Forums > Windsurfing   Gps and Speed talk

Flying the fin??

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Created by paddymac > 9 months ago, 3 Dec 2011
DarrylG
WA, 495 posts
15 Dec 2011 10:01PM
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Vector Carbon Canefire 40 cm

Te Hau
479 posts
16 Dec 2011 5:35AM
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DarrylG said...

A fin angled at 45 will also give a bit of lift I guess




Way too many pies!

Te Hau
479 posts
16 Dec 2011 5:39AM
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Side note, if there is such a thing as Heavy Ground Effect under a windsurf board, has anybody ever tried watersking with a windsurf board, if the HGE Hypothesis is correct it should be possible get True Flight. No?

Kinda like a Hoverboard? Except they don't work on water (unless you've got power)

....................

Naa Barn, waterskiing's never gonna hover it.....
'cos you haven't got the lift from the sail !

Bonominator
VIC, 5477 posts
16 Dec 2011 2:44PM
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barn said...

kato said...


Its an interesting discussion Barn. Just remember any pract tests with the car will have to take the disturbed air flow into account.



That was the thinking behind the plywood splitter, mounted on some high roof racks it should be possible to get the board in some clean air.


My money is still of Slowly as some skills just carn,t be explained by science.


Bet on me and you might be able to replace that 1993 Bic Saxo you cruise around on.

Just to clear some things up, as I think there has been a bit of nitpicking as far as definitions and whatnot, my first point was that you cannot generate enough lift from the sail to take the weight off the board.. I understand there can be some, but there is a difference between claiming 100% and a few kilos. No matter how important those kilos are.

I believe I proved that, and everyone seems to agree (from the silence). Although I am the only one who has put a 'Ballpark' on that lift, at 10-20kg...

Putting my money where my mouth is.


The second point raised is how the board generates the remaining lift, Slowy has pointed out that the lift comes from the 'Heavy Ground Effect' from the first 2m of the board. An interesting point, but I am also skeptical this is a source for much lift. I'm guessing in the ballpark of 5kg at the mast track.


The centre of lift should be around the mastbase, acording to the specs on a Ground Effect foil http://www.dsto.defence.gov.au/publications/2058/DSTO-GD-0201.pdf


I ain't really concerned about the efficiency of a foil in Air or water, as I doubt there is much lift at the nose anyway.

Anyway, so far I think I understand how the airflow is supposed to interact with the nose of the board. I dunno if my pictures are helpful but sometimes words are confusing.


This is a board doing 30knots with a 10knot breeze at nose height, giving us 31 knots of apparent wind at 18 degrees. The airflow above is in white, and below is in yellow. I feel these lines are being fairly aerodynamically generous, there would be more turbulence IRL.

----------------------------

Side note, if there is such a thing as Heavy Ground Effect under a windsurf board, has anybody ever tried watersking with a windsurf board, if the HGE Hypothesis is correct it should be possible get True Flight. No?

Kinda like a Hoverboard? Except they don't work on water (unless you've got power)


Touche mon ami on that sweet ballpark uplift figure!

Lessacher
89 posts
18 Dec 2011 2:25AM
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If you look on the pic.of West Kirby,you see that the board in front looks
against the wind. Maybe 10°. I worked a 22cm fin so,that she is in front
on the right side of the tuttle base.
The end of the fin is on the left side of the tuttle base. angle is 7°.The fin do
the work. How always,but the board looks to the finish.We need right and left fins.
It works good. I reduce a little the brake of the board. Wolfgang

Roar
NSW, 471 posts
19 Dec 2011 9:46AM
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here is a real simple test - next time its 30 knots of wind just put your board on the roof racks and point it into the wind - wait and see how long the board stays there

dont forget to bring a tape measure to you can measure the distance away from the car it lands.

barn
WA, 2960 posts
19 Dec 2011 8:37AM
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Putting it on the roof won't have the HGE.

