hey decrepit thanks for the info I did enjoy your comments on the last post on fins
I'd like to be over 40 kn with these fins I realise this is going to take time "especially in NSW lets face it we struggle to get a 10.7 on the plane most weeks !!!
decrepit
I'd love to know more on how you work out these percentages
I have got on with one side of the fin with 2 layers of fiber glass woving 4 oz
and 2 layers of carbon and glass mix becase it looked good in the shop
do the rest tomorrow
does anyone know how to do the leading edge and trailing so its nice and neat ?

thanks for the foil Decrepit I'll try and follow it .. if you wish to PM me with more insights please feel free I'm loving having a go at this stuff and learning so much
cheers leo

Jesper that looks nice mate... good to see manufacturers listening

not had chance to give the delta a blast yet does look the goods

Select to expand quote

decrepit said...

lelos12345 said...

>>>>>>
NOW FROM WHAT I CAN GATHER IT NEEDS TO BE THICKER FOR IT TO BE STIFF

Or have a layer or two of carbon down it.

THE HIGH POINT SHOULD BE AROUND THE QUATER MARK BACK FROM LEADING EDGE
LEADING EDGE SHOULD BE SHARP AND STRAIGHT TO THE TIP

Hmm don't think so, my fins are around 40% to 45%. It's hard to get a fine leading edge with the thickest part so far forward.

Here's a 9% eppler foil yoyo gave me that I use for my fins.

BASS 10MM THICK AT AROUND 110 MM WIDE

that's about 9% thickness to flow ratio, should give a stiff powerful fin, but you could go down to 7.5% to 8% if you use carbon to stiffen it.
A lot depends how fast you plan on going, some of my fins are 9% and are good to 36kts, but much faster I suspect they may have ventilation problems.
If you're going over 45kts, 9% will probably start cavitating.

This has become a very interesting thread. Decrepit in that fin profile diagram towards the trailing edge it has a concave, what is the idea behind this how does it help the fin. I'm thinking it helps the water join together more smoothly and helps it exit the fin, reducing drag and possible spin out as well. It would also give the fin a very slightly thicker working profile ratio?
The wide point also seems to be back a bit.
Why don't you get a female mold cnc cut at the 7.5% and lay it up with carbon?

The eppler is pretty much the profile Boogie used on his custom carbon composite (C3) X2 fins some years back. I think they were originally 9% but by the time he found the best rake angle of 30 degrees they were probably 7.5% in waterflow direction.

Martin van Meurs got over 50knots on his 21 X2. This profile is better than any NACA profiles I've run on Xfoil but not as good as the custom ones Chris and I designed. Whether those advantages transfer fom the computer to the real world is open to debate.

after a good days work sat here itching big time !!
lets hope this SE holds for tomorrow
from a flat fin with quite a bit of flex I decided to give it more of a foil and a bit of strength thanks for the foil Decrepit ! i did my best to follow it
my first go at glassing went well thanks to FGI in Brookvale giving me a whole heap of roving some carbon some just glass. but good quality and free !!
got some resin a pair or two of gloves ... this is it
hope to test it tomorrow its hell stiff i put 4 layers of carbon and 6 of glass
lets hope for some better speeds 31.80kn before hope for 37kn
so the lenth of the bottom is 115mm and at its thickest now is 14mm does that make it 8.2%?
just based on numbers the less the % the quicker ?
and if so would shortening the width to 110mm would make it 7.8% and quicker ?
the fin is super stiff now so can i go shorter ? and does the hight of the fin alter the numbers .. sorry for being dum if someone can clarify that would be GREAT!!
comments welcome as i'm just learning cheers

That looks great Leo,
Now lets see if I can remember the questions.

Yes, chord (chord is the line at right angles to the center line of the foil) to thickness ratio, is the thickness divided by chord by 100, so your 115 and 14 is 12.17% a bit too thick.

Thicker isn't necessarily slower, it's all about lift to drag ratio.
A lot of literature recommends 12% to 14% as giving the best lift to drag ratio.
The higher lift of a thicker foil allows a smaller foil to be used.

This is fine at low speeds, it's what a lot of yachts use for their rudders and keels.
But with windsurfers we have the problem of the fin being close to the surface, and the potential of much higher speeds.

The higher chord to thickness ratios produce more lift, this also increases the high and low pressures around the fin. So a thicker fin is much more likely to suck air.

Then there is the problem of cavitation, this where the low pressure produced is low enough for the water to vaporise, this causes extra drag.

Not sure when these problems will start with a 12% foil, but be prepared for interesting times much over 30kts.
Must go for now back later.

