# Collapsible Flettner Rotor Project

Discussion in 'Projects & Proposals' started by Yobarnacle, Jun 4, 2014.

1. Joined: Nov 2011
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### YobarnacleSenior Member holding true course

I understand how you arrived at the rotor dimensions. I can follow the math, though my understanding of the aeronautics is dim and strained.

To begin boxing the compass of relative wind directions, I have taken some graphical shortcuts.

I used the rotor calculator Rwatson posted, and I read that equal or greater lift is provided from the 30 degrees abaft thru 10 degrees forward of the beam. Actually, a little mental interpolation concludes adequate lift available to maintain 6 kts or better thru 20 degrees forward of beam. Closer to the wind the lift starts diminishing with rapidity.

I did a little apparent wind vector polar graph. At 6 kts and true wind 20kts from 120 deg relative, the apparent wind is 102 deg rel at 17.5 kts.
The reduction in wind velocity is likely compensated for by the increased lift as you approach beam on, so I'm confident at least 6 knots is manageable.

As apparent wind draws forward the apparent strength increases. Again, providing more lift. The conclusion is, this 1 meter dia rotor would propel the vessel at 6 kts or better through 100 degrees of relative wind direction. (50* either side).

Assuming sailing closer than 30* to the wind not a realistic expectation and tacking downwind is a normal speed enhancing stratagem, again 30 degrees either side of dead downwind. A total of 120 degrees more need not be calculated.
All together, I just reduced the workload through 220 degres of arc.
Only need to calculate the remaining 140 degrees which is symmetrical so only calculations for 70 degrees of arc need be performed.

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2. Joined: Nov 2011
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### YobarnacleSenior Member holding true course

Here's a blank rapid plot radar work sheet for anyone wanting one.

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3. Joined: Nov 2011
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### YobarnacleSenior Member holding true course

Oh! 17.5 kts relative wind is 20mph in the calculator a couple posts back. Just in case somebody thinks I'm mixing apples and oranges again!

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### YobarnacleSenior Member holding true course

Here is Per Brohalls nomograph for power, resistnce, range ect for Albin 25.

She requires little force to make 4 kts.

If she can sail close hauled at 4 kts, that's GOOD!

I'll do some figuring on 6 kts and see how far she falls short with the rotor.

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5. Joined: Nov 2011
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### YobarnacleSenior Member holding true course

Here is sorta what she'll look like, I think. The tripod spars are same height as original sailplan.

The two forward mast supports will have roller furling genoas attached to aft side. Not tacked to deck. All tension of luff is along the spar.
It's my answer to downwind. Wing and wing.

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6. Joined: Nov 2011
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### YobarnacleSenior Member holding true course

daiquiri
In the calculations you did, and again I thank you, there are several things I'm not understanding.
I follow how you arrived at the diameter and height of rotor. I'm not seeing where in your figures you compute the RPM. suddenly it's there.

Likewise with the PROT. Power to rotate I presume. Suddenly prot has the value 3413 watts. Conversion to HP I understand. 746 watts per. You rounded a little.

I imagine the RPM and the PROT must of had some different labels in the calculations is why I don't see them.

Another question for anybody. In the rotor calculator, there is lift and drag. So windage on the rotor is already accounted for. The boats resistance in the water 1200n we get from Per Brohalls graph.
isn't there wind resistance on the above water hull, house and rigging that needs to be compensated for with power? Wind abaft the beam probably negligible or even helps rather than hinders. but once the apparent wind is forward of the beam, how do I calculate or allow for that impediment?

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Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

You are correct. The wind resistance of the above-water structure should be included in the calcs, as well as the induced resistance of the keel. The later one is due to the fact that rotor produces not only the drive force but also the side force, and the keel has to produce lift in order to balance this side force. By producing lift, it also produces an additional drag, which is called "induced drag", which has to be added to the drag curve published by Per Brohalls.

The drive force is calculated as Fdrive=Faer cos(phi), while side force is Fside=Faer sin(phi), where phi is the angle explained in my calc sheet. So the lift of the keel has to be equal to Fside.

But, in order to calculate the resistance due to the keel, I would need to know what does the keel look like? Is it the full-length shallow keel, or will there be a daggerboard or similar?

The calculation of the aerodynamic resistance is explained pretty well in this discussion: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/aerodynamic-drag-coefficients-38479.html

Cheers

8. Joined: May 2004
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### daiquiriEngineering and Design

RPM comes from the Omega value in my calc sheet. Omega is the so-called "angular velocity", and is measured in radians per second (rad/s). One full turn has 2Pi radians, so omega/(2 Pi) gives revs per second. Multiply this by 60 seconds and you get revs per minute (RPM).
Hence:
RPM = Omega * 60 / (2Pi) ​
Simple, isn't it?

Cheers

9. Joined: Aug 2007
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### rwatsonSenior Member

Have you guys notice the table at the bottom of the calculator that gives the forces at different wind angles ?

note too, that the majority of drag on the rotor is at right angles to the wind. So, in some cases it actually assists the direction of travel.

Its a bit clunky, but if you enter the rpm that matches one of the columns calculated in the top half of the sheet (into C32 ) it will choose on of the performance columns on row 18 ( 100 - 1000 rpm ) and calculate the vectors at different angles.

In the attached example, the biggest propulsive force is 10 degrees behind right angles.

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10. Joined: Aug 2007
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### rwatsonSenior Member

You know, on reflection, this might be a real gold mine. I suspect that crazy rotor driven sailing ship would attract a lot of attention from shore. Both Costeau and Flettner got huge crowds when they arrived in the US.

With a brightly painted rotor, on a public holiday, you might get some interested sponsors, and a few news articles explaining the physics behind the idea, could make the whole exercise an educational experience for people of all ages - whether the rotors were actually better or not than sails.

Now, if you can just find a sponsor whose logo looks good spinning at 200 rpm

Kai Rabenstein likes this.
11. Joined: Aug 2007
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### rwatsonSenior Member

Very innovative

Just for illustrative purposes, i have attached some pictures from
http://www.boatdesign.net/forums/bo...r-rotor-ship-launched-24081-2.html#post393067

giving some detail on the build detail an actual rotor installed on a similar sized boat.

It might be worth perusing for more brain food on the topic.

Edit - Hooray for Google books. i accidentally found the Popular mechanics article online about the "Tracker"

I put it here for future reference

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12. Joined: Nov 2011
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### YobarnacleSenior Member holding true course

I'm surprised nobody has mentioned the 'yellow' rotor on my sketch looks like it's illuminated. I don't want a yellow rotor. I was trying to hint it might be internally lit. High visibility. Each of my kids when they first began to drive, were given a huge yellow stationwagon. A high visibility TANK!

In addition I'm a big believer in AIS for yachts. Let's you be SEEN!

13. Joined: Aug 2007
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### rwatsonSenior Member

Thanks for your invaluable contributions on this thread Mr D. I am learning heaps.

I enjoyed that thread on Resistance you provided.

I wonder if one day you could add some calcs to the dreaded 'rotor calculator' to deduct approximate PROT from the estimated drive forces.

That is always an important part of the system.

14. Joined: Aug 2007
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### rwatsonSenior Member

If I had a rotor ship, I would want to show it off ( I think )

On second thoughts, you could have an animated picture that worked by displaying several frames as the rotor worked.

Maybe you could do something with the attached image

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15. Joined: Nov 2011
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### YobarnacleSenior Member holding true course

Hand symbols can mean different things in different cultures. The american "A-OK' and the roman "thumbs up" are both obscenities in a few cultures.

Maybe the Vulcan "live long and prosper" hand symbol, which I can't do.

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