A board is just over 1m2 in area, there is a limit to the amount of lift it can generate in free air.

This is a Shaka in 30knots, half way around your body weight is committed to the sail, and the nose of the board is pointing directly into wind, starting at a small angle, then an extreme angle, and then back down again.. I can guarantee there is not crazy lift from the board in free air, it's very controllable (the sail, less so).




Ian K
WA, 4048 posts
19 Dec 2011 9:45AM
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The wing loading for a hanglider is 6.3 kg per square metre

en.wikipedia.org/wiki/Wing_loading

and a typical hang glider of 14.4 sq metres has a best glide speed of 50 kph ~ 30 knots for an all up weight of ~ 110 kg.

deltaplane.brozs.net/doc/scandal_spec.pdf

The picture of a hanglider further back in this thread shows the similarity of design to a windsurfer sail - so a similar wing loading is quite likely - ie about 30 kg of lift all up. But I think we'd arrived at a figure something like that earlier on in the thread when we put the idea of being able to be largely supported by the sail to bed.

The amount of lift that can be provided by the board is still being sorted out, but on the way an application of the bluff body drag formula might be useful. Drag = Area * Cd * 1/2 rho V squared.



This is the force the board feels once the nose is fully lifted and it's square to the wind.

in 30 knots = 15 m/sec, air density of 1.2 kg per cubic metre, the Cd for bluff bodies is usually around 1. The drag formula gives 13 kg for a 1 sq metre board. That's about what it feels like trying to get it on the roof of the car. It's all drag of course so not very useful, but interesting to compare that with the aerodynamic lift of the hang glider in similar wind speed, ie. 6.3 kg/m squared.

So the question is. If a purpose designed wing in 30 knots generates 6.3 kg of lift per square metre how much more lift, if any, does a compromised wing ( the board) develop in 30 knots with ground effect?

The previously referenced article

www.dsto.defence.gov.au/publications/2058/DSTO-GD-0201.pdf

gives a clue but is not definitive

"WIG craft have been championed on the basis that they are more efficient than
equivalent aircraft and quicker than equivalent marine vessels. The efficiency argument
is somewhat speculative. While theoretically an improvement in efficiency is gained by
flying in ground effect, this efficiency is reduced by design compromises required of the
WIG craft. Such compromises include strengthened hull structures, reduced aspect
ratios and larger control forces. The degree to which total efficiency is improved can
only be determined by the direct comparison of optimised designs of equivalent WIG
and aircraft. Only through such a comparison would the value of the improved
efficiency and the cost of gaining this efficiency be determined."



slowboat
WA, 553 posts
19 Dec 2011 11:18AM
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Ian K said...
ie about 30 kg of lift all up.

You can also estimate this from a simple free body diagram made from a picture... both methods provide a similar result. So I agree thats about the right figure. ~1/3
Ian K said...
the Cd for bluff bodies is usually around 1.

errr why do you think its a bluff body? I dont think so.

Assuming its a streamlined body, with only a slight camber, at 5 deg AOA its likely to have fully attached flow and a Cl in the 0.5 range...

30kts speed, at 90deg to the wind of 20kts = 18.5m/s apparent wind speed.
Area = 1.2m^2 (~2m in the air on a 70cm wide board- average width of 60 over that length)
L = Cl.p.V^2.A/2 = 0.5*1.2*(18.5^2)*1.2/2 = 123N ~= 12.5kg.

ANd now to the curly one of ground effect... consider that in our case the board is sitting over the water at a small fraction of the chord- considering the apparent wind angle- we are talking a few %. Thats going to have a huge impact on the lift coefficient.
http://www.ghdstudios.com/radacraft/fig1.html
www.se-technology.com/wig/html/main.php?open=aero

My guess is it will be at least double.