Thanks decrepit thats great so ill give it a blast tomorrow see how i go..
so if i wanted to drop down to 9.5 i would ..if i was getting too much lift .. reduce thickness.
if lift was ok i would shorten the chord accordingly ?

one more.. with the new Vector fins the tip is very fine and pointy
with snides 22 it looks the opposite what effect does the tip of the fin have ?

my black fin the first one i made is stubby fat and round and it holds real well down and up wind 37.07 second time out in marginal winds ...what's your theory did i just get lucky ?

sorry for pestering just intrigued by it all

So to get back to your foil Leo, making in narrower to 110 mm would increase the chord to thickness ratio, not the way you need to go with that foil.

With that thickness in theory you can go very short, and reduce drag quite a lot, but unfortunately that will probably just make it more prone to ventilation, unless your in super smooth waters.

Actually of course, it's a weedy so the base length isn't the chord, the chord length will be shorter.
There's some confusion, (in my mind anyway) as just how the chord to thickness ratio and flow to thickness ratio relate. Intuition says that the flow to thickness ratio is what the water "sees" and should be the one to use, but some of the literature says otherwise.

So as yoyo says the foil he sent me is 9%, and on one of my fins I've made this the flow ratio, I've done 36kts. I'll need to qualify that it's asymmetric, so it's only 9% on one side, this may make a difference.
For the long fin of Benders I modified with some carbon I used the foil along the chord, bringing the flow back to about 7.5%. Bender reports the fin now works fine.

Rider

yes, the concaves allow to the 2 streams to merge with less turbulence.

Not sure about it affecting the "working profile".
Yep the thickest point is about 45%, this allows a more even curve, and a thinner entry, spreading the pressure differences out more. It's more likely to stall at higher angles of attack, but it's much faster at low angles.

I'm not into manufacturing, and all my fins are different.
if enough people wanted the same fin and contributed to the costs, it's a possibility.

thanks so much u are a wealth of knowledge be in touch tomz for sure !

Fin tip, Hmmm, that's where the turbulence caused by high pressure side trying to get to the low pressure side occurs. This causes a vortex coming off the fin tip, (induced drag)

What's his name who designed the spitfire wings and helped to win our side WW2 was probably the first to work this out. getting the right taper towards the wing tip, reduces the drag. Unfortunately the amount of taper needed increases with fin rake.
So the fine pointy tip of the new vector, looks like it's been designed with this in mind.

But coming back to the old ventilation problem, the more the fin lift is closer to the surface, the more likely it is to ventilate.
I think this is why a lot of fins carry area as deep as possible. It's all swings and roundabouts.

I've heard a few reports of the venoms being susceptible to spin out in chop, but great in flat water.

Yes you may have just been lucky, but theory doesn't all ways correspond to practice! (especially mine, I'm no expert, I've just been playing around with speed weed fins since I joined the GPSTC)

Hi decrepit
K after testing the fin in 14-18 kn. "not what i had in mind!!"
I rigged 7.5m Ka CA sp 50 with the 22 cm spweed

bit big ? maybe but not too bad all in all, if i was looking for a light wind fin it was nice only 28.65kn on the day as the wind after 45 mins did its usual disappearing act
after blowing 35 kn. all nite

so came home not disappointed but more determined to do more work on it

so now the chord is 12mm length 145mm with a flow of 8.2% if i've got that round my head right ?

Ive also brought back the high point 65mm shaped and tapered the leading edge to high point more gradual as the epler profile.

did a bit of work to the back edge too to try n get that concave to the trailing edge
few nicks to clean up but all in all I think it will be better

I did love the stiffness it had from the first time i sailed it .. i hope i"ve not lost too much

comments welcome

before n after

Bummer, I was hoping you'd try it at 12% in 25kts, just to test my theories.

Thinking back on how I formed them, it basically went along these lines.

most fins are between 7% and 9% flow to thickness ratio.
12% to 14% is supposed to have the best lift to drag ratio.

Why aren't windsurfer fins this thick??????
Ahhhh, they'll probably suck air down.

You can see how shaky this logic is.
Perhaps there's just a culture among fin makers that says thin fins are fast.
Or perhaps thin fins are cheaper to make.

I've never tried a 12% fin, maybe it would hang in to 40kts.
Now we'll never know.

Any way, I think you'll be safer on that as it is now.

U should of said i would of been happy to of tested it ! r well i can always bring it back up
theres wind tomorrow NE I'll test it even if i don't need a weed fin lets see what it will do... keep u posted
thanks for the tech info i have a greater understanding

What tools are you using for the shaping? I'm looking at having a go myself but I can't think what to use to get the concave trailing edge. Aside from the obvious - sand paper. Are you using hand tools, power tools....
Are you just eyeballing the shape or is there more to it than that?
Must be very rewarding when it all comes together.

