Variable Geometry Wing Sails

[VGWS1]

Hi,

I am new to SailNet and have been prompted to join by reading many posts about the virtues and drawbacks of wing sails. Along with my brother, I have been involved in wing sail research and development for more than 20 years. Unfortunately, due to time and budget limitations this has been far from a full time pursuit. However, I have had a considerable amount of experience with wing sails and also had a lot of success. We have reviewed a considerable amount of literature and I believe we have understood the limitations of some of the research. It is only through the combination of research from many authors that the theoretical and practical performance of wing sails can be understood. If this is then backed up by practical experience, it provides confidence that the theory is correct.

So there are some myths I would like to rebuff:

Myth 1: Thin cambered foils produce less drag than thick foils.
This is simply not true for the practical range of operation of a sail on a yacht. Wing sails will produce comparable lift and in almost all circumstance considerably lower drag (by as much as an order of magnitude). There is a plethora of research to back up this statement. Much of the research is misquoted by people who either have not understood it or look no further than the piece of information that backs up their position. In the correct configuration (of thickness, camber, angle of incidence, etc) thick foils outperform thin foils in the practical range of wind speeds experienced in a yacht.

Myth 2: You cannot fit a wing rig to an existing boat without major modifications.
I own a 25’ light displacement monohull. I bought the boat second hand and other than moving the chain plates to support a rig with swept back spreaders I needed to make no modifications to the boat. I have also had similar rigs on an 18’ skiff, a 20’ trailer yacht, a 28’ displacement yacht, a hobie 18 and a windsurfer.
My current rig is fractional and has one set of spreaders (though this is not a limitation). It uses conventional sheeting and vang arrangements. It can be reefed while sailing. It can support headsails and spinnakers. My current sail plan is a 23 m2 wing main, a 10 m2 conventional headsail and a 45 m2 asymmetrical spinnaker.

Myth 3: Wing sails are too heavy.
The rigs I have used on the above boats carry only a marginal weight penalty over a conventional rig. The extra weight is attributable to having two sets of battens. The mast is of comparable weight to a conventional mast – due mainly to being able to carry a much larger mast section (and thus reduce wall thickness). The sails are only marginally heaver (even though there are two skins) as the wind load on the sail is shared between both skins, as are leech and foot loads.


I agree with many of the posts that many wing sails in use today are heavy, complicated and have practical limitations. However, this does not need to be the case.

Below is a photo taken from under the boom (while the boat was on the hard stand). I hope it reproduces well enough for you to see both the leeward and windward skins on the sail. The photo shows the rig with very little camber and around 12% thickness (i.e. nearly 400mm between skins on a 3.3m foot).

The rig can be easily set up in many configurations. The diagrams below show some of these.

Thickness is controlled by mast rotation, camber by outhaul.

So, in summary, I would like to contribute to the discussion on wing sails. I know that they can be a practical and economic alternative to conventional rigs. I also believe we are at the very beginning of the next performance curve.

Cheers

[WanderingStar]

Thanks for sharing. Interesting stuff, though I'm a very traditional sailor.

[VGWS1]

WanderingStar,

Thanks for the reply. I also appreciate traditional sailing and have recently been sailing of a 40' gaffer and thoroughly enjoying it. There is a spectrum and every place on it is as valid as all the others ;-)

We should continue to explore but not forget the past.

[WanderingStar]

I agree. I was thrilled to watch the last America's Cup match, and I enjoyed sailing a Nacra cat on my last vacation. My own boats are all wood, two of them are sprit rigged. It's interesting to see modern sails come back to square tops!

[VGWS1]

Anything is better than a triangle ;-)

[turbulicity]

Can you elaborate on why you think triangle form is bad. Or in general,
why do you think minimizing drag of the sail is important or even necessarily a good thing. I can see that drag would contribute somewhat to heeling and leeway but I am not sure the gain with a wing sail would be that significant.

More than the drag issue, the appeal of the wing sail I think is delaying stall to potentially higher angles of attack, thus getting more lift. In a run, you utilize
drag, so how does wing sail perform downwind?

