Basher said.. It would indeed be great if that were actually true.


And also accused the bloke of making sh!t up

OK, to change the subject slightly, and to stop bashing Basher.
And because I want to skite a bit.
Here's a home made fin I chucked together a few years ago. Grab a western red cedar weather board off cut, some carbon fibre scraps from an aircraft manufacture, and a short length of 5mm X 20mm stainless steel.


It's 17cm deep and rakes at 50 degrees, is starboard asymmetric, 9% on the thick side and 8% on the thin side.
On Wednesday this fin got me to 40kts. Fastest I've ever sailed.
www.gpsteamchallenge.com.au/sailor_session/show?date=2022-02-02&team=2

At the start the blue paint cover the whole fin apart from the stainless. 37km later this is what the abrasive weed does. You can see the wear starts almost from the base.
I was taking a short cut back upwind through very thick surface weed, this fin handles that well as long as you're planning. You can water start in it, but there's a lot of yaw, until you pick up speed and shake off the weed.
But the downwind course had surface weed but not real thick, and there was some shallow stuff at the start of the course. So this fin worked a treat, more efficient than a full on delta in light weed, but raked enough to get through the thick stuff when needed.
And to get this 77year old, 70kg 165cm sailor to 40knots, is obviously fairly fast.
It also behaved itself very well, not that I went into much chop, but in the smooth didn't put a foot wrong, just felt solid and slippery.

well thats worth bragging about ,well done decrepit ....it is a interesting looking fin .

but it has done the job you know your stuff .

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decrepit said..

And to get this 77year old, 70kg 165cm sailor to 40knots, is obviously fairly fast.

Imagine how fast you will be when you grow up

^^^ I think he should try to get to 177cm

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decrepit said..

And to get this 77year old, 70kg 165cm sailor to 40knots, is obviously fairly fast.
It also behaved itself very well, not that I went into much chop, but in the smooth didn't put a foot wrong, just felt solid and slippery.

Thats impressive, getting a PB at 77 and over 40 knots.

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Mark _australia said..

Basher said.. Fat fins are really slow and are only really of any benefit to beginners with poor technique.

You said more than just fat fins are slow.

His problem is jumping in with his "superior knowledge of all things windsurfing" before understanding the rationale behind the original post. Its both irrigating and funny.

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Mark _australia said..
^^^ I think he should try to get to 177cm

I did contemplate stilts, but thought I'd loose a certain element of control.
Then I made a board with a wide high hump on the back, to get me more leverage, over the sail. But there was also more leverage over the fin, needing a bigger fin than I wanted to go fast. So I cut the hum off, then it was hard to get going, because of lack of volume. Gave it away in the end.

Love the stainless strip down the leading edge...now how would I do that in a production environment?

It's a bit more complicated than it looks. There needs to be a rebate on the inside of the stainless for the Carbon/fibreglass to butt up against. I've tried the easy way, just fairing the layup on top of the stainless, but it's much too thin at the boundary. The epoxy doesn't grip well enough, and bits just flake off.
It would be good if the leading edge could be moulded in something very abrasive and impact resistant.
Stainless is great for both of those, but it's a bugger to work.

You don't need it at Lake George, weed there just polishes the fin. But over here, G10 and carbon fins, end up with a serrated leading edge after a few km, they need a light wet and dry after each session.

My dentures are made with moulded titanium, but I suspect that wouldn't be a cheap way to make a leading edge.

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PhilUK said..

Mark _australia said..

Basher said.. Fat fins are really slow and are only really of any benefit to beginners with poor technique.

You said more than just fat fins are slow.

His problem is jumping in with his "superior knowledge of all things windsurfing" before understanding the rationale behind the original post. Its both irrigating and funny.

Haha, I suggest you read the original post, and then the first reaction, which was straight after. Perhaps you should slag off that poster too.
I would never claim to have 'superior knowledge of ALL things windsurfing' but I have been in the watersports industry all my life, dealing with a cross section of wind-powered craft. My race results also speak for themselves although few of us compete nowadays.

