Anti Vortex Panels

This is just the old story of trying to use the hull of a boat as a leeway preventer. 19th century stuff.

Todays modern multihull designers and indeed the designers of the canting keel monos, separate the functions of hulls and keels.
This allows each component to concentrate on that which it does best.

Keels (un-ballasted), daggers and centreboards of proper planform, area and section for the job, are the most efficient means for leeway resistance..

Hulls with smooth rounded hull sections have minimum wetted surface and form drag. With a waterline L/B ratio suitable for the intended purpose they can be highly efficient at what they do best. Floating the whole shebang.

 

Normally I do not participate in forums, because I have to less time.
But here an exceptional answer because I "invented" them.
Yes anti vortex panels work. We sailed with our Pelican against as example a Shuttleworth Spectrum. The Spectrum has as you know dagger boards. We sailed as high on the wind as the Spectrum. This was witnessed by other multi hull sailors who where on board of our boat and the Spectrum.
As I outline in the article the work. They will not work so good on hulls with round cross sections.
Otherwise the theory is valid. I am a professional aircraft designer. I studied physics and aerodynamic.
But see above.
We improved also a French multi chine catamaran. The boat goos now with 8 degree better to wind wards.
I have had till now no opportunity to test them in tanks. But see above and this practical test proved my theory.
By the way this answer will stay an exception !!

Bernd Kohler
K-designs
France

 

Hi Bernd,

I'm going to ask some questions, although you indicated you won't reply. Just how critical is the shape and location of these panels? Did you experiment with various setups or did a theory dictate setup used? Would you expect the panels to perform the same as dagger boards in the same design, i.e. a like for like comparison? Why are they not a foil section?

I am just curious, if they can be shown to work as well as dagger boards the potential application is quite wide.

Thanks
MBz

 

I'm sure them sharks are thrilled that we stopped at rubber paint jobs and not thier hides.

All yoking aside, about those anti-vortex winglets on a hard chined trailer sailor. a sharpie lets say, one can argue for trailer friendly design and thin water boating potential. Providing you don't knock'em off pulling up to Lord Fletchers for a nooner.

Hmm, can a properly engineered retractable horizontal daggerboard-style winglet provide for uninterrupted cabin volume?:idea:

Cheers,
Bob

 

8 degrees better? 8 degrees higher? Same boat speed with better VMG?

From 58 degrees off the true wind to 50 degrees off the wind would be an 8 degree improvement.

From 50 degrees off the true wind to 42 degrees off the wind would also be an 8 degree improvement.

What L/D is needed to sail at these angles?

Is it probable that the vortex off the leeward hull is such a large part of the total drag of the boat, that eliminating it will reduce drag enough to see this increase?

I find this claim very hard to believe. Not saying it is impossible, just hard to believe.

 

IMHO Bernds comparison of the Pelican with anti vortex panels VS Spectrum with daggerboards, is like comparing Apples with Oranges.

The only true test would be two identical Cats with the different foils.

A series of runs on all courses, with the helmsmen swapping over would provide much more reliable results..

Again I would like to hear Richard Woods take on this.

 

Yes! Sounded like Richard was sailing like against like.

I'd be quite impressed if the concept came anywhere near dagger boards actually, that alone could make it attractive for a cruising sailor.

 

I wanted the anti vortex foils to work. I fitted them to my Day Sail to Russia Strider Club so I used them in close company to two sisterships for about 3000 miles. As I said before, I didn't notice any real difference in sailing performance between the three boats.

Certainly the way the rig was set up made more of a difference. All three boats had identical sails from the same sail maker and all had Z Spar masts. Obviously all three boats came out of the same moulds.

So for once I think this was a fair comparative test.

You can see the boats sailing on my Day Sail to Russia video available, of course, on my website. The anti-vortex panels were on the blue boat.

Hope this helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

So essentially your saying they are as good as center boards?

 

If you mean do I think that. Then NO I don't.

The Strider Club has low aspect ratio keels. The standard Strider has daggerboards. The hulls are otherwise the same (OK one has a different rudder arrangement and they usually, but not always, have different rigs)

In other words they are as close to being identical boats except for the LAR keel or daggerboard as anyone has done.

The daggerboard boat is ALWAYS the better sailing boat. Faster, points higher and pitches less.

But it has less load carrying because the keels add buoyancy; and more bother, because you need to adjust the boards; and more expense, because while the daggerboard cases cost about the same as the keels to make, the boards cost more than you think.

Does that answer the question?

You can see more on my website, look on the Strider and Shadow pages and also at the FAQ's and Hull Shapes sections.

www.sailingcatamarans.com

Richard Woods of Woods Designs

 

Hi Richard,

All I am trying to establish is some comparsion bench mark. So is saying they are similar to LAR keel a fair comment, less the buoyancy benefit of the LAR keel. Or would you say they are not up to the LAR keel sailing ability? I'm just talking rough guide, like I said I'm a little surprised they work that well.

Thanks for the reply.

Cheers
MBz

 

Sorry for the delay in replying, we've just come back from shopping. As we live on an island that meant a 3 hour sail to the nearest big supermarket and another 3 hours back again. Beats taking the ferry every way you look at it!

There are three ways to prevent leeway.

You can use just the hull, use LAR keels or use boards.

Using just the hull is OKish if the boat is fast (like a beach cat) but for a bigger cruising cat you won't have very good performance on any point of sail and it will be poor at best to windward. LAR keels are better and boards are best.

I don't think adding a antivortex plate will really help a boat relying solely on the hulls to prevent leeway. You'd be much better off fitting keels. Better still to fit boards.

It would be easy to get an old Hobie, say, and experiment with anti vortex plates.

Don't forget that the rig balances the foils. So there is no point in improving one without improving the other. So you're wasting money if you have hi-tech sails on a LAR boat.

I hope that answers your query because I don't know what else to add.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks Richard.. It all helps, I'm really just trying to get a feel for what these things might offer. Thanks for your patience.

Cheers
Mbz

 

Richard.
Living where you are at present, do you have any observations regarding "Bad Kitty", which is a round bottom Cat with CBs. It's about 25 yrs old I believe.?

 

I'll point out that the same sort of horizontal foils have been used successfully on Matt Layden's sharpie microcruiser, Paradox. This is a heavy boat slightly under 14 feet, and credible observers confirm that the boat goes reasonably well to windward.

In my view, the most daunting problem with these horizontal fins is that no completely convincing theory exists that explains their efficacy.

When I originally drew my 16' beachcruising cat, Slider, I intended to try them. The advantages seemed substantial, particularly in such a small boat with seating within the hulls and a modest sail plan-- there was little room for boards in the cockpits, and the boat would not be a high-performance cat in any case.

But because I could not find a plausible (to me) explanation for their effect, and because windward performance was very important to me, I eventually figured out a way to install a reasonably high aspect daggerboard case in one cockpit, so that it didn't destroy my seating arrangement.

The boat goes reasonably well to windward even with the board up, so perhaps I shouldn't have lost my nerve. Still, with the board down, balance improves, and tacking angle decreases.

My belief is that the horizontal foils have received very little attention from designers, so that they are at an early stage of evolution. Apparently they work. We don't yet know how well they work, because few boats are so equipped, and little thought has been given to improving their performance.

Ray