anti vortex board

[44'cruisingcat]

Originally Posted by vanvan450

BERN KOHLER SAYS , I COULD SAIL AS CLOSE TO THE WIND AS MONOHULL WITH HIS 11 METERS CATAMARAN PELICAN (WITH ANTI WORTEX PANEL),,, ANY ONE WITH AN EMAIL FOR BERN,,,, AND WE COULD HAVE HIS OPINION,,,,

We sail at around 30' apparent for best VMG. That's higher than a lot of cruising monos.

[longjonsilver]

As i understand this, these antivortex panels are mounted on the bow (5-6% before center of effort) of a cat with no centerboard or minikeel. They are designed to make the low aspect ratio hull operate at higher efficiency. If this is true then they would also be of use on cats with centerboards. No?

I don't understand the 5-6% part - 5-6% of what? The waterline? They are shown on the bow. No? That certainly is not 5-6% of waterline before center of effort! Perhaps he means their length should be 5-6% of waterline length? dunno. please clarify

Another question is how would one install them on a tri? The inside of both floats and both sides of the main hull?

jon

[a64pilot]

Originally Posted by Guy

These are not end plates they are winglets. Every new jet has them.

Yes most of the newer jets do have winglets, which do differ from end plates.
But do you know why?
Firstly and most importantly they look sexy and modern and that helps sell airplanes.
Secondly to make any real significant difference in drag, the wing has to be at a high angle of attack, the closer to stall, the more effective they are. Somewhat surprisingly most jets at cruising altitude are at high angle of attack, pretty close to stall, yes at 400+ MPH they are close to stall, the air is that thin up there. So they do work on jets, in very thin air at high altitudes cruising close to stall speed, the other aircraft that have them do so because they look sexy and the marketing dept says they help sell airplanes, at best though they do no harm, they do no good though on a regular General Aviation aircraft.

However do any of our sailboat airfoils operate at high angles of attack, close to stall? Maybe the rudder when your turning hard, but the rest of the time the end plate or winglet if you will is just excess drag.
I would have thought maybe vortex generators on rudders of high performance boats to keep the boundary layer attached at high angles of attack maybe, but I haven’t seen any.
Maybe water is so dense, and certainly is incompressible that it doesn’t follow the laws of aerodynamics, maybe more like supersonic airflow?

[model 10]

I thought winglets provided lift perpendicular to the their cord? The center of lift is somewhere above the fuselage. If so they would increas drag. Little fancy wingtips and end plates are different.

[Markusik]

When researching leeway-reduction strategies, I came across this article. Excited, I threw myself into the topic.

I summarize the discussion from boatdesign.net; anti-vortex panels are useful when relying on an asymmetrical, or a single chine "box" hull shape for leeway reduction. Bernd uses an asymmetric hull form.

Asymmetric and box chines for leeway reduction have their charms; no protrusions (except the anti-vortex panels themselves), little draft, beachable, no active management required. They work for smaller craft like beach cats (with their high SA/D). Once into cruising size boats though, the two hull forms run into problems.

On a larger boat, the asymmetric hull seems to limit performance; higher wetted surface, less buoyant, it just isn't as efficient a hull shape as the canoe. I've read reports of great difficulty tacking in asymmetric cruisers.

A box chine hull offers leeway reduction while maintaining a relatively efficient, high buoyancy hull at the expense of a modest increase in wetted surface. The problem with these comes when the cruiser moves off paper and into the water; the complex curves typical of the modern GRP cat hull lend strength and rigidity. The box chine lacks these, and must be seriously overbuilt (read; heavy and expensive) to prevent "oil can" motions with the associated risk of delamination.

Again, I'm only paraphrasing what the folk over at boatdesign concluded; anti-vortex panels with asymmetric and box chine hulls are fine for smaller craft, but suffer when expanded to cruising boats. There are reasons why mini-keels, with daggerboards a distant second, dominate the world of production boats.

Now if only leeboards weren't so goofy looking...

[a64pilot]

Originally Posted by Guy

I thought winglets provided lift perpendicular to the their cord? The center of lift is somewhere above the fuselage. If so they would increas drag. Little fancy wingtips and end plates are different.

They perform the same or similar function as far as lift generation and drag reduction.
A properly designed winglet, when it is in the design parameters, IE high angles of attack, will reduce drag, and increase lift. It does this by interfering with the formation of the wing tip vortex.
An end plate does the same thing, just has more area to it and therefore more drag, however the increase area also can add greatly to yaw stability.

A few years ago Mooney Aircraft introduced their new models of aircraft, (Mooney has been forever famous for the fastest, most efficient aircraft with the least power for decades) which had upswept wing tips, not down like was the standard, forever. Lopresti who designed the new wing tip was being interviewed for one of the aviation magazines and of course the buzz was what’s with the upswept wing tips, how much more efficient are they, how much speed do they add? During the interview he kept on about how his wife had such a beautiful nose and the wing tips reminded him so much of her nose.
Bottom line, the upswept wingtips are a styling feature, they do nothing at all aerodynamically. Almost all Mooney’s have no wing tip enhancement at all, the wing just ends, and if Mooney could get .1kt out of a wingtip, they would have had one decades ago.

An Ag plane is a different animal and the function of a winglet on an Ag plane is to reduce the wing tip vortex in order to not interfere with the spray pattern. The aircraft in that photo as you likely know is an S2R Thrush, most likely a 1340. I’m pretty familiar with a Thrush and wing tip devices.

On edit, that photo is obviously doctored, the vortex is displaced, and no Thrush has a vortex that large, maybe a passenger jet in the landing flare, but no Thrush.

