Hi guys m just making sure:

Asymmetrical Speed Fins: In a Starboard/Luderitz fin which side is flatter? sailor side or sail side of the fin?

Cheers,

I would say always sailor side, because its the other side that gives you the lift

Never used one but would say the flat side would be on the 'sail side' and curved side on 'sailor side' as that would produce lift in the opposite direction to the sail and therefore push against your feet giving lift.

if it's the other way (same as sail shape) the board would be pushed downwind away from your feet.

low pressure is around the curve side
- high pressure is on the flat side - lift goes towards the low pressure.

I use a starboard assy most of the time.
It works like this.
Think of an aircraft wing, the curve is up the flat is down, so lift is in the direction of the curved side.
A fin has to resist the downwind force of the sail, so the fin lift has to be in the opposite direction to sail lift, so curved side towards sailor.

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decrepit said..
...t, so curved side towards sailor.

that's why I am so slow !

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Stralen said..
Hi guys m just making sure:

Asymmetrical Speed Fins: In a Starboard/Luderitz fin which side is flatter? sailor side or sail side of the fin?

Cheers,

The flatter side is on the sail side,

Curve to windward - flatter to leeward. Seems the best way to think of it for me?

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decrepit said..
I use a starboard assy most of the time.
It works like this.
Think of an aircraft wing, the curve is up the flat is down, so lift is in the direction of the curved side.
A fin has to resist the downwind force of the sail, so the fin lift has to be in the opposite direction to sail lift, so curved side towards sailor.

All true. But for me, it's easier to visualize this by thinking of the sideway pressure from the rear foot. If it's too much for a fin, the fin spins out to leeward. So the lift from the fin has to counteract the sideway pressure from the back foot. Of course, the reason why the back foot has to exert sideway pressure is the downwind force of the sail. Since that is in front of the center of lateral resistance when planing, the nose would turn downwind without sideway pressure from the back leg.

Reading this again and I thought it is relevant to point out that the most used and successful Assy speed fins in real world speed spots are not entirely flat on the high pressure side. With the exception of a one way canal, like the one at Luderitz, we must sail upwind again, as least some of the way, to have another run. We quickly found that the degree of asymmetry was quite important to allow this, and the Assy fins still needed to have some modest performance on the 'other' tack. Personally, I found that the Time Machine, KA and Tribal Assy fins had a sweet spot upwind at around 20-25 knots. They didnt like going much faster than that on the wrong side at all! But they worked every bit as well (as far as I could tell) on the downwind speed run on their correct tack as the few much more radical Assy fins I tried.

A good example of this was a 18cm MXR fin I tried at Lake George was back in around 2013. It was really almost completely flat on the high pressure side. It was 'sweet as..' on the downwind run, but I was struggling to sail upwind again even on the mirror smooth water. The slightest ripple would have me spun out! It was a very effective 'Canal' fin, but not very useful when you were required to sail upwind again.

What is needed is a reversible self adapting asymmetrical fin. I stumbled across the work of Simon Fagg who in 1997 wrote as part of his degree a paper on" the reversible and infinitely variable camber windsurf fin". The concept is brilliantly simple and I can't help thinking with 3d printing the idea can be pushed further. He seems to have spent a lot of time on the theory, computer analysis and building the fin but almost no time testing the fins he made. Then he appears to have disappeared.

To save everyone the time reading the paper, the rear third of the fin is rigid, as is a 'mast' in the leading edge. The forward 2/3 of the foil is made with flexible material that can rotate around the mast. Easy and cheap nowadays to 3d print such a thing in situ. When the sailor applies force to the fin the pressure distorts the foil to create more curve in the foil towards the sailor and rotates the leading edge into the flow thus increasing the lift to drag ratio. When changing direction the fin distorts accordingly.

His paper here for those interested
eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf

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Flex2 said..
What is needed is a reversible self adapting asymmetrical fin. I stumbled across the work of Simon Fagg who in 1997 wrote as part of his degree a paper on" the reversible and infinitely variable camber windsurf fin". The concept is brilliantly simple and I can't help thinking with 3d printing the idea can be pushed further. He seems to have spent a lot of time on the theory, computer analysis and building the fin but almost no time testing the fins he made. Then he appears to have disappeared.

To save everyone the time reading the paper, the rear third of the fin is rigid, as is a 'mast' in the leading edge. The forward 2/3 of the foil is made with flexible material that can rotate around the mast. Easy and cheap nowadays to 3d print such a thing in situ. When the sailor applies force to the fin the pressure distorts the foil to create more curve in the foil towards the sailor and rotates the leading edge into the flow thus increasing the lift to drag ratio. When changing direction the fin distorts accordingly.

