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Old 16-07-2006, 09:47   #1
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Vortex Generators Vs. Cross keels

I like Norman Cross trimarans, but do not care for their draft. In and attempt to remedy this, I have been thinking of removing the Cross keel and putting a dagger board in a Cross 36. But I have been re-reading Thomas Firth Jones book, Multihull Voyaging, pg 76 and he spoke to Vortex Generators, mounted perpindicuar to the bow to reduce draft and leeway when pointing. Has anyone had direct experience with this, or can recommend some further reading. Of course I like this idea, no moving parts.

Also any thoughts about losing the Cross keel and adding the dagger board in the main hull. Thanks

Pat Ross
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Old 16-07-2006, 10:01   #2
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Pat,

I've had a passing interest in this subject since stumbling across a link on the internet while looking for something completely different. This link is a white paper on "Anti Vortex Generators"????? He lists 5 references in this paper and claims that reduced hull turbulance has a lot to do with hull tracking.

http://www.ikarus342000.com/Avorart.htm

This peaked my interest because of their claim of reduced leeway on catamarans with multichined hulls, like my Catalac.

Since then, I've searched the net for more info from time to time, and have found nothing else written on the subject!

When I have more time, I'd like to track down the fella who wrote this web page and ask him for examples of the boats utilizing "anti vortex" generators.

Rick in Florida
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Old 16-07-2006, 10:10   #3
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Thanks Rick, this is an excellents start. I have put a few feelers out on other sites, I will let you know what I find out.

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Old 16-07-2006, 13:15   #4
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There is more data on fitting plates on the bottom of a rudder. There is a class of smaller boats in northern australia, that I believe are all modified this way. They are designed to overcome the stern squat, which will cause a turbulent flow and reduce your speed.

see http://www.well.com/~pk/ACtmFP.html#nelsons
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Old 15-11-2006, 00:33   #5
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Pat;
I have a 28 ft Cross Tri and it has a centerboard in the main Hull. It is on the starboard side under the settee so does not intrude in the middle of the cabin. I have just gotten the boat and it seems to sail very well with the board down.

Kelly.
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Old 15-11-2006, 19:42   #6
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Find a copy of The Cruising Multihull by Chris White. It has a few drawings showing some ideas about dagger board trunks. The key is to have some sort of sacrificial impact mechanism to absorb the impact in case of grounding. The vortex generators are a variation of a winged keel if I am not mistaken. I remember a story about them being fitted to a Wharram cat so his website may be one to check out also.
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Old 16-11-2006, 06:39   #7
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The vortex generators are horizontal structures attached to the inner hulls. The look like triangles.

The fella who wrote the original article did put up some photos. I felt he didn't explain the effect very well and was light on the construction details. Pat Ross, who started this thread, emailed me that he was in contact with the author of the paper, and that was the last I heard of it.

I'm still very interested.

Rick in Florida
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Old 16-11-2006, 17:08   #8
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Rick, the author of that paper is Bernd Kohler. He is also a designer, of course.

http://www.ikarus342000.com/Catpage.htm

I have emailed him and joined the K-design Yahoo group asking for more info on these panels, but to no avail as of yet. He did respond saying he would address them in a post on the Yahoo group.

http://groups.yahoo.com/group/k-designs/

As a fan of asymmetric hulls, I found this very interesting and simple.
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Old 19-11-2006, 07:25   #9
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Frank

I'm just catching up with this now. Back in July, Pat Ross had emailed me as I had given him Kohler's web site. We coresponded for two weeks and he had copied me me on Kohler's emails to him concerning the anti vortex generators.

Thanks to your link, I now see that Pat had bought a set of plans and is building one of Kohler's designs. I think these designs utilize the anti vortex generators. Pat is also the moderator of that yahoo group and is obviously very involved.

I'm interested in the possibility of adding these to my hulls. In his original work he states that they will reduce leeway substantially and work well on multichine hulls.

Apparently that early work was the foundation for a line of catamarans he's now designed and actively promoting. I'm left with my original questions unanswered as to how to design a pair of them and add them to my hulls.

Rick in Florida
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Old 19-11-2006, 08:01   #10
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Rick,
Yes, I have considered them for my boat as well.... therefore getting rid of the daggerboards...(I know, I know... you say why would I ever get rid of the daggerboards.. ;-)...)
I am wondering about aspect ratios and surface area to be effective. It is difficult to identify proportions from the picture on the website.
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Old 19-11-2006, 08:17   #11
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Exactly. It sounded promising, but without more information that's all it is, a promising idea.

Rick in Florida
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