Soft Wing Sail

Hello Pericles.

You are right. This is the way we usually use to compare efficiency, however, when dealing with wing sails there are some more questions.

The first question is how you measure sail area of a wing. Is it right to use the cross section area of the wing or you should calculate all around sail cloth area.
Omer wing sail cross section sail area is about 85% of the original sail area of the same (standard) boat one can find at the manufacturers brochure. If you calculate all around sail area, it will sum up to 170%.

To make it a bit more complicated, with Omer wing sail you need no spinnaker. Boat speed down wind is better than the standard boat using spinnaker. No good way to compare if you use only the criteria of sail area.

Another very important element is upwind performance. With Omer wing sail you can point up wind much better than with the standard sail. Is it not also an element of what we mean by efficiency of a sail (or rig)?

Finally, if you ask me, it is only a question of a definition that has not been done yet for wing sails.

Ilan

 

Ilan,

Sailing upwind! If you can point higher than a conventional rigged boat and equal or improve boat speed, then that is a very good measurement of efficiency. As for measuring both sides of your Soft Wing sail, I don't think that is necessary. We only measure one side of a conventional sail.

As for cross section of the Soft Wing, if you mean wing chord cross section, then from what I have read, the Mean Aerodynamic Chord MAC needs only to be what you choose to make it, based upon your knowledge of aeronautics. However, it seems the MAC is not critical as wind speeds are low compared with aircraft and loss of lift is not going to cause an aircrash.

I really like what you have created. In terms of innovation, you are up there with Maltese Falcon and I wish you the very best of good luck.

Regards,


Pericles

 

Ilan:
You have done an interesting job of developing the wing-sail concept.
You wrote:
<<To make it a bit more complicated, with Omer wing sail you need no spinnaker. Boat speed down wind is better than the standard boat using spinnaker.>>

IMHO, in a non-planing, monohull boat where the wind condition is below that sufficient to generate hull speed (often the case in real world sailing,) there is no substitute for square area downwind. Could you please explain your claim in engineering terms?
Thanks and best regards,
- Bill Hansen

 

Basically because the wingsail has a proper aerodynamic section shape (i.e. it has a third dimension which is thickness) it is more efficient than a conventional sail. My research has shown it to be between 1.5 and 1.8 times more efficient, so wingsail areas can be reduced by that same amount to equal the conventional sail.
I’m attaching two articles The PDF one is part of a thesis – read the section entitled Wing versus sail.
The other word document I got from the net somewhere (apologies because I cannot give the author credit ) and that gives a more straight forward answer.
Take care!

 

Down wind sailing with a wing sail

The wing rotates spontaneously into the wind and the wings leading edge is always facing the wind.
This is also the case in down wind sailing, where the leading edge is facing backwards and the boom is pointing forward (please have a look at www.omerwingsail.com ,gallery, there is a picture that shows it. Also have a look at Wings Vs. Sails / air flow and driving force. There are some drawings that make it clearer).
This way, you keep on using the lift force of the wing, instead of using drag like we do in sails and spinnakers.
In general, lift is more efficient than drag, becaus it is bigger and has relatively no turbulence.
Hope I made my self clear although it is not in engineering terms.

Ilan

 

Other Wing Sail discussions

...a few other wing sail discussions

http://boatdesign.net/forums/showthread.php?t=16911

http://boatdesign.net/forums/showthread.php?t=6698http://boatdesign.net/forums/showthread.php?t=6698

 

Ilan:
Maybe I'm not understanding you or making myself clear so I'll challenge you to a race:
Equal non-planing monohull 'lead mines.'
We'll both have identical wing sails and sail a downwind run in light-to-medium wind.
I'll also have a spinnaker. But, you won't...
OK?

I'm the last to denigrate wing sails having spent a lifetime making them and using them. I just have a problem with the claim that a double-sided wing sail eliminates the need for adding projected area (spinnakers) downwind on a traditional mono-hulled sailboat.
- Bill

 

Bill,
According to my experience (which is yet limited since I have just finished the last version of the wing sail), it is right to say that there is no need for a spinnaker.
In the few times I "raced" (not officially) down wind, the boat was at least as fast as others flying a spinnaker, at wind angles of plus minus 30 degrees and more.
If you have to sail directly down wind, you can always sheet in, have the wing perpendicular to the wind, and use the wing for drag the way you use a spinnaker.
Since the wing in this case is almost flat (you can of course trim the wing to a small camber as well), the effective sail area used for drag is very large.
Best regards
Ilan

 

Ilan:
Ok. That's your story and you are sticking to it.
Congratulations on a nice construct of a wing sail in any case.
FYI, here is a way we did it 20 years ago:
Sail System #4,704,979
http://patft.uspto.gov/netacgi/nph-...TI.&OS=TTL/"sail system"&RS=TTL/"sail system"
Best regards,
- Bill

 

Lift/drag ratio is a very important metric because it directly relates to the speed of the boat. However, it is best to include all the windage of the hull, rigging and crew when calculating the lift/drag ratio, because that is the number that determines the boat's performance. Because of the windage, its best to operate the rig at a higher lift than is optimum for the rig alone.

Another metric might be the lift/drag ratio times the ratio of the metacentric height divided by the height of the rig's center of effort. This would bring in the element of the boat's stability, because in the higher wind ranges what matters is how efficient the rig is for a fixed heel angle.

I think both of these metrics show that the optimization of the rig has to be considered in the context of the whole boat as a system, and not the rig in isolation.

 

tspeer,

You mention rigging drag and angle of heel as two factors in calculating efficiency. As you can see on the Omer site the Soft Wing sail operates on a free standing mast, which it appears,turns easily so as to point the leading edge into the wind. That means less rigging drag. As for angle of heel, the Soft Wing mast is 3 metres taller than the standard Elan 37 mast, but the sail plan area was 15% less. That was found to be too much, so the area was reduced another 10% down to 23.5% of the original conventional sail plan.

Even so, Ilan still reefs down at 15knots true to keep the boat upright. In light airs the boat exceeds wind speed. As the boat is in Turkey, perhaps a forumite or two will contact Ilan for a boat ride.

Pericles

 

Pericles

I see that you really know all about Omer wing sail.

Also you are right about less windage, however it is not only because of pointing into the wind. The boat has no stays, therefore much less windage (aerodynamic drag on stays is usually much bigger than what we think it is).

You are very very welcome to have a ride (and to know each other). I am in Marmaris / Turkey. Just let me know when.

Ilan

 

Take care, indeed. The Word document is utter rubbish.

Here is the highest lift (CLmax=3.06), and highest lift/drag ratio (600:1), single-element (no slot) section I 've ever seen. What does it say about the notion that thickness itself makes for a better section? Or that the difference in distance along the two surfaces is what produces lift?



If the Word document were correct, a staysail would produce no lift. It's a mystery to me why people perpetuate aerodynamic theories that are so obviously contradicted by everyday experience.

 

Tom

I totally agree.

Back to soft wing sails;
In soft wing sail, it is impossible to have very high CL numbers. You can't have this very nice, smooth and exact airfoil shape.

I think that the aerodynamic benefits of wing sails over sails are in the following order:
A; better L/D
B; the airfoil is in the right angle of attack and and the air flow is smooth in all wind directions.
C; the leading edge eliminates mast drag
D; airfoil camber control.

Ilan

 

Tom, not wanting to cross swords with you, but the word document certainly makes a lot of sense to me. But then I’m not as versed as you in foil theory. Could you possibly elaborate a little?