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Fins - stiffness

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Created by joe windsurf 31 days ago, 18 Dec 2018
joe windsurf
1418 posts
18 Dec 2018 8:24AM
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in the past i purchased fins based on length, area, shape, rake and thickness
(the matching finbox is obvious)

just lately i started looking at stiff fins versus soft
(that is nothing to do with Freud or ...)

has the thinking changed regarding soft fins ??
at first people complained about the JP SLW 56 cm fin being too soft
now i am reading soft fins are great for pumping onto a plane ...

so, should one have soft and stiff fins for a board like a JP SLW (large freeride or race) and for formula boards ??

stehsegler
WA, 2930 posts
18 Dec 2018 12:19PM
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Have a look at this link for explanation in regards to fin stiffness:www.k4fins.com/fin-guide/

olskool
QLD, 942 posts
18 Dec 2018 5:41PM
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Hey Joe, im no guru but heres my experience with fins on Starboard 380 Phantom. Ezzy 9.5lion. Me 96kg.Used mainly on reaching course. Started with a
Drake R19 Race 480NR. Great fin in light winds. Quite flexy n can be pumped onto the plane easily. All good until about 13-15kts. Then that flexiness becomes a disadvantage. In chop n current it becomes squirrelly. Is hard to control the lift it produces. It darts everywhere.
Other fin i use, now my favourite on RB
Drake R13 Race 520NR its a bit thicker foil. Nowhere near as flexy. Doesnt pump onto the plane as easily as R19 480. But the more powerful longer foil of R13 520 gets going easily n stays in the game in higher winds with much better control.
I was also having issues with fins for my Ahd Freediamond 149 n Ezzy lion 9.5m Kept spinning the fin 52cm Select freeride n 52cm Select S10 elite. Changed to Select Vmax3 57cm. (Stiff fin) Well that changed the whole ride. No more spin. Every puff of wind energy is now converted to forward drive. Comes outta the blocks like a slingshot up to about 18kts wind Then it starts lifting n railing. Best fin i own for the application. By far.

Imax1
VIC, 1613 posts
19 Dec 2018 10:11PM
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I once asked the same kind of question in regards to what is better to get heavy me on to the plane. A stiff fin to push against or a flexy one that creates more lift when bending . Just about started a full blown argument between different theory's . Got no definitive answer .
My personal theory is to try heaps of fins and keep the onesi like

joe windsurf
1418 posts
19 Dec 2018 7:15PM
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^ believe that seems to be the correct answer
get one of each

check out Ander's fins

olskool
QLD, 942 posts
20 Dec 2018 6:30PM
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Back to the flexy vs stiff fin debate. On 130-150litre n flatwater im sold on stiff fins. JP Supersport, MFC H1, Vmax3. Not into the squishy let go of a flexy style fin.eg Tectonics Raptor. Just my preference....

ClausF
22 posts
20 Dec 2018 5:22PM
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According to Gasoil Fins (free translation from French):

Hardness H (Hard):
Behavior: The fin will not bend easily, allowing the board to settle and gain control. In return, the effect of lift is limited and the board is less free on the water.

Hardness M (Medium):
Behavior: The fin bends moderately in tip. This provides an easily perceptible and usable foil effect, the board plans more freely on the water but the control is not impaired.

Hardness S (soft):
Behavior: The bottom third of the fin bends with an important angle, and provides a strong lift effect. The board will sail on the fin with a reduction of the wet surface area, but board control will suffer more or less seriously, especially if the wind rises..

Imax1
VIC, 1613 posts
20 Dec 2018 8:51PM
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ClausF said..
According to Gasoil Fins (free translation from French):

Hardness H (Hard):
Behavior: The fin will not bend easily, allowing the board to settle and gain control. In return, the effect of lift is limited and the board is less free on the water.

Hardness M (Medium):
Behavior: The fin bends moderately in tip. This provides an easily perceptible and usable foil effect, the board plans more freely on the water but the control is not impaired.

Hardness S (soft):
Behavior: The bottom third of the fin bends with an important angle, and provides a strong lift effect. The board will sail on the fin with a reduction of the wet surface area, but board control will suffer more or less seriously, especially if the wind rises..