I use a 100mm right angle grinder with a rubber backed sanding pad attachment, with a fine pad
Long continuous sweeps, with a lot of care. This will reshape the foil and hollow out the trailing edge no probs.
Any slight over shoots I bog back up and use a foam hand sanding pad to finish off with.

I've printed out that foil shape in a large range of sizes, small enough one end for the tip and big enough the other for the base.
Then I use a tiler's profile gauge to check the shape, about every centimetre.

Select to expand quote

lelos12345 said...

U should of said i would of been happy to of tested it ! r well i can always bring it back up
theres wind tomorrow NE I'll test it even if i don't need a weed fin lets see what it will do... keep u posted
thanks for the tech info i have a greater understanding

Nah leave it as it is, unless you feel it's not stiff enough. Another chance will come along.

Look forward to hearing how it works, that's the important bit.

I use automotive spray can undercoat/filler as a final coat, fills any gaps in the fibers, and can be wet and dried with 400 or 600 to a nice smooth finish.

Without a solid even colour it's hard to see imperfections in the finish.

Select to expand quote

decrepit said...



Not sure about it affecting the "working profile".

Well I figured that the convex part of the foil would create lift and the concave part would not. So I thought that the convex part would be the working part. I looked at the foil and just thought that the working area is quite thick compared to its length.

Select to expand quote

R1DER said...

decrepit said...



Not sure about it affecting the "working profile".

Well I figured that the convex part of the foil would create lift and the concave part would not. So I thought that the convex part would be the working part. I looked at the foil and just thought that the working area is quite thick compared to its length.

Well the part that's concave would normally be straight, don't think that will make a lot of difference.

Select to expand quote

Must be very rewarding when it all comes together.

hey Ian
I only use sand paper to begin with i use a file but would love to have a go with a grinder
the process for me with sand paper is laborious but so rewarding .
just make sure you go get a good paper suit and mask or ull itch like hell for days ... and that was without a grinder !!
enjoy !!

Select to expand quote

decrepit said...

lelos12345 said...

U should of said i would of been happy to of tested it ! r well i can always bring it back up
theres wind tomorrow NE I'll test it even if i don't need a weed fin lets see what it will do... keep u posted
thanks for the tech info i have a greater understanding

Nah leave it as it is, unless you feel it's not stiff enough. Another chance will come along.

Look forward to hearing how it works, that's the important bit.

hopefully a bit of a NE today should be able to give it a blast down the runway
If its good I'll finish it off with spray putty make it look real nice !!

I use automotive spray can undercoat/filler as a final coat, fills any gaps in the fibers, and can be wet and dried with 400 or 600 to a nice smooth finish.

Without a solid even colour it's hard to see imperfections in the finish.

hopefully a bit of a NE today should be able to give it a blast down the runway
If its good I'll finish it off with spray putty make it look real nice !!

Eppler section is more than just a benchmark. It was developed in the heyday of computer CFD by the hydrodynamics guru Richard Eppler. He factored a lot of things into the design and its been used for a lot of hydrofoil applications since- including many popular windsurfing fins in more recent years. As a "best foil for windsurfing" its hard to beat.

Thickness/chord- A thicker section is not necessarily bad. In terms of cavitation, generally the thinner you go, the faster you can go before cavitation sets in- assuming that the section shape is designed for that thickness (scaling down a section is OK but not optimal). The consequences of going thin are that the boundary layer is more easily stressed near the leading edge due to the more rapid change in curvature there. This restricts the maximum angle of attack before the foil separates near the leading edge- which triggers ventilation. Its also a lot more sensitive to machining/finish accuracy. The standard thickness/chord for G10 slalom fins is 10%. This is a good compromise for low speed performance, whilst maintaining adequate stiffness from the material, and still can push speeds into the mid-high 40s without cavitating. Some fins are even thicker. I've got some 13% ones that I pushed over 43kts back in the early days of GPS. That was the trigger for my investigating cavitation. Mal Wright and I both went on that mission at the time despite the "it cant happen" pretty much universally from the industry (including some "big name" designers). Now its a buzz word.

For reference, the KA Asymmetric fin that I designed has a thickness to chord of ~9%. Its proven many times that this is not too thick to do ~50kts and I conservatively designed it to do this speed. That was when peaks of 46kts were the fastest speeds being achieved in places other than the canal.

The concave on the trailing edge (reflex) changes the pressure distribution to be more gradual on the low pressure side where the pressure increases (recovers) towards the trailing edge. A flat, or convex trailing section will have a lower pressure gradient initially (ie pressure increases to that of the surrounding water more slowly), with a steeper recovery towards the trailing edge- the pressure distribution is convex.

