Anti Vortex Panels

[Richard Woods]

To Old Sailor

I raced on Bad Kitty in the Swiftsure last weekend. We were first boat to finish, just beating Dragonfly an ex-Aus F40 cat. Third to finish was a F25C which won on corrected time by 1 minute. We were second. But since it was a 20hr race I consider that a dead heat.

After the first 3 multis came the monohulls

The best bit of the race was not doing 22 knots through Race Rocks, but overtaking the big monohulls (75 ULDB's like Rage and the SC52's) to windward in light winds.

We have a video to prove it which I hope will get to youtube soon.

So yes. I know Bad Kitty!

It looks tatty and is as you say 25 years old. But it is well sorted, the rest of the crew knew how to sail it to its potential. Providing nothing breaks it usually beats the local Farriers, even the raciest of them.

More on my website soon

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Chris Ostlind]

I'm not at all convinced that Matt Leyden's chine runner solution on his Paradox design is anything at all like Kohler's anti-vortex panels on his cat work.

Paradox depends on the leeward side of the boat to be decently submerged while beating for the chine runners to enter as substantive lateral resistance mechanisms... so, essentially, they are on the outside of the hull. The windward (or inner) runner is at the surface, or completely out of the water when sailed normally upwind.

By contrast, Kohler's anti-vortex setup is on the inner, or windward side, of the working hull and operates in a completely different fashion. His boats are sailed essentially flat with but a hint of the heel experienced in a Paradox at the same sailing angles to the wind.

Paradox gains increased resistance from the chine runner when the hull is at its deepest heel angles and Kohler's design does not.

Now, if Paradox were to be sailed flat all the time (which isn't going to happen) then, maybe, the windward chine runner could be called into play to some very small degree.

[tspeer]

Originally Posted by oldsailor7

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've raced against Bad Kitty on the VanIsle 360. It's competitive with the F31's and F9's. If somewhat more Spartan in accommodation.

The boat may be 25 years old, but it's constantly being rebuilt. They have no reservations about taking a sawsall to it to make significant changes. It was upgraded with nice, new boards (and trunks) a several years ago.

[Richard Woods]

Ah! but both new boards broke sailing over to the start of the Swiftsure, due to hitting logs in the fast flowing Frazer river (snow melt time). So we used the old ones

Spartan! HA!

and WET!!!

Richard Woods of Woods Designs

www.sailingcatamarans.com

[oldsailor7]

Thanks Richard and tspeer for that update. I hope Karl Utoff is OK.

[Meanz Beanz]

That makes sense Chris. I suspect I am on your ignore list but for what its worth I generally like your posts when your not bitching with Lord over foils... I suspect I am also on his ignore list as well

Rayaldridge, I find it interesting that an adequate explanation of how these things work is not available. It suggests there maybe a good deal of unexplored potential here....I bet a swag of you guys would love your own test tank

Its a pitty Bernd will not engage on this topic, we have a shortage of people who have actually played with these things. To many ideas get harsh treatment to soon IMO, we should look for any possible positives here and learn from them... you might not end up embracing the idea but you certianly will build knowledge. Then again I'm not a designer so what do I know

[Ikarus342000]

[quote=Richard Woods;205602]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!

I feel pressured also to give a final word to the anti vortex panel discussion. Here we go.
Basically Richard is right.
Dagger boards should work better as anti vortex panels. (But we sailed «against» a lot of catamarans with dagger boards and non pointed higher to windward as our boat with sole anti vortex panels.
They will be better as low aspect ratio keels. The stability of catamarans with LAR´s is lower as with anti vortex panels or dagger boards (dagger boards in down position). See stability diagram. Same boat, one with a keel the other with anti vortex panels.
They perform best on hulls with trapezoid, V and sharpie cross sections as outlined in my article.
The have the best effect on hulls with an aspect ratio of 1 : 12 or better.
Dimensioning them needs a bit of understanding of hydrodynamic. Butting some piece of material under the hull and hope that they work is not sufficient.
Ours have a Clark Y section at the leading edge going over to a Kaspar section. I found out that the delta shape generates the least resistance. The length is about 12 % of the L.w.l.
It would be best to have them on each side of the hull. But this would be in practical. Harbor walls e.t.c.
Strength, ours had to show that the are strong. Our boat was in the ice. Everything was okay so far till the wind changed and we have had to get the boat out. The ice was about 10 cm thick. It was difficult to lift the boat but in the end we managed. You can see on the picture the outline of the hull and anti vortex panel. See picture.