A square sail would extract more force for a given height but you can't make them as high as the triangles (too high center of effort, too much heeling)

I kind of mixed up two different topics but thanks for bringing these up.

[VGWS1]

Triangular plane forms have poor spanwise flow characteristics. Spanwise flow is induced when the plan form of the wing (or sail) does not have a constant chord and is not infinitely long (in other word on all wings or sails). The consequence of span wise flow is tip vortices and trailing edge vortices caused by the flow on one side of the section leaving the trailing edge at a different angle to the flow on the other side. This translates to drag. Spanwise flow can be reduced by making the plane form more eliptical or by putting end plates and/or vortex fences on the section to limit the spanwise flow. An elipitcal plan form on a high aspect ratio section is about the best. A short triangle is about the worst. A square top is better than a triangle because the span wise flow is less and also because the square edge at the trailing edge provides a point where the vortex can spin off - thus reducing oscillation of the vortex at the end of the section and thus reducing the size of the wake from the section. This is a simple explanation and drag can be modified by many aspects of the final configuration.

So now onto why is drag such an issue. In any sintuation where the true wind is fwd of the beam (and possibly aft of that depending on sheet angles, boat speed, etc) the drag of the sail will reduce the forward driving force of the sail and increase the healing force. So in these circumstances less drag is favourable. Drag is ultimately what limits pointing ability (simplisticly put). So the low drag coefficients that can be obtained from thick cambered sections can significantly improve pointing ability.

When running drag is the principal driving force and is pretty much down to presented area.

In a broard reach both lift and drag will assist in driving the boat. So in such circumstances it a foil configuration that gives high lift and drag is best. This can be achieved with a moderately thick, highly cambered foil.

So flexibility in the foil shape is important. The ability to vary thickness and camber can provide the ability to set the sail up to provide the good lift and drag characteristics for all points of sail. When on the wind a wing sail is an advantage, when running it is not a disadvantage as long as you can hoist running sails (like spinnakers).

Also, as you rightly point out a thick section is less prone to stall. It is also less prone to leading edge separation on the high pressure side at low angles of attack. In general a thick section has a much wider operating "slot" than a thin section. In circumstances where the boat is pitching or the wind is gusting, this will mean that the sail stays powered up for a greater percentage of the time.

[turbulicity]

I agree. I think the consensus is that the wing sail is likely to improve windward performance at the expense of downwind performance. So as in almost any other topic in sailing, it is a trade-off. I think a lot can be gained by concentrating on minor design modifications at the leading edge (luff-mast joint) of traditional sails. In any slight angle of attack, flow is separating from the get go. Cloth fairings covering this area, turbulence trippers at the leading edge etc. comes to mind.

As for the sail plan shape, most common main sails have roaches so they are not triangular but closer to semi ellipse. You want this taper to reduce the center of effort. Otherwise it means more heeling force and substantially more loads on the rigging.

[VGWS1]

The point is you can have the best of both worlds. The wing sail I use can give a very wide range of section shapes from a thick, moderately cambered section (12% thickness, 4% camber) to a thin, highly cambered section (thick leading edge with >10% camber). The consequence of this is that the sail can be set up easily for any point of sail.

The pictures below represent two such configurations. The first picture shows the leeward side of the top of the sail in an upwind configuration. A 100mm long tell tail can be seen streaming aft just behind the mast and sail transition. There is another above in the Kevlar panel below the top batten. This type of flow pattern is established very easily and is easily maintained. The improvement in pointing ability that this configuration offers is substantial.

The second picture shows the windward side of the sail in a cambered and relatively thin configuration. Here you see the tell tails just below the blue depth stripe. On a broard reach this configuration is very powerfull.

We often trim the boat by just making changes to the mast rotation (which increases or decreases thickness). The wide range of section shapes we can produce allows us to carry the sail into much stronger winds before a reef is required. However, it works reefed as well.

We are able to produce section shapes that are advantageous in all points of sail other than dead running. And dead down hill we have no disadvantage. In any event we are using spinnakers in off wind scenarios that over double the sail area.