Perhaps we should stick to the topic. And in this case the truth is that fat fins are slow. The optimum thickness is usually a function of foil width unless the foil is asymmetric. With most foils the thickness to chord ratio is between 12% and 9% but 12 % is already pretty slow for a windsurf board. There's a further discussion to be had over the chord thickness ratio as it changes over the length of the fin, head to fin tip.
(And I'm pretty sure there have been previous Seabreeze threads on this subject...)


The good news is that a reasonable sailor can immediately feel if a replacement fin is draggy - compared to one you had used in the same board before.
With care, a fin that is too thick can also be thinned out - as long as the inner laminate doesn't lose necessary strength, if for example it has a lighterweight core.

We have a windy weekend coming up here, so I'll give this some more thought.

Basher... You DO know the fin subject on this thread concerns weed fins designed for specific surface weed, don't you?
So SHALLOW fin depth is paramount.
Thin fins need more surface area..wide is slow, deep drags weed.
Nobody is saying a thick fin is fast in all conditions.

Progress report .
Had a ripper of a day on the fin in shallow water .
Huge lightwind board , NP Evo 8.6 . A solid 15 kts .
It handled big me pushing hard on the fin with ease creating lots of lift . No spin out .
Short wide fat fins definitely work for grip , lift and handling .
Now for the speed ,
I was cruising at 25 kts and peaked at 27 over light ,medium chop . It could definitely go faster . I couldn't feel any extra drag at that speed because the board and sail was so big anyway . Not bad for light wind .
It feels as grippy as a 50 cm pointer and almost as lifty. But it doesn't have that nice lively springy lifting feel in light wind that a long fin has . No short fin does .
It was designed for a smaller board and more wind which I haven't tried yet . It could be too much . Watch this space . I may have to make a smaller one . 27 cm , 15 mm thick ???
As for really fast speed , I will never know , I'm too slow .
For shallow water general blasting , wide and thick Fangy concept is the best . By far. People here are using a 28 cm Fangy on extra wide superlightwind boards with grip to spare . It's the perfect choice for shallow water .
And we don't even have weed !
Compared to Fangys, mine has a lot more grip than a FF 24 and slightly less than FF 28 . Creates more lift than FF28. FF 28 probably faster top speed and does weed .
Long live fat fins despite naysayers.

Showing how much a long fin flexes .
I know its a formula fin , but I bet with my weight I flex a 55cm pointer just like that , possibly more . Also in the vid its looks flat and not a lot of wind .
I'm no fin expert and probably wrong , but ,
I'm presuming when a long fin hits chop , it bends like that , then snaps back when the board gets light over the other side . This snapping back , ( like the mast bending back on a sail ) , would create more lift , keeping the board light and floaty and more even over chop . That's what it feels like is happening under my back foot when on a long fin . A Fangy or mine doesn't feel like that . Its more of a solid feel .
I understand for pure speed on flat water a stiff fin is better .

The other thing is, that sort of flex is fine on an upright fin, but as the rake on a fin increases the effect of that flex, gets progressively negative. At 45deg it's just drag. My test of a weedy, is put it over my knee and try to bend it. If it moves more than a mm or 2 it won't be much good. That's one of the reasons fangys just feel solid, they are designed to be absolutely stiff.

Good notes for weed fins.
Criteria different for blades.

Im ordering a slab of 16 mm G10 to make a smaller all solid version. ( Except for the flange ).

Hope you won't be doing that with a sanding block!

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decrepit said..
Hope you won't be doing that with a sanding block!

I'd need a bloody big sanding block
Manual milling machine .
I could do a build with pics , but it would be pretty boring .

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Imax1 said..
Progress report .
Had a ripper of a day on the fin in shallow water .
Huge lightwind board , NP Evo 8.6 . A solid 15 kts .
It handled big me pushing hard on the fin with ease creating lots of lift . No spin out .
...