I understand this , but what's better for getting heavy me planing . Once going there is not a lot of difference to lighter guys , it's the getting out of the water that's a lot harder . It seems about 2 kts difference to my lighter mates. When it hits 15 kts I have no disadvantage. It's that all to common 11 to 13 kts range.
I find a super stiff , wide , chopped down formula fin works best for me. 52cm.

joe windsurf
1418 posts
20 Dec 2018 7:02PM
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Imax: once the fin is chopped, did you round the bottom or keep it flat ??

Imax1
VIC, 1613 posts
21 Dec 2018 6:57AM
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joe windsurf said..
Imax: once the fin is chopped, did you round the bottom or keep it flat ??


Yes I swept the fin back starting about 200mm from the tip. And reshaped the front foil in that area so the fin was getting thinner at the tip . Still very stiff though. Feels dead but gets me going early.

jusavina
QLD, 1151 posts
21 Dec 2018 3:45PM
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Get a foil

sailquik
VIC, 4217 posts
24 Dec 2018 8:52PM
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There are many different types of fins and uses for them, and there is no rule that covers all the different types.

For instance. Speed fins must be super stiff. When you are working with a tiny angle of attack at top speeds, half of a degree of flex or twist completely wrecks the designed flow.

Wave fins can work really well if they have lots of twist and flex for maneuverablity and they generally operate at quite slow speeds.

etc, etc.

Roo
612 posts
25 Dec 2018 4:23AM
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sailquik said..
When you are working with a tiny angle of attack at top speeds, half of a degree of flex or twist completely wrecks the designed flow.

etc, etc.


Twist is exactly what you want, just has to be designed into the fin to work properly. Funnily enough I have a machine that measures both twist and flex.

Ian K
NSW, 2752 posts
26 Dec 2018 11:39AM
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Roo said..

sailquik said..
When you are working with a tiny angle of attack at top speeds, half of a degree of flex or twist completely wrecks the designed flow.

etc, etc.



Twist is exactly what you want, just has to be designed into the fin to work properly. Funnily enough I have a machine that measures both twist and flex.


I'd have thought for speed sailing (not that I was an exceptionally fast speed sailor) that if your fin was working at a tiny angle of attack you should reduce the fin size so it will be working closer to the angle where lift to drag is a maximum?



A windsurfing fin probably doesn't get its best L/D at 7 degrees like this one but there'll be a maximum at some angle. Why is it so hard to google graphs like this of actual windsurfing fins?

You probably don't need flex if sailing in very flat water. Flat water should have correspondingly low turbulence so you won't need flex to absorb changing angles of attack as you speed through eddies. If twist at the tip was a good thing for speed sailing in low turbulence water you could lock it in with an asymmetric design.

sailquik
VIC, 4217 posts
28 Dec 2018 10:20PM
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Roo said..


sailquik said..
When you are working with a tiny angle of attack at top speeds, half of a degree of flex or twist completely wrecks the designed flow.

etc, etc.



Twist is exactly what you want, just has to be designed into the fin to work properly. Funnily enough I have a machine that measures both twist and flex.




That is great Roo, and it will be interesting to see what you can achieve in slalom type rough water conditions where flex and twist seem to be a big advantage for board handling.
I think you will eventually find that the stiffest fins will be the fastest on a speed course, as we did.

sailquik
VIC, 4217 posts
28 Dec 2018 10:37PM
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Ian K said..
I'd have thought for speed sailing (not that I was an exceptionally fast speed sailor) that if your fin was working at a tiny angle of attack you should reduce the fin size so it will be working closer to the angle where lift to drag is a maximum?



A windsurfing fin probably doesn't get its best L/D at 7 degrees like this one but there'll be a maximum at some angle. Why is it so hard to google graphs like this of actual windsurfing fins?

You probably don't need flex if sailing in very flat water. Flat water should have correspondingly low turbulence so you won't need flex to absorb changing angles of attack as you speed through eddies. If twist at the tip was a good thing for speed sailing in low turbulence water you could lock it in with an asymmetric design.