Ian K said...
The previously referenced article

www.dsto.defence.gov.au/publications/2058/DSTO-GD-0201.pdf

gives a clue but is not definitive

"WIG craft have been championed on the basis that they are more efficient than
equivalent aircraft and quicker than equivalent marine vessels. The efficiency argument
is somewhat speculative. While theoretically an improvement in efficiency is gained by
flying in ground effect, this efficiency is reduced by design compromises required of the
WIG craft
. Such compromises include strengthened hull structures, reduced aspect
ratios and larger control forces. The degree to which total efficiency is improved can
only be determined by the direct comparison of optimised designs of equivalent WIG
and aircraft. Only through such a comparison would the value of the improved
efficiency and the cost of gaining this efficiency be determined."


not to mention the height/chord has to be pretty big to avoid crashing into waves, so ground effect is limited for these types of vessels. Comparing apples and oranges.

Ian K
WA, 4048 posts
19 Dec 2011 12:52PM
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^

I'd used a bluff body in reference to the previous post commenting on how they get hurled off a car roof, Cd rather than Cl. But the formulas have the same form, no argument there.

Not sure what you mean by

slowboat said...

ANd now to the curly one of ground effect... consider that in our case the board is sitting over the water at a small fraction of the chord- considering the apparent wind angle- we are talking a few %. Thats going to have a huge impact on the lift coefficient.



What has been decided on the angle of the apparent wind close to the water? Probably need some one to run some telltales on a small flexible inverted mast at the nose of a board one day.

The second reference you gave has the Cl increasing from 0.8 to 1.1 at a clearance of 0.5 the chord - which might be about 50mm. Parts of the board near the nose where it has more clearance, will have less of a gain, those closer to the wet area... will be getting wet.

There's no disagreement on the theory, just a matter of estimates for the Cl, the apparent wind angle, and the benefit of ground effect in this case. Might have to rig up that experiment 6 or so posts back. Any problems with the validity of that proposed experiment? Might have to hang the tell tales first to get a valid angle of attack.

I think "At least double 12.5kg" is a fairly optimistic application of the theory. I'll go for less than 10, wouldn't be surprised if it's closer to 5.


sausage
QLD, 4873 posts
19 Dec 2011 3:11PM
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Te Hau said...

Way too many pies!


Hey, I resemble that

Very interesting (and intellectual) thread too.

barn
WA, 2960 posts
19 Dec 2011 1:44PM
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sausage said...


Very interesting (and intellectual) thread too.


(we can fix that)

HOW ABouT THIS<

I HAV aN IDEA,

LIEK,

WE'LL PUT,

for EXTRA LIFT>>

AN AUTO GYRO PROP ON THE NOSE!!111!!!!1111ONE!!!




WHAT DO YOU THINK,, GYRO CAPTAIN???

petermac33
WA, 6415 posts
19 Dec 2011 3:02PM
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This conversation is complete double dutch to me.

Not one sentence can i make head nor tail of.

All i know is,if you sheet in,in the gusts,you go faster or you go over the handle bars.

Te Hau
479 posts
19 Dec 2011 6:32PM
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sausage said...

Te Hau said...

Way too many pies!


Hey, I resemble that

Very interesting (and intellectual) thread too.


Alright...way too many sauasages

slowboat
WA, 553 posts
19 Dec 2011 7:07PM
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Ian K said...
The second reference you gave has the Cl increasing from 0.8 to 1.1 at a clearance of 0.5 the chord - which might be about 50mm. Parts of the board near the nose where it has more clearance, will have less of a gain, those closer to the wet area... will be getting wet.


err more like 0.05 of chord (which is more like 1m )Sometimes two wrongs combine to get the right number... But then look at the area from the mast track back that is sitting at 1 or 2 cm off the water, with a chord of ~1m in the apparent wind direction... The first graph shows that things get funky under 5% of chord. Lift/drag goes thru the roof. I dont think thats just the drag disappearing.

Te Hau
479 posts
19 Dec 2011 7:07PM
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petermac33 said...

This conversation is complete double dutch to me.

Not one sentence can i make head nor tail of.