Very happy to read the fin has been a winner for you Imax. Can't wait to see the next iteration

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fangman said..

Imax1 said..
Progress report .
Had a ripper of a day on the fin in shallow water .
Huge lightwind board , NP Evo 8.6 . A solid 15 kts .
It handled big me pushing hard on the fin with ease creating lots of lift . No spin out .
...

Very happy to read the fin has been a winner for you Imax. Can't wait to see the next iteration

I have to thank you fangman for the idea . I just copied your fin and made it for weedless water . If it wasn't for you , we'd be struggling with one of those Deltas

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Imax1 said..
Im ordering a slab of 16 mm G10 to make a smaller all solid version. ( Except for the flange ).

16mm x 320mm x 245mm , $ 116.50 inc del .
Ouch !

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Imax1 said..

Imax1 said..
Im ordering a slab of 16 mm G10 to make a smaller all solid version. ( Except for the flange ).

16mm x 320mm x 245mm , $ 116.50 inc del .
Ouch !

Ouch indeed! Are two sheets of 8mm epoxied together any more economical? ( do -able on the basis that the strength needs to be in the outer layers of the laminate, not so much the middle?)

^^^
I get your drift , but I wouldn't think it's cheaper . Too late now . Will look into it next time .
Brain pick if I may .
I want to have basically the same outline profile only smaller , slightly sharper at the tip , cos it worked .
Thinking of keeping it fat 15 mm instead of 17 mm .???
Do you think I should have a sharper leading edge than a FF 24 . The fin above is FF 24 blunt . It's at 30 deg rake so I don't get any lift from the front edge or do I . ???
Ill keep the thickness profile the same as my first fin because it works so well .
Im not just chasing a bit more speed , but I need it to still have lift and grip for heavy me on a 74 wide board .
I will take your advice as gospel and if anything goes wrong , you owe me $ 116.50

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Imax1 said..
^^^
I get your drift , but I wouldn't think it's cheaper . Too late now . Will look into it next time .
Brain pick if I may .
I want to have basically the same outline profile only smaller , slightly sharper at the tip , cos it worked .
Thinking of keeping it fat 15 mm instead of 17 mm .???
Do you think I should have a sharper leading edge than a FF 24 . The fin above is FF 24 blunt . It's at 30 deg rake so I don't get any lift from the front edge or do I . ???
Ill keep the thickness profile the same as my first fin because it works so well .
Im not just chasing a bit more speed , but I need it to still have lift and grip for heavy me on a 74 wide board .
I will take your advice as gospel and if anything goes wrong , you owe me $ 116.50

Will you accept a cheque for $116.50? I promise it's in the mail.
My thoughts, which are worth slightly less than a cheque in the mail: Please remember my experiments have been with 50degrees plus rake, so my findings may not extrapolate to lesser rakes.
Outline - a double ellipse (eg Spitfire wing) is the most efficient shape, however, the necessity to get enough lifting surface in a small span can negate any gains here. Overall, anything you can do to reduce the tip vortex intensity and the concomitant drag is a good thing.
Thickness depends on root length. With wide base fins, as low as you don't go below 8% max chord thickness, you should have a good basis for benign handling. One or two per cent more will give more lift and a wider operating margin in turbulent flows. (grip). I don't know for sure, but I guess it's possible a higher aspect fin with less rake may be a little different?
Leading-edge lift - assuming you mean vortex lift generated on the leading edge, the amount of lift generated by this method will be very low. Lifting vortexes on the leading edge require a sharp leading edge and an angle of attack closer to 15-20 degrees. They are also unsteady flows in themselves and not suited to choppy conditions.
At thirty degrees rake, you will need a leading-edge radius to chord ratio of below 0.003 to generate a vortex of sufficient intensity. By its very nature of creating leading-edge separation, this sharp leading edge is not amenable to attached flow parallel to the foil and will lead to a fin that is unpredictable in its behaviour as it swaps between unsteady vortex lift and the standard pressure lift models. Theoretically, the sharpest you should go to ensure no vortex-like separation is a ratio of 0.007 of leading-edge radius to chord. In practice, I find 0.01 better. Eg a chord of 200mm has a leading-edge radius of 2mm/diameter of 4mm. As a general rule the wider the radius, the wider the tolerance of unsteady flow/turbulence causing sheet cavitation. The downside is an increase in drag ( but the efficient and usable portion of the AoA/drag bucket is wider) Swings and roundabouts...