As I understand it, that is what Mal Wright called the "Drag Bucket". He did some calculations which showed that at top speed the AoA of his fins was around 1 degree.

The issued we found was that the fins that turned super fast and slippery at high speed,(great L/D) were the hardest to get to 'drop in' to the 'drag bucket'. In everything, there are compromises.....

While I was chatting with Mal about fins the other day I realised that I have not used any of the TM fins since I was at Luderitz in 2013.
The reason is that pretty much everywhere I speed sail these days requires at least 25 degrees of rake to shed weed, (even Sandy Point!) and the most of the best spots need 40-50+ degrees. Those fins are nothing even remotely like the TM fins @ 12 degrees..

And yes, using the smallest fin I can get going, and maybe back upwind on, seems to really work for me, especially if it is asymmetrical, which decreases the AoA even more.
I think I larger fin will have a lower angle of attack at any given speed. But it is combination of L/D that we are seeking. Mal was the master at doing insane speeds on his insanely stiff TM V8 assy 23 cm fins at tight wind angles that none of the rest of us could get anywhere near, except on broader angles in more wind.

Pacey
WA, 57 posts
28 Dec 2018 10:05PM
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sailquik said..
As I understand it, that is what Mal Wright called the "Drag Bucket". He did some calculations which showed that at top speed the AoA of his fins was around 1 degree.

The issued we found was that the fins that turned super fast and slippery at high speed,(great L/D) were the hardest to get to 'drop in' to the 'drag bucket'. In everything, there are compromises.....

While I was chatting with Mal about fins the other day I realised that I have not used any of the TM fins since I was at Luderitz in 2013.
The reason is that pretty much everywhere I speed sail these days requires at least 25 degrees of rake to shed weed, (even Sandy Point!) and the most of the best spots need 40-50+ degrees. Those fins are nothing even remotely like the TM fins @ 12 degrees..

And yes, using the smallest fin I can get going, and maybe back upwind on, seems to really work for me, especially if it is asymmetrical, which decreases the AoA even more.
I think I larger fin will have a lower angle of attack at any given speed. But it is combination of L/D that we are seeking. Mal was the master at doing insane speeds on his insanely stiff TM V8 assy 23 cm fins at tight wind angles that none of the rest of us could get anywhere near, except on broader angles in more wind.


The drag bucket is typically not very wide, so if you reduce the area of your fin you will increase the angle of attack and lift coefficient needed to produce the required amount of lift and make it harder to get into the drag bucket. If you are reducing area by reducing chord while keeping span the same, you will also increase the amount of induced drag at about the same rate that you reduce profile drag due to the reduction in area for a given amount of lift.

Similarly, having an asymmetrical fin doesn't really reduce the angle of attack of the fin to the water, as the asymmetry makes the angle of zero lift go negative. The actual angle between this negative angle of zero lift and the angle needed to produce the lift required is about the same as for a symmetrical fin, the difference is that the water flow is better aligned with the centreline of the board and the drag bucket of the fin is hopefully centralised around the angles of attack most commonly used during sailing.

The best way to get good L/D is increasing span (i.e. fin length) as much as possible, not by reducing area to a minimum. Unfortunately that increases the overturning moment generated by the fin and conflicts with the amount of righting moment that can be provided by the feet on a narrow board. Deep fins are also not compatible with the the shallow water depths available on many of the best speed runs.

In aircraft design where high L/D was crucial (large airliners trying to increase fuel efficiency, sailplanes), wingspan used to be crucial. These days, winglets have changed the situation, allowing shorter wingspans to generate higher L/D ratios, without increasing the bending moment at the wing root. Maybe there is a case in speed sailing for a small winglet on the windward side of the fin for the favoured run direction to increase the effective span of the fin, reduce the induced drag, and increase the overall L/D ratio of the fin.

This is similar in a lot of ways to what Neil Scheltema has been doing with adding winglets to conventional fins. While some people have interpreted these as being mini-hydrofoils, in my view they work primarily to increase the L/D ratio of the fin while providing damping of pitch and heave motions. Neil's winglets have been symmetricalport and starboard,so the fin works equally on both tacks, but this isn't necessary for a speed fin. There is no reason not to have a winglet just on one side, particularly on an asymmetrical fin.

