Anti Vortex Panels

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

 

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.

 

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.

 

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

 

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

 

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

 

Thanks Bernd

 

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.

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

 

Yes, it sounds like it.

 

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.

 

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.