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Imax1 said..
The way I see it is thick foils make more drag and grip , so you can use a smaller foil which is less draggy . Got to be a balance there somewhere .

BALANCE. excactly the right word for the creation of all mechanical items built for a specific purpose.

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Paducah said..
I've followed the Fangy threads for a long time with interest as one of my favorite sailing vacation spots is notoriously shallow.

Following in the theme of this thread, would it be worth trying to reshape and fatten an existing weedie with what we've learned from the Fangy project? I live too far to get one plus always up for a time consuming project and totally lack Imax1's skills and available tools. I'd love to get rid of the 48cm G-10 boat anchor that's my current big board weedie.

Australian sailors get fins shipped from Europe all the time. Get an original Fangy fin, you'll still have the satisfaction of giving it a polished finish

Some thoughts.
I think flanges are more benificial on heavily raked fins. upright fins work well enough without them.

The theory that water has to go faster over a deeper camber to meet the rest of the water column at the same time has been debunked, the flow over the heavy camber lags the flow coming over the shallower camber.

Home made fins can be made several ways. I'm now creating the foil from endgrain palm, it's very light and has excellent compressive properties, this then gets several layers of carbon over the top for it's tensile strength and stiffness. This is a classic beam construction. compressive strength internally tensile strength external. because they are raked fins stiffness is very important. This blade then has a moulded base, and built up flange.
If I'm feeling very keen it may also have a stainless steel leading edge to cope with some of the very abrasive weed we sail through.

Other sailors here, mould there fins in home made moulds. so far only one other has invested in a CNC machine, to cut his fins out. Fangy of course gets his cast in aluminium,

^^^
Gots to have pics

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Imax1 said..
^^^
Gots to have pics

Hmmm, now you're asking, I'm sure I had a thread on here a while back, I'll see what I can find.


OK, seems my thread is too old, but I do make an appearance here.
www.seabreeze.com.au/forums/Windsurfing/Foiling/Foil-wing-build#2291941
scroll down to about the 7th post, and there's a few pics of the endgrain palm being shaped, and ready to carbon.

As a small aside - Lift generation: This fellow gives one of the better lectures I have seen and I believe the simplest explanation. He also takes the time to debunk why previous theories don't stack up. (You can watch at 1.25x speed)




This one is a bit heavier but good if you are really keen on this stuff. This delves more into the physics of the fluid particles themselves and their behaviour as a result of Newton's 2 and 3 laws.



Suffice to say Lift in 3D appears to be far more complex than we generally thought and as far as I can tell, not everyone agrees on the fine detail

decrepit said..
Some thoughts.
I think flanges are more benificial on heavily raked fins. upright fins work well enough without them.

The theory that water has to go faster over a deeper camber to meet the rest of the water column at the same time has been debunked, the flow over the heavy camber (edit woops XXXXXX) Leads the flow coming over the shallower camber.

Home made fins can be made several ways. I'm now creating the foil from endgrain palm, it's very light and has excellent compressive properties, this then gets several layers of carbon over the top for it's tensile strength and stiffness. This is a classic beam construction. compressive strength internally tensile strength external. because they are raked fins stiffness is very important. This blade then has a moulded base, and built up flange.
If I'm feeling very keen it may also have a stainless steel leading edge to cope with some of the very abrasive weed we sail through.

Other sailors here, mould there fins in home made moulds. so far only one other has invested in a CNC machine, to cut his fins out. Fangy of course gets his cast in aluminium,

I have lost count of the times naysayers have told me " that doesn't work" even as the said thing that "doesn't work" is patently in use and working. Weird.

Don't know if this has been talked about on here?
finfoil.io/

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fangman said..
As a small aside - Lift generation: This fellow gives one of the better lectures I have seen and I believe the simplest explanation. He also takes the time to debunk why previous theories don't stack up. (You can watch at 1.25x speed)

This one is a bit heavier but good if you are really keen on this stuff. This delves more into the physics of the fluid particles themselves and their behaviour as a result of Newton's 2 and 3 laws.

Suffice to say Lift in 3D appears to be far more complex than we generally thought and as far as I can tell, not everyone agrees on the fine detail

Hmmm interesting theory but the only contradiction to Bernoulli's equation is the pressure difference is caused by curvature in the streamlines not a difference in speed between the streamlines. Aeronautical engineers and pilots have been going by the principal of bernoulli's equation for 120 years now and It sounds like professor babinsky's lecture is still a theory at this stage and needs to be proven beyond a shadow of a doubt. Also I will add that on Bernoulli's principal, they have gone from a plane made from sticks and rags that flew 120 metres at less than the speed a horse can run to Concorde which can circle the globe at twice the speed of sound carrying 100 plus passengers in blissful comfort. This can only come from a valid principle.