Maybe it's time I built one and tried it out.

Ben1973
134 posts
29 Dec 2018 1:24AM
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joe windsurf said..
in the past i purchased fins based on length, area, shape, rake and thickness
(the matching finbox is obvious)

just lately i started looking at stiff fins versus soft
(that is nothing to do with Freud or ...)

has the thinking changed regarding soft fins ??
at first people complained about the JP SLW 56 cm fin being too soft
now i am reading soft fins are great for pumping onto a plane ...

so, should one have soft and stiff fins for a board like a JP SLW (large freeride or race) and for formula boards ??


I stuck a 50cm tectonic raptor in my fox 140, awesome fin for my 95kg and 8.6 sail. Gets going quick and the back of the board seems to ride higher with it

sailquik
VIC, 4217 posts
29 Dec 2018 9:09AM
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Select to expand quote
Pacey said..



sailquik said..
As I understand it, that is what Mal Wright called the "Drag Bucket". He did some calculations which showed that at top speed the AoA of his fins was around 1 degree.

The issued we found was that the fins that turned super fast and slippery at high speed,(great L/D) were the hardest to get to 'drop in' to the 'drag bucket'. In everything, there are compromises.....

While I was chatting with Mal about fins the other day I realised that I have not used any of the TM fins since I was at Luderitz in 2013.
The reason is that pretty much everywhere I speed sail these days requires at least 25 degrees of rake to shed weed, (even Sandy Point!) and the most of the best spots need 40-50+ degrees. Those fins are nothing even remotely like the TM fins @ 12 degrees..

And yes, using the smallest fin I can get going, and maybe back upwind on, seems to really work for me, especially if it is asymmetrical, which decreases the AoA even more.
I think I larger fin will have a lower angle of attack at any given speed. But it is combination of L/D that we are seeking. Mal was the master at doing insane speeds on his insanely stiff TM V8 assy 23 cm fins at tight wind angles that none of the rest of us could get anywhere near, except on broader angles in more wind.





The drag bucket is typically not very wide, so if you reduce the area of your fin you will increase the angle of attack and lift coefficient needed to produce the required amount of lift and make it harder to get into the drag bucket. If you are reducing area by reducing chord while keeping span the same, you will also increase the amount of induced drag at about the same rate that you reduce profile drag due to the reduction in area for a given amount of lift.

Similarly, having an asymmetrical fin doesn't really reduce the angle of attack of the fin to the water, as the asymmetry makes the angle of zero lift go negative. The actual angle between this negative angle of zero lift and the angle needed to produce the lift required is about the same as for a symmetrical fin, the difference is that the water flow is better aligned with the centreline of the board and the drag bucket of the fin is hopefully centralised around the angles of attack most commonly used during sailing.

The best way to get good L/D is increasing span (i.e. fin length) as much as possible, not by reducing area to a minimum. Unfortunately that increases the overturning moment generated by the fin and conflicts with the amount of righting moment that can be provided by the feet on a narrow board. Deep fins are also not compatible with the the shallow water depths available on many of the best speed runs.

In aircraft design where high L/D was crucial (large airliners trying to increase fuel efficiency, sailplanes), wingspan used to be crucial. These days, winglets have changed the situation, allowing shorter wingspans to generate higher L/D ratios, without increasing the bending moment at the wing root. Maybe there is a case in speed sailing for a small winglet on the windward side of the fin for the favoured run direction to increase the effective span of the fin, reduce the induced drag, and increase the overall L/D ratio of the fin.

This is similar in a lot of ways to what Neil Scheltema has been doing with adding winglets to conventional fins. While some people have interpreted these as being mini-hydrofoils, in my view they work primarily to increase the L/D ratio of the fin while providing damping of pitch and heave motions. Neil's winglets have been symmetricalport and starboard,so the fin works equally on both tacks, but this isn't necessary for a speed fin. There is no reason not to have a winglet just on one side, particularly on an asymmetrical fin.

