail to moderate roll.


Quote:
Originally Posted by philSweet View Post
The main environmental factor that I had in mind was apparent wind angle. In reference to your example of wind speed- Power extracted may increase faster than power consumed (I'm holding rotor speed/wind speed constant) as wind speed increases, or it may not. If the vessel is primarily driven by the rotor then it will accelerate, and the vessel's speed increase will contribute to the power calculation. If, instead, the throttle to the props is reduced and constant speed is maintained at reduced fuel consumption, I don't know if power out increases faster than power in (percentagewise). I spent a good bit of time trying to figure this out, but the data doesn't seem to be out there. Magnus force increases with the square of the wind speed. Energy extracted will thus increase with the square of the wind speed if all else is constant. Rotor torque will increase with speed. The torque increase will be greater than linear but I haven't found a reference. I'm going to guess its about 1.7th power based on turbulent flow. The rotor power is torque times rotational speed. So drive power would be a function of the 2.7th power of rotor speed. Thus I think that drive power increases faster that extracted power unless you let the boat accelerate. None of this matters much if the power out is ten times power in at the design sweet-spot. Of course you can choose to apply any amount of torque you wish, but that complicates the formula for predicting Magnus force. The reason I'm spinning up the rotor as wind speed increases is that maximum lift to drag seems to occur at a fairly constant rotor to wind speed relationship. As I said previously, I think this is a flawed method of measurement and comparison, but it is how the tests I could find were structured.

Apex, your post implied a constant rotor speed. When all is said and done, this might turn out to be the best control system design. On or Off. Build it light an run it at its structual limit. The real power out always being more the the power consumed when switched on. Likely to have the shortest ROI this way.
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  #102  
Old 09-02-2010, 02:17 PM
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philSweet philSweet is online now
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I missed the gust response issue completely. Thanks.
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  #103  
Old 09-02-2010, 02:37 PM
Clarkey Clarkey is offline
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I think it is these differences from conventional sails/wings that make rotors interesting.

There is some great footage of the Buckau sailing on the Pathe news archive website by the way. Performance looks good - remember it only has a 45hp diesel, somehow I can't imagine it going that fast if it was just hooked up to a propellor! Also check out the flags in some of the views - pointing pretty well!

Several more clips at the bottom of the page, you might have to sit through a couple of ads.

http://www.britishpathe.com/record.php?id=21533

ETA: the people writing the subtitles have no idea how it works.





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I missed the gust response issue completely. Thanks.
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  #104  
Old 09-02-2010, 03:22 PM
tspeer tspeer is offline
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Originally Posted by apex1 View Post
Reportedly she did perform well. No real wonder, the "turbo sail" is nothing else but a Flettner rotor. Cousteau developed a new name, nothing else.
Neither Cousteau's catamaran, Moulin a Vent, nor Alcyone used a Flettner rotor or anything like it. The Alcyone used a turbosail, which had a fixed cylinder and a flap with sharp trailing edge that could be positioned at any point around the cylinder. It used boundary layer suction to avoid separation on the cylinder. This is an entirely different principle of operation from the Flettner rotor. It is essentially a conventional wing with a very large thickness ratio. It has more in common with a Griffith airfoil than a Flettner rotor.

The Flettner rotor sounds great when you see the large lift coefficients. But the problem is the rotor produces a lot of drag. And sailing performance depends on the lift/drag ratio. Take a look at Figure 5 from NACA-TN-228, "The Flettner rotor ship in the light of the Kutta-Joukowski theory and of experimental results". The rotor can produce a section lift coefficient of 9, but has a minimum drag coefficient of 0.5! The maximum sectional lift/drag ratio is less than 8, which is an order of magnitude less than for a wing section. You can make the wing 10 times bigger and still have less drag, and the benefit of the large lift coefficients vanishes.

The paper claims a much reduced heeling moment from the shorter rotors, but that doesn't account for the impact on induced drag. The induced drag depends on the square of the span for a rotor, just like a non-rotating wing. So if the span reduced considerably - almost a must for practical reasons with the rotating machinery - then the induced drag skyrockets.

The Flettner rotor has one big advantage as a sail: you can turn it off. The maximum lift at a given rotor rpm increases with wind speed, then flattens out and does not increase appreciably for higher winds. So in a blow, the ship can regulate the maximum force by controlling the rpm. The other advantage of a rotor is it doesn't obstruct much deck space, compared to the sweep of a boom.

The turbosail would share many of these advantages, as the additional sweep of the turbosail trailing edge is not great and the boundary layer control can be turned off to stall the rig. But it would have high windage compared to a feathered wingmast. And it would incur the induced drag penalty of using a smaller span than a conventional rig if made to the same proportions as Alcyone.
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  #105  
Old 09-02-2010, 03:29 PM
EuroCanal EuroCanal is offline
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Originally Posted by philSweet View Post
The strangest thing to me is that after five hours of looking, I couldn't find 1 single data point on a Fletner Rotor test that provided the minimum data needed to make an engineering decision. As a minimum one needs-
Rotor geometry
Rotor speed
Wind speed
Magnus force
Drag force
Applied power
Any comparisons with other forms of propulsion need to be done in terms of work (or power), not forces. This has led to a lot of confusion so far with respect to sailboat comparisons. You cannot use lift-to-drag ratio in any meaningful way with respect to Fletner rotors. These are forces. You must use energy. A Fletner rotor is not a sail for the same reason an aircraft engine is not a wing. The sail and wing do not consume power and can be analyzed on a force vector diagram. The rotor and the jet engine cannot be analysed as force vectors because the input power cannot be represented. A better analogy would be to compare the rotor to a heat-pump or air conditioner that transports more heat energy than it consumes and whose efficiency depends on the environmental conditions. The rotor transports more wind energy to the ship than it consumes. The ratio of energy transport to energy consumption depends on environmental conditions.
Rotor geometry - it's a cylinder

Rotor speed - (s) as fast as possible without stalling the air flow, increases in proportion to V.

Wind speed - (V) any

Magnus force - L = rho . V . G (per unit length)
rho = fluid density
V = relative wind speed
G = pi^2 . d^2 . s
d = rotor diameter
s = rotational speed

Drag force - D = rho . V^2 . d / 4 (per unit length)

Applied power - almost nothing - just enough to overcome skin friction

This assumes ideal flow. You can get more accurate results using 'FoilSim III' which you can use on this site:

http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html

Typical result, for a 5m high cylinder, 2m diameter:

Windspeed___Rotorspeed____Lift______Drag
_100 kph____75 rpm____8,328 N____3,717 N
__80 kph____60 rpm____5,338 N____2,148 N
__60 kph____45 rpm____3,002 N____1,075 N
__40 kph____30 rpm____1,334 N______402 N
__20 kph____15 rpm______340 N_______74 N


Checking the formulae above for 100 kph gives:
V=27.78 m/s
s=1.25 rps
rho=1.224
L = 1,678 . 5 = 8389 N
- surprisingly close!
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