His paper here for those interested
eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf

One of the other things you may consider when you 3D print your fins Flex is to topographically optimise the structure to get the strength and flexibility you need. We use it on the drones we build and have had success in creating morphing wings. When Simon did his work back in the 1990s there were a few of us looking at variable camber foils. I did some initial work with Curtis, Jeff Magnan and Peter Thommen on them.

Interesting Roo, might have to hit you up if/when I get around to looking at this more deeply. However, in the meantime Decrepit has introduced me to a completely different idea which is much easier to implement and test but also offers asymetrical performance. He made a few of these fins in his time and reckon they make a big difference but all suffered structural issues. I think I've come up with a way to make them more buildable/reliable...standby.... should have prototype out soon.

Hello Flex 2,
"He seems to have spent a lot of time on the theory, computer analysis and building the fin but almost no time testing the fins he made. Then he appears to have disappeared."
I spent a bit of time with Simon in Maui in the early 90's.
He went from there to WA and did engineering work and then on to Melbourne where he did a medical degree and last I heard is now working as a Doctor there.
A clever fella.

Nice info Te Hau. Do you know if Simon continued windsurfing? Any contact details?. PM me if anyone has a clue. So many good ideas from these old timers (Simon probably 10 years younger than me though) that were ahead of their time that perhaps technology available wasn't ready for them. Maybe the tech is ready for them now?? Simon didn't even have GPS available to compare fins, just touchy feely subjective stuff which doesn't cut it in engineering world. Would be great to bring them back into the fold to get their take on things nowadays.

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Flex2 said..
What is needed is a reversible self adapting asymmetrical fin. I stumbled across the work of Simon Fagg who in 1997 wrote as part of his degree a paper on" the reversible and infinitely variable camber windsurf fin". The concept is brilliantly simple and I can't help thinking with 3d printing the idea can be pushed further. He seems to have spent a lot of time on the theory, computer analysis and building the fin but almost no time testing the fins he made. Then he appears to have disappeared.

To save everyone the time reading the paper, the rear third of the fin is rigid, as is a 'mast' in the leading edge. The forward 2/3 of the foil is made with flexible material that can rotate around the mast. Easy and cheap nowadays to 3d print such a thing in situ. When the sailor applies force to the fin the pressure distorts the foil to create more curve in the foil towards the sailor and rotates the leading edge into the flow thus increasing the lift to drag ratio. When changing direction the fin distorts accordingly.

His paper here for those interested
eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf

I tried one about 10 years ago, Remi from the Windsurfing shed was selling them, the fin acted like the flaps on an aircraft, so whatever tack you were on the fin shape would shift to become asymmetrical on that tack

Something like these choco? The yellow one I sort of get but the green one?? I guess they weren't that great??

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Flex2 said..
What is needed is a reversible self adapting asymmetrical fin. I stumbled across the work of Simon Fagg who in 1997 wrote as part of his degree a paper on" the reversible and infinitely variable camber windsurf fin". The concept is brilliantly simple and I can't help thinking with 3d printing the idea can be pushed further. He seems to have spent a lot of time on the theory, computer analysis and building the fin but almost no time testing the fins he made. Then he appears to have disappeared.

To save everyone the time reading the paper, the rear third of the fin is rigid, as is a 'mast' in the leading edge. The forward 2/3 of the foil is made with flexible material that can rotate around the mast. Easy and cheap nowadays to 3d print such a thing in situ. When the sailor applies force to the fin the pressure distorts the foil to create more curve in the foil towards the sailor and rotates the leading edge into the flow thus increasing the lift to drag ratio. When changing direction the fin distorts accordingly.

His paper here for those interested
eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf

Brought back some memories that paper, seems just like yesterday. As Daffy just pointed out to me, I was one of the reference sources Simon cited in his paper. I had forgotten all about that! One of the sources we all referenced back then was Marchaj, he did some very insightful work that helped in the design of sails as well as fins. I still have his book on my bookshelf and refer to it quite often.

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Flex2 said..
Something like these choco? The yellow one I sort of get but the green one?? I guess they weren't that great??
www.seabreeze.com.au/img/photos/windsurfing/20830565.jpg' />

Nothing like that Flex it looked like a normal fin but from memory had rods running down the center(built inside) of each section connected by a clip on top of the tuttle head, it created a pivot pivot point down the center, I'm sure someone must have one lying around. I've sent an email to Remi so hopefully he may have a pic or still has one.

You guys are bonkers. Luv reading what boundaries are being pushed by you all and the reference to who did what ages ago. Keep it up fellas

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choco said.


I tried one about 10 years ago, Remi from the Windsurfing shed was selling them, the fin acted like the flaps on an aircraft, so whatever tack you were on the fin shape would shift to become asymmetrical on that tack

I remember them, never seen one but I liked the idea.