Maybe it's time I built one and tried it out.




Great insight! Thanks Andrew.

Yes, the main reason I use shorter fins is that I was having control issues with the deeper ones.

These days, sailing in weedy water also changes things. The deeper fins get more drag from the weed.

It seems quite obvious that the very low aspect ratio "Delta' type fins will have a very inferior L/D ratio, but they seem to be able to enable reasonably high speeds anyhow in the very flat, weedy environment. Is it possible that the weed actually changes there water flow over the fin, producing something like the winglet effect??

And what is the effect on L/D ratio when one rakes a conventional, narrower chord fin back to 45 degrees?

joe windsurf
1418 posts
29 Dec 2018 8:20AM
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Ben1973 said..


joe windsurf said..
in the past i purchased fins based on length, area, shape, rake and thickness
(the matching finbox is obvious)

just lately i started looking at stiff fins versus soft
(that is nothing to do with Freud or ...)

has the thinking changed regarding soft fins ??
at first people complained about the JP SLW 56 cm fin being too soft
now i am reading soft fins are great for pumping onto a plane ...

so, should one have soft and stiff fins for a board like a JP SLW (large freeride or race) and for formula boards ??




I stuck a 50cm tectonic raptor in my fox 140, awesome fin for my 95kg and 8.6 sail. Gets going quick and the back of the board seems to ride higher with it



the supplied fin - SV F-Series 440 was not enough ?? for the FOX 140 with 8.6 (severne sail i presume :-)

Ben1973
134 posts
29 Dec 2018 9:01AM
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It worked fine and I still use it when well powered up on lumpy water but I was looking for something a bit racier for the lighter days.
F44 fun fin, easy to sail matches the idea of the board perfectly
Raptor 50 bit more to push against to get the board up and going, racier feel, better upwind.

I really like the feel of a race board but some days you just want to have a easy blast, The fox is a joy to sail and switching between the 2fins gives me both options.

I did have my doubts about the F44 and a 8.6 Overdrive with my 95kg but was supprised how well it worked, it's quite a fat section which I think helps

Ian K
NSW, 2752 posts
29 Dec 2018 2:23PM
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sailquik said..

Ian K said..
I'd have thought for speed sailing (not that I was an exceptionally fast speed sailor) that if your fin was working at a tiny angle of attack you should reduce the fin size so it will be working closer to the angle where lift to drag is a maximum?








As I understand it, that is what Mal Wright called the "Drag Bucket". He did some calculations which showed that at top speed the AoA of his fins was around 1 degree.




Probably the same thing.
This curve plots Lift to drag holding speed constant and varying the angle of attack.

If you hold lift constant and plot lift to drag against speed you get the bucket shaped curve.

One degree doesn't sound like much of an angle of attack. Be interesting to see some curves for actual windsurfing fins.

Maybe in shallow weedy water the loss of lateral L/D of the short stubby fins is more than overcome by the gains in vertical L/D of the hull in shallow water. Do you use wider boards in the weedy speed sailing locations?

ZYX
52 posts
4 Jan 2019 10:00AM
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Pacey said..
...While some people have interpreted these as being mini-hydrofoils, in my view they work primarily to increase the L/D ratio of the fin while providing damping of pitch and heave motions. ...Maybe it's time I built one and tried it out.

Are you saying that the smaller winglets may work better compared to the existing winglet high speed fins?
&t=2s
if you noticed swept forward winglets on this video. May be for a reason?
Do you have a theory supporting one sided winglet design for an asymmetrical fin? Or it is coming from air travel experience watching most of the airplanes have winglets up in order to have minimum required ground clearance. Some airplanes have winglets up and down. On what side do you think to have a winglet on an asymmetrical fin? How would you set an angle of attack in case of a one side winglet and on each of the winglets if you think an asymmetrical should have two winglets?
In case of one winglets or two winglets with opposite angles of attack how would you attache this winglet to the tip of the fin which is some 1mm thick considering you want it for high speed, which results in high unbalanced bending moment. I know how it is done structurally on the video above because I have this fin but with smaller (15mm) wiglets. Base on the methods of attachment of mini-foils shown on this forum structural integration of the unbalanced winglet to the fin will be a problem if you want to keep reasonably small frontal area, which is critical for high speed performance.

seanhogan
2969 posts
4 Jan 2019 3:20PM
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video title : Fin for High speed !!!
that was so painfully slow to watch

Pacey
WA, 57 posts
4 Jan 2019 10:31PM
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XYZ said..