A concaved trailing edge section puts the lifting centre slightly further forward as a result of the reduced aft pressure magnitude, and the lifting centre is more constant over angle of attack changes. This is a useful feature when you are limited by structural factors that require the lifting centre to remain stable over a range of angles.

For windsurfing using fins of the standard dimension, there are some advantages in flattening the trailing region. It does not impact significantly on predicted drag, but makes it a lot easier to manufacture and finish. It also has some low speed advantages.

So if you arent willing to go to town on CFD, testing, and theory, just stick with the eppler section and you cant go wrong.

the guru has spoken^^^^^

Select to expand quote

Troppo said...

the guru has spoken^^^^^

He may have spoken! but i need subtitles to understand what he just said

To demonstarte my understanding of what Chris said, I just transalted it to Japanese................. That will give you a direct experience of my understanding


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Read phoneticallyDictionary - View detailed dictionary

Wicked!! cheers hardie !

Thanks for this slowy, nothing like getting it from the horse's mouth.

Select to expand quote

slowboat said...

<<<<<<<

>>> I've got some 13% ones that I pushed over 43kts back in the early days of GPS.

There you go lelos, maybe you should have left it at 12%.

Select to expand quote

For reference, the KA Asymmetric fin that I designed has a thickness to chord of ~9%.

So slowy, can you comment on the difference between chord to thickness and flow to thickness ratios?
As the Ka at about 35 degrees is less than 9% flow to thickness.

Select to expand quote

slowboat said...

WOW. Mr Boat's theoretical knowledge is just as impressive as his sailing!
... and beautifully written too.

But what does it all mean?

...and does it solve the problem of where to get a good weedy?
I could use one too.

Easy peasy

Grab a reasonably good slalom fin and rebox it at 45°

Thin high speed slalom fins don't seem to work to well

Nothing more satisfying than taking a hacksaw to a Techtonics, although the smell is diabolical.

If you're a lazy basket like Bender you'll use a drop saw

I shudder to think how many fin's I have introduced to "my leetle friend" Mr Hacksaw

I've also created a bit of a hack happy monster with young Pepe as well

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decrepit said...

Thanks for this slowy, nothing like getting it from the horse's mouth.

slowboat said...

<<<<<<<

>>> I've got some 13% ones that I pushed over 43kts back in the early days of GPS.

There you go lelos, maybe you should have left it at 12%.

For reference, the KA Asymmetric fin that I designed has a thickness to chord of ~9%.

So slowy, can you comment on the difference between chord to thickness and flow to thickness ratios?
As the Ka at about 35 degrees is less than 9% flow to thickness.

ha ha yup seems that way... I did take it for a run yesterday but i was on the 7.5 AGAIN ! and it is too small. It definatley felt better before in stiffness and lift it seemed much more stable and in control.
I'll do more testing if the wind Ever blows ... might go into formula fins !

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Ian1 said...

What tools are you using for the shaping? I'm looking at having a go myself but I can't think what to use to get the concave trailing edge. Aside from the obvious - sand paper. Are you using hand tools, power tools....
Are you just eyeballing the shape or is there more to it than that?
Must be very rewarding when it all comes together.

For reshaping G10 fins I use a cutter bar from a wood workers 4 sider (thicknesser) machine. Its about 4cm wide, 35cm long and 5mm thick, with a good sharp edge.
I use it as a scraper and drag it down the fin.
Much more accurate than grinding and very easy to move thick point back.
For a concave trailing edge just grind the desired curve into a cutter bar and resharpen it.
Most woodworking shops have old cutter bars which are too far gone for high speed machining but good enough for scraping.
Scraping is reasonably accurate and a bit of a lost art. Used to be where the mains and big ends in your car (or your aeroplane) were likely to have been scraped when they were fitted.
Good enough for that, good enough for a windsurf fin.

sounds good can we get some pics of these tools ?

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Te Hau said...

Ian1 said...

What tools are you using for the shaping? I'm looking at having a go myself but I can't think what to use to get the concave trailing edge. Aside from the obvious - sand paper. Are you using hand tools, power tools....
Are you just eyeballing the shape or is there more to it than that?
Must be very rewarding when it all comes together.

For reshaping G10 fins I use a cutter bar from a wood workers 4 sider (thicknesser) machine. Its about 4cm wide, 35cm long and 5mm thick, with a good sharp edge.
I use it as a scraper and drag it down the fin.
Much more accurate than grinding and very easy to move thick point back.
For a concave trailing edge just grind the desired curve into a cutter bar and resharpen it.
Most woodworking shops have old cutter bars which are too far gone for high speed machining but good enough for scraping.
Scraping is reasonably accurate and a bit of a lost art. Used to be where the mains and big ends in your car (or your aeroplane) were likely to have been scraped when they were fitted.
Good enough for that, good enough for a windsurf fin.