Some remarks to letters.
CFD would help to understand them better for the lay man. By the way nobody asked this for low aspect ratio keels. A lot of these low aspect ratio keels are very crude designed. To fat and stubby to work as keels. Look at the practice. Why are so many catamarans today are not sailed to windward but use the engine ?
Comparison aerodynamic and hydrodynamic. On the university in Delft (and not only there) water test tanks where used for supersonic flight tests series. Because not many supersonic wind tunnel exists.
The Density of Air is 1.293 kg/m^3 at 0°C and 1 atm
Density of Water 998 kg/m^3 at 20°C. So the speed of the water in the tank can be around 771 times slower for the same test.
What you can do in the one direction you can do also in the other.
Of course the viscosity is different as well as the Reinolds numbers. But the measurements are valid.

Coming back to anti vortex panels, or end plates or what ever we call them.
The are by far the cheapest solution as leeway reducers.
To have a shoal draft boat, one of the attraction of multi hulls and spoil this with keels is a bit funny to say at least.
The work even on mono hulls. For example Matt Layden´s small and clever designed «Paradox» uses them very successful as well. Somebody posted already a picture of of the under water ship of the boat, Thank you.
When you surf the net you see more and more end plates on low aspect ratio rudders. (what else is a hull, as a low aspect ratio very big rudder ?). Sometimes the execution of the end plates is very crude, but the owners are happy to have a more efficient rudder.
At last, can the anti vortex panels as described in my article improved? Yes the can. The article was meant to describe the working principle and give an idea for a new, better? solution for a more efficient leeway reducers. We proved that the work on our Pelican and that is all.

Cheers

Bernd Kohler

[Meanz Beanz]

Thanks Bernd

[rayaldridge]

Originally Posted by Meanz Beanz

Rayaldridge, I find it interesting that an adequate explanation of how these things work is not available. It suggests there maybe a good deal of unexplored potential here....I bet a swag of you guys would love your own test tank

It might just be that I'm not smart enough to understand the various theories that have been put forward, but I see problems with all of them. Of course, as Chris implied, it may be that Paradox's foils work on a different principle than Pelican's. Much of my confusion regarding the various theories I've seen has arisen from an effort to reconcile them.

Originally Posted by Meanz Beanz

Its a pitty Bernd will not engage on this topic, we have a shortage of people who have actually played with these things. To many ideas get harsh treatment to soon IMO, we should look for any possible positives here and learn from them... you might not end up embracing the idea but you certianly will build knowledge. Then again I'm not a designer so what do I know

I have to say that when I was considering using the foils on Slider, before I chickened out and went with a daggerboard, that Bernd was very helpful, and was willing to answer many questions regarding the foils. He was helpful enough to recommend a size and shape, complete with drawings, and I'm not even a customer. It predisposed me to think that he is a designer with whom it would be a great pleasure to work. I also corresponded with Matt Layden, who was equally generous with advice. Matt has built a model cat, and designed a beachcruising trimaran for the guy who organizes the Everglades Challenge.

My feeling is that the horizontal foil is a fertile area for further development.

Ray

[Meanz Beanz]

Originally Posted by rayaldridge

My feeling is that the horizontal foil is a fertile area for further development.

Yes, it sounds like it.

[markdrela]

As others have mentioned here, the panels are roughly equivalent to winglets. And by far the best orientation for a winglet, from a purely induced-drag view, is horizontal. i.e. as a span extension. Structural considerations, specifically minimizing wing bending moments, is what favors the vertical winglet.

So unless there's a draft rule in effect here, mounting one of these anti-vortex panels vertically like a keel or daggerboard would be far more effective than mounting them horizontally. It will produce more heeling moment, but on a cat this should have a very small effect on the overall moment balance.

BTW, whatever the mounting orientation, it would be beneficial to put a proper leading edge on the panel. LE vortex separation is effective, but in terms of L/D an attached LE flow is better.

[oldsailor7]

Back in the day (1971), I built a 24ft Trimaran by a world acclaimed designer, specifically configured for the New York - Bermuda race. Since this is a deep ocean voyage I applied the KISS principle and went for a fixed fin keel.

The race is usually a high speed dash in rough water for the first part and then a slow and very light wind sail thru the Sargasso Sea.

To this end I configured the keel fin with the plan form of the Concords wing and a 6% supersonic section.

I reasoned that at high sailing speeds the low aspect ratio shape would be very efficient. At slow speeds in flat water and very light winds the large vortices produced by the Concord planform at high anlgles of attack would enable better windward progress progress despite the poorer L/D ratio.

This worked very well in practice, but in the confines of Toronto harbour the fin keel was prone to grounding damage, and the following year I reverted to a lifting daggerboard, with an area of 2% of the projected sail area (mainsail and fore triangle), a NACA0008 section with a 3:1 aspect ratio.

For Harbour racing this compromise turned out just fine.