All i know is,if you sheet in,in the gusts,you go faster or you go over the handle bars.


Yep, I reckon some of these boyos may be getting baffled by science.....

what I know is that my boards get up in the air in the bear offs and hover with the whole lot out of the water.The faster I go the more lift I get off the nose and I build my boards with varying nose areas just to use this effect.
I'm up to speedy #8 now and mostly playing with this effect.
The fastest runs I've done have been hover jobs and I don't feel the chop 'cos the board's not in the water.I call that sail lift and nose lift.
I've also (for other sailors) modified a number of older longer slalom boards for using as speed boards.
They all have the problem of too much nose lift in big gusts and by shortening the nose and reducing the area this problem comes totally under control so I guess that indicates that nose lift is real and it is a sizeable force.
No science, no theory just the real world , chop it up and see.

slowboat
WA, 553 posts
19 Dec 2011 7:11PM
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yep mike- thats looking at it from the other angle... build it and see. The fastest boards from the last 5 or 6 years were designed to exploit aerodynamic nose lift.

Wineman
NSW, 1412 posts
19 Dec 2011 10:49PM
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Te Hau said...

Yep, I reckon some of these boyos may be getting baffled by science.....

what I know is that my boards get up in the air in the bear offs and hover with the whole lot out of the water.The faster I go the more lift I get off the nose and I build my boards with varying nose areas just to use this effect.
I'm up to speedy #8 now and mostly playing with this effect.
The fastest runs I've done have been hover jobs and I don't feel the chop 'cos the board's not in the water.I call that sail lift and nose lift.
I've also (for other sailors) modified a number of older longer slalom boards for using as speed boards.
They all have the problem of too much nose lift in big gusts and by shortening the nose and reducing the area this problem comes totally under control so I guess that indicates that nose lift is real and it is a sizeable force.
No science, no theory just the real world , chop it up and see.



Well that's gone & done it!

How to destroy a perfectly good theorising, hocus pocus thread by being practical & actually building AND testing somefink.

What's poor Barn gonna fearorise or preach on now.
(Maybe go sailing )

barn
WA, 2960 posts
19 Dec 2011 8:40PM
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Wineman said...

Te Hau said...

Yep, I reckon some of these boyos may be getting baffled by science.....

what I know is that my boards get up in the air in the bear offs and hover with the whole lot out of the water.The faster I go the more lift I get off the nose and I build my boards with varying nose areas just to use this effect.
I'm up to speedy #8 now and mostly playing with this effect.
The fastest runs I've done have been hover jobs and I don't feel the chop 'cos the board's not in the water.I call that sail lift and nose lift.
I've also (for other sailors) modified a number of older longer slalom boards for using as speed boards.
They all have the problem of too much nose lift in big gusts and by shortening the nose and reducing the area this problem comes totally under control so I guess that indicates that nose lift is real and it is a sizeable force.
No science, no theory just the real world , chop it up and see.



Well that's gone & done it!

How to destroy a perfectly good theorising, hocus pocus thread by being practical & actually building AND testing somefink.

What's poor Barn gonna fearorise or preach on now.
(Maybe go sailing )


Hey Wineman, where did I say any of this lift wasn't important?

I'm simply disputing the claim that 100% of the weight can be lifted by a windsurfer without any force going through the wetted area..

I was first told it can be taken from the sail. I debunked that. Then it turned into HGE and I was told that I would need to do a handstand on a board to keep it from taking off at 70k...

Now I'm just after a Figure of lift. The ballpark figure is from my 5kg to Slowies 25kg of lift from the board..

Extraordinary claims require extraordinary evidence.. So far it seems everybody has backed off their claims except me..


(And I can't go sailing, I'm injured)

Wineman
NSW, 1412 posts
19 Dec 2011 11:54PM
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We're all luvin' it Barn.

That was my weird way of encouraging you to continue.

So far 3667 reads of this topic - it's got us all (except Petermac) interested.

barn
WA, 2960 posts
19 Dec 2011 9:24PM
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^^ hahah I don't need encouragement!