[Thumbs Up]

I don't thin I've followed this entire thread but I was going to weigh in earlier. I have twin stern hung rudders and I have seen photos of hitchhiker rudders with end plates (small delta wings). I am tempted to add them to mine. I am worried about the extra strain to the rudder brackets but would want to make them as large as practical because I think that they could help control pitching. Maybe I could make them detachable underwater in case I don't like them. What about a couple of those doelfins that they make for outboards. I would be interested in ideas about some type of fin near the bow as well. My double ended hull shapes need something to dampen the motion in choppy conditions.

[Thalas]

The test with the rudder in the article would have meant more to me if they measured both the lateral force delta and drag delta between wing and no wing. At 15kts (on the high side for most wind powered boats that aren't purpose built racers, but slow relative to the vortices we know can kill prop efficiency), I wonder how much rudder force while turning is from the low pressure side via the Bernoulli effect and how much is brute force contact with the high pressure side. At low speed, if Bernoulli isn't really an issue, then any vortices would seem to have minimal impact on performance whereas the extra drag is always present.

[model 10]

It would depend on what kind of rudder you had. A skeg rudder would be more about a low pressure area. A rudder on the end of a full keel would be more a deflection thing.

[a64pilot]

Originally Posted by Guy

It would depend on what kind of rudder you had. A skeg rudder would be more about a low pressure area. A rudder on the end of a full keel would be more a deflection thing.

I’d say this is a high performance thing, which pretty much rules out full keel barn door rudders

[Thumbs Up]

I'm not sure what you would call this rudder, the top part is more deflection but the bottom part more of a pressure thing. The idea is to put a delta shaped end plate on the bottom of them...

[LeeV]

Originally Posted by vanvan450

please read first the article,,,,, before writing,,,

I’ve read it, and looked at the photos and videos of Bernd’s boat and several of his designs. Your best best is to ask on his forum, to which I still belong. I had the plans to build a Voyager 40, but then a health reason stopped that. My friend Patrick is about 2/3 finished building a Voyager, and is on the forum with lots of photos. Ask the builders there.

Regards,

[vanvan450]

Originally Posted by a64pilot

They perform the same or similar function as far as lift generation and drag reduction.
A properly designed winglet, when it is in the design parameters, IE high angles of attack, will reduce drag, and increase lift. It does this by interfering with the formation of the wing tip vortex.
An end plate does the same thing, just has more area to it and therefore more drag, however the increase area also can add greatly to yaw stability.

A few years ago Mooney Aircraft introduced their new models of aircraft, (Mooney has been forever famous for the fastest, most efficient aircraft with the least power for decades) which had upswept wing tips, not down like was the standard, forever. Lopresti who designed the new wing tip was being interviewed for one of the aviation magazines and of course the buzz was what’s with the upswept wing tips, how much more efficient are they, how much speed do they add? During the interview he kept on about how his wife had such a beautiful nose and the wing tips reminded him so much of her nose.
Bottom line, the upswept wingtips are a styling feature, they do nothing at all aerodynamically. Almost all Mooney’s have no wing tip enhancement at all, the wing just ends, and if Mooney could get .1kt out of a wingtip, they would have had one decades ago.

An Ag plane is a different animal and the function of a winglet on an Ag plane is to reduce the wing tip vortex in order to not interfere with the spray pattern. The aircraft in that photo as you likely know is an S2R Thrush, most likely a 1340. I’m pretty familiar with a Thrush and wing tip devices.

On edit, that photo is obviously doctored, the vortex is displaced, and no Thrush has a vortex that large, maybe a passenger jet in the landing flare, but no Thrush.

ANY ONE WITH BERN KOHLER EMAIL¡¡¡¡¡ THANKS

[vanvan450]

any one with bern kohler email please,,,,

[catsketcher]

There is a reason for end plates.

Gday all

I used to sail a Nugget trimaran 35 years ago when I first got into multis. I had problems with the centre board so sometimes I couldn't use it. When I tried to go to windward on my slab sided Nugget tri I would get these interesting 10cm diameter vortexes spinning off the base of the leeward float. They were only there when the board was up. When the board was fixed they went away.

Obviously what is happening is that the hull sides are being used to generate side force. Being a very low aspect foil they have a lot of tip loss. Back then there was a bit of talk about Wharram owners putting on fences (end plates) to reduce tip loss and stop the vortex. (Actually Bernd should call them vortex dampers to be more precise). I remember reading an article from a bloke who put them on his Wharram, he went out sailing to check out how they worked and he couldn't tell much difference. Then when one fell off he still couldn't tell. People were quite interested in these devices back in the mid 80s.

I saw the same vortexes on a minikeel cat I used to sail on as well. How the addition of a second low aspect foil could improve a first one is hard to understand. According to the articles I read, end plates can often be a substitute for increased span, just some aircraft use them so they can get into small gates without increasing wing length.

As to Bernd sailing well upwind, so can a Hobie 16 and other dagger free cats, reduce drag above water, use better sails etc. All these will help. There is never a single design attribute that makes a boat sail well, drag and lift come from the whole package. A bit of lipstick is not going to make your heavy, baggy sailed high cabin cat leap to windward.

As to the end plates on a Hitchhiker rudder. These were designed in the 80s and back then everyone was putting wings on anything they could (go Ben Lexcen) but they also helped pitch damping on the tall and low prismatic double ended hulls. I seem to recall that they also helped the boat sit better on soft sandy beaches with a bit of support at the back end.

cheers

Phil