Hope that helps mate, but please remember I have no formal training, just a keen interest, and so may be guilty of applying the wrong theory to the right scenario and vice versa. Now, where did I put my chequebook and quill?

Sent from my iCarrierPigeon

Wow, now that's info .
Some hi-tech stuff that's making me feel dumb .
After reading it half a dozen times , I think I know what your talking about .
With my outline at its widest its 9% thick at 15mm. Ill try to keep that ratio almost all the way up . Or should I go thinner at the top 5cm or so ?
At 165mm at its widest ill go 3.3mm diameter . According to your calcs. Ill segment up the fin to keep that ratio .
Thanks for that info , at least now I'm not flailing in the dark .
As a fluke I was very close to those ratio figures in my first fin . With no knowledge . What an ass
Now ill have a better chance at fluking another fin .
I very much appreciate your time and knowledge

My fins have a constant thickness to chord ratio, usually 9%, from base to tip, but thinking about it. -----
Plane wings normally reduce the ratio at the tip, because they want the stall to start at the tip. That way it can be controlled, possibly similar with a fin. If the tip stalls, you loose a bit of grip that can be felt, and stopped by pulling the back foot in. If it stalls at the base, then you'll have ventilation running up the whole fin and instant spin out.

That makes sense, does every body agree???

May have to do some subtle fin mods.

A couple of questions from an amateur.

Are plane wing tips that way because they have one on each side and when turning sharply the outside tip is travelling a lot faster than the inside wing tip?

Why have a very swept back design of fin if there isnt weed?

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decrepit said..
My fins have a constant thickness to chord ratio, usually 9%, from base to tip, but thinking about it. -----
Plane wings normally reduce the ratio at the tip, because they want the stall to start at the tip. That way it can be controlled, possibly similar with a fin. If the tip stalls, you loose a bit of grip that can be felt, and stopped by pulling the back foot in. If it stalls at the base, then you'll have ventilation running up the whole fin and instant spin out.

That makes sense, does every body agree???

May have to do some subtle fin mods.

I suspect the position of the wingtip in relation to the root would be important in this case. The lack of flex doesn't help in inducing some wingtip pre stall. In any case, a tip on a highly swept wing where spanwise flows predominate presents a different can of worms. I did thin the top of a few foils to see if I could influence the behaviour for good or bad. I could not pick any real world difference, and all I could say was that when the fin let go there was not much difference in warning or retrievability.

PhilUK I am no aeronautic expert so I will leave that for those with more expertise who can answer more succinctly. Suffice to say, as a generalisation, there is no great advantage for most speed fins to be raked other than that required for structural and performance requirements, as it reduces efficiency. Hopefully, the smart fellas will jump in here and answer this properly.

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decrepit said..
My fins have a constant thickness to chord ratio, usually 9%, from base to tip, but thinking about it. -----
Plane wings normally reduce the ratio at the tip, because they want the stall to start at the tip. That way it can be controlled, possibly similar with a fin. If the tip stalls, you loose a bit of grip that can be felt, and stopped by pulling the back foot in. If it stalls at the base, then you'll have ventilation running up the whole fin and instant spin out.

That makes sense, does every body agree???

May have to do some subtle fin mods.

Yeah thats what I've always done

No idea but it makes sense