After the 30knot scenario broad reaching in 25knots, I can move onto sailing deep downwind at 45knots in 45knots of wind at SP on a 40cm wide board, that'll get interesting..

If my injury doesn't heal by next year I may have to take up this leisurely sport of Speedsailing, then I won't be all talk..

mr love
VIC, 2345 posts
20 Dec 2011 7:59AM
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You need evidence?

Sandy Point 2010.
Mr Love takes off from the East bank on the Luv Mus Small, crosses the inlet and bears off hard down the run just at the end of the dunes. The wind is quite square , probably 30 knots and the water super flat. Next thing I know is Mr Love . 4 to 5 feet in the air, still in normal sailing position, nose of the board maybe 20 to 30 degrees from Horizontal, but no looping here , just sailing 4 feet off the water.
Mr Love bails, lands on his arse and if not for the wetsuit would have received intense colan cleansing. GPS reported the incident took place at 41 knots of board speed.
Witnessed by Seahorse who was standing on the bank right where it happened and who could not stop laughing for the rest of the day.

Where did all that lift come from????? 90kg me , 5 kg board, 10 plus KG rig, 4 feet plus in the air, almost mirror flat water, no ramps to launch off here.

barn
WA, 2960 posts
20 Dec 2011 9:36AM
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mr love said...

You need evidence?

Sandy Point 2010.
Mr Love takes off from the East bank on the Luv Mus Small


Hold it, this sounds like a personal account, the most notorious form of evidence possible.. There was even a Church Of Speedsailing disciple there to witness this aqua miracle!!



Now, we have all been lifted out of the water, but usually that is just transfering forward speed into vertical lift, and doesn't last long. Sustained sailing 4 feet above the water would of course contradict the HGE hypothesis..

----

I note there is some Anti-Science factions forming, this usually happens in any Church when the Science contradicts Doctrine... I'll just note that whenever anybody is in a plane a few thousand feet in the air, that planes is up there thanks to Scientific theory.. It works..

mr love
VIC, 2345 posts
20 Dec 2011 1:11PM
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No hypothesis from me about what type of lift it was, forward motion transfered into vertical lift ect ect, thats why I asked you rocket scientists 'where did that lift come from", but it happened. And yes it didn't last long, I s..t myself and bailed.
Where did you get the latest photo of the PiT crew, didn't think we had released that one yet!!! Knew I left my sandles somewhere.

Ian K
WA, 4048 posts
20 Dec 2011 10:33AM
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Te Hau said...

petermac33 said...

This conversation is complete double dutch to me.

Not one sentence can i make head nor tail of.

All i know is,if you sheet in,in the gusts,you go faster or you go over the handle bars.


Yep, I reckon some of these boyos may be getting baffled by science.....

what I know is that my boards get up in the air in the bear offs and hover with the whole lot out of the water.The faster I go the more lift I get off the nose and I build my boards with varying nose areas just to use this effect.
I'm up to speedy #8 now and mostly playing with this effect.
The fastest runs I've done have been hover jobs and I don't feel the chop 'cos the board's not in the water.I call that sail lift and nose lift.
I've also (for other sailors) modified a number of older longer slalom boards for using as speed boards.
They all have the problem of too much nose lift in big gusts and by shortening the nose and reducing the area this problem comes totally under control so I guess that indicates that nose lift is real and it is a sizeable force.
No science, no theory just the real world , chop it up and see.


I'm not getting baffled by the science, I'm a scientist, in a previous life I used to design wind tunnels.

We're just using unfamiliar terminology that makes it seem more complicated than it really is. All you really need to be able to do is visualise pressure. Blow up a ballon a few times and get the hang of pressure.

Then if you can arrange for the pressure of air or water on one side of a solid object to be more than the other you get lift.