Pacey said..
...While some people have interpreted these as being mini-hydrofoils, in my view they work primarily to increase the L/D ratio of the fin while providing damping of pitch and heave motions. ...Maybe it's time I built one and tried it out.




Are you saying that the smaller winglets may work better compared to the existing winglet high speed fins?
&t=2s
if you noticed swept forward winglets on this video. May be for a reason?
Do you have a theory supporting one sided winglet design for an asymmetrical fin? Or it is coming from air travel experience watching most of the airplanes have winglets up in order to have minimum required ground clearance. Some airplanes have winglets up and down. On what side do you think to have a winglet on an asymmetrical fin? How would you set an angle of attack in case of a one side winglet and on each of the winglets if you think an asymmetrical should have two winglets?
In case of one winglets or two winglets with opposite angles of attack how would you attache this winglet to the tip of the fin which is some 1mm thick considering you want it for high speed, which results in high unbalanced bending moment. I know how it is done structurally on the video above because I have this fin but with smaller (15mm) wiglets. Base on the methods of attachment of mini-foils shown on this forum structural integration of the unbalanced winglet to the fin will be a problem if you want to keep reasonably small frontal area, which is critical for high speed performance.






From cms.education.gov.il/NR/rdonlyres/D9F6FC7B-A508-43C8-BB34-5C6D8AE0346D/178686/Understanding_Winglets_Technology.pdf

"Whitcomb optimized his original winglet design for the cruise speed and lift coefficient of a typical jet transport. He fitted the wingtip with a comparatively small lower winglet located near the wingtip leading edge and a much larger upper winglet farther aft. Whitcomb later concluded that the upper/lower winglet combination produced very small reductions in induced drag compared to the upper winglet alone and that it complicated ground clearance problems. Very few aircraft, as a result, presently use the upper/ lower winglet combination. Only upper winglets have been fitted to business aircraft, with virtually no exceptions."

I think an ideal single sided winglet on an asymmetrical fin would be of the blended winglet design, a nightmare to manufacture, but stronger and more efficient.


BEACHSTART
NSW, 68 posts
8 Jan 2019 10:02PM
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Sorry to say but I agree with Sean .
I watched a lot of windsurfing video every week and this one was slow. Keep up the training!

ZYX
52 posts
Monday , 14 Jan 2019 8:09AM
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I think the note about the board speed is sarcastic. Probably because you have just started to learn about wiglets and started to think how to start a productive and meaningful talk to approach a value added intention in writing down an outline of the action plan to plan talking to right people who can contribute expert opinion for building a prototype out of fiberglass laminated plywood. And it turned out frpgear has done winglets already.
I agree that we do not see the comparison of speed on the video: with winglets and without winglets on the same size fin and the same setup. We just see that it has been already done, but the results have not been reported to us. This is you opportunity to make your own winglet fin and please us with the results.
Even if winglets add speed it would be impossible to see the difference on a video. From the article about airplane wiglets you can see that 7% of efficiency is the maximum you get from the winglets. I think 1-2% is more common. 7% is on high loaded wings. Sometimes they use existing wing for a higher loaded airplane for cost reduction. When the airplane is designed initially it will not have a winglet because more span is better. Unless the optimal span exceeds requirements. The same with fin winglets. It is better to increase fin size than add winglets. In other words winglets will help to fix high induced drag on poor fin design.
In my own experience with winglets I notice more stability at high speed. I have seen someone on a forum (do not remember where) had small winglets and noticed better board stability on higher speed. I assume if the board remains more stable to goes faster. But I noticed that flexible fins deliver similar result, like my custom slalom 38cm fin which is only 8mm thick on the head.



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"Fins - stiffness" started by joe windsurf