But one big constraint on all this is as a particle of fluid, air or water, moves into a region of higher pressure it must slow down. All a particle of fluid, say a 1cm by 1 cm by 1cm cube can see is its neighbours, if the one in front is pushing back with higher pressure than the one behind it'll slow down.

You've probably seen the equation F=ma the harder you push something the quicker it accelerates. This equation is called Newton's second law of motion. It's pretty intuitive, we all know you've got to push heavy things harder to get them up to speed.

The fluid dynamicists modified Newton's equation to allow for the flexible nature of fluids, the force becomes a pressure gradient, friction became viscosity, they still refer to it as the Navier-Stokes equation because that sounds more complex than "F=ma for fluids".

So in specially designed wings the air over the top speeds up a bit and the air below slows down a bit. But you can only speed air up by a small percentage, and slow the air down below by a small percentage, otherwise smooth flow becomes unfeasible. The more wind speed you have to start with the more you can slow the air down without upsetting smooth flow.

That's why the wing loadings for various aircraft are much higher for faster planes, you can afford to drop or raise the pressure further without correspondingly slowing or speeding the airstream up or down by more than a few percent.

The hang glider - wingloading 6.3 kg per square metre goes about 30 knots
Spitfire 158 kg per square metre 300 knots
A380 663 Kg per square metre even faster

So ground effect or not I have trouble seeing any wing of 1 square metre developing much more than 6.3 kg of lift efficiently in 30 knots. In attempting to generate 25 kg the air on the underside would be brought to a stand still or the air on the upper surface coaxed to do more than 50 knots, or a combination of both, unlikely, I can't visualise a flow pattern consistent with this.

yoyo
WA, 1646 posts
20 Dec 2011 2:51PM
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In 2006 Sam Parker did a table top similar to Martin in front of us as Chris and I were walking back up the bank. In his case he got about 2-3 feet up, board stayed horizonal and he landed it. Initially I thought he must have hit a bit of chop that wasn't apparant. On reflection, the bank was quite steep and I think the wind bounced off the surface. An inverted microburst so to speak.

Talking to him afterwards, admiring his skill I got the impression he was just as surprised as us, froze for a second by which time he had landed, so he continued on as if nothing had happened.

Te Hau
479 posts
20 Dec 2011 4:29PM
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So ground effect or not I have trouble seeing any wing of 1 square metre developing much more than 6.3 kg of lift efficiently in 30 knots. In attempting to generate 25 kg the air on the underside would be brought to a stand still or the air on the upper surface coaxed to do more than 50 knots, or a combination of both, unlikely, I can't visualise a flow pattern consistent with this.


6.3kg of lift would obviously never get the gear out of the water, so why does it happen and how do we hover it all for 1-300 metres at a time?

ikw777
QLD, 2995 posts
20 Dec 2011 11:46PM
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Te Hau said...

DarrylG said...

A fin angled at 45 will also give a bit of lift I guess




Way too many pies!


I didn't know Jack Black windsurfed!

Roar
NSW, 471 posts
21 Dec 2011 12:53PM
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remember there are other forces involved than just the board creating the lift.

When the sail is raked back and the gap between the board and sail is closed the sail itself is generating significant lift which is pullign up thru the mast base.

The amount of lift needed from the board itself is probably failry minimal. on a lot of baords the biggest problem speed sailers have is actually keeping the boards to stay down and flat.

You also need to take into acount the amount of lift the fin is generating.

looking from where the force is applied by both sail and fin the sail would be doing most of the lifting and the fin only really lifts the last 25% of the board.
Getting this balance right would give the board the right angle of attack to generate the ground effect that makes it feel like its hovering.

Too much angle and the board will flap up and down - too little wont see enuf air getting in under the board to do the job and it will slam thru chop.

da vecta
QLD, 2512 posts
21 Dec 2011 1:54PM
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At these speeds too there is a small amount of Vortex Shedding coming from your arms, legs and even body.



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Forums > Windsurfing   Gps and Speed talk


"Flying the fin??" started by paddymac