I think you are wrong, just following the smoke trails shows Bernoulli isn't on the money. And aeronautics has been doing this for a long time.
Yes, the lift drag formulas work, but they don't rely on Bernoulli

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decrepit said..
I think you are wrong, just following the smoke trails shows Bernoulli isn't on the money. And aeronautics has been doing this for a long time.
Yes, the lift drag formulas work, but they don't rely on Bernoulli

The later demonstration of the smoke trails with the thick foil and the thin foil shows the thick one is a laminar foil with virtually same chamber top and bottom and the thin foil is non laminar. If the thick foil has the same shape at the bottom as the thin one, then does it not produce more lift at the same speed?, whether or not it is produced by curvature or speed of the streamlines. Why do aircraft with thick wings have much lower stall speeds than aircraft with thinner more high performance wings?. As professor babinsky himself says earlier on in the lecture, the royal airforce does in fact rely on and teaches bernoulli's principle to this day. Its an interesting topic for sure as it does directly relate to windsurfing fins. I need the wind to return im thinking too much. Damn windless days!

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mob dog said..

decrepit said..
I think you are wrong, just following the smoke trails shows Bernoulli isn't on the money. And aeronautics has been doing this for a long time.
Yes, the lift drag formulas work, but they don't rely on Bernoulli

The later demonstration of the smoke trails with the thick foil and the thin foil shows the thick one is a laminar foil with virtually same chamber top and bottom and the thin foil is non laminar. If the thick foil has the same shape at the bottom then does it not produce more lift at the same speed?, whether or not it is produced by curvature or speed of the streamlines. Why do aircraft with thick wings have much lower stall speeds than aircraft with thinner more high performance wings?. As professor babinsky himself says earlier on in the lecture, the royal airforce does in fact rely on and teaches bernoulli's principle to this day.

Using Imax's brilliant suggestion of watching videos at 1.5 time real speed ( and a bit of skipping) I got up to 19 minutes where he makes the assumption that the flow is incompressible and frictionless. In which case Bernoulli's principle is spot on.

What's the problem with Bernoulli anyway? It's just a more convenient way of using F=ma for a fluid. We all know to be a little careful of F=ma if there's too much friction involved.

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Ian K said..

mob dog said..

decrepit said..
I think you are wrong, just following the smoke trails shows Bernoulli isn't on the money. And aeronautics has been doing this for a long time.
Yes, the lift drag formulas work, but they don't rely on Bernoulli

The later demonstration of the smoke trails with the thick foil and the thin foil shows the thick one is a laminar foil with virtually same chamber top and bottom and the thin foil is non laminar. If the thick foil has the same shape at the bottom then does it not produce more lift at the same speed?, whether or not it is produced by curvature or speed of the streamlines. Why do aircraft with thick wings have much lower stall speeds than aircraft with thinner more high performance wings?. As professor babinsky himself says earlier on in the lecture, the royal airforce does in fact rely on and teaches bernoulli's principle to this day.

Using Imax's brilliant suggestion of watching videos at 1.5 time real speed ( and a bit of skipping) I got up to 19 minutes where he makes the assumption that the flow is incompressible and frictionless. In which case Bernoulli's principle is spot on.

What's the problem with Bernoulli anyway? It's just a more convenient way of using F=ma for a fluid. We all know to be a little careful of F=ma if there's too much friction involved.

Yes I hear where your all coming from. I was taught bernoulli's theory and applied it practically in gaining my private pilots licence about 30 years ago so im set in my ways but academically minded people are priceless in that they have the ability to look deeply at established theories and question everything and usually do come up with alternate theories and sometimes prove the established theory to be wrong, advancing the collective knowledge of humans as a whole. But there is also another kind of person that contributes equally to technological advancement that I admire just as much, maybe even more, the person who just builds the damn thing, tries, fails, refines, tries, fails, refines etc, like a bulldog with a bone and eventually improves something with persistence alone. When I started windsurfing in 2001 someone said to me a fin is a fin what else could they do to it and I thought yes your right. Now in 2022 as I watch a foiler pass by nearly 1 meter in the air in perfect silence on a highly technologically advanced carbon foil, I can now see how wrong I was to just think that statement to be true. There is always room for improvement, in just about anything. Theoretical+practical= improvement!

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mob dog said..

Ian K said..

mob dog said..

decrepit said..
I think you are wrong, just following the smoke trails shows Bernoulli isn't on the money. And aeronautics has been doing this for a long time.
Yes, the lift drag formulas work, but they don't rely on Bernoulli

The later demonstration of the smoke trails with the thick foil and the thin foil shows the thick one is a laminar foil with virtually same chamber top and bottom and the thin foil is non laminar. If the thick foil has the same shape at the bottom then does it not produce more lift at the same speed?, whether or not it is produced by curvature or speed of the streamlines. Why do aircraft with thick wings have much lower stall speeds than aircraft with thinner more high performance wings?. As professor babinsky himself says earlier on in the lecture, the royal airforce does in fact rely on and teaches bernoulli's principle to this day.

Using Imax's brilliant suggestion of watching videos at 1.5 time real speed ( and a bit of skipping) I got up to 19 minutes where he makes the assumption that the flow is incompressible and frictionless. In which case Bernoulli's principle is spot on.

What's the problem with Bernoulli anyway? It's just a more convenient way of using F=ma for a fluid. We all know to be a little careful of F=ma if there's too much friction involved.

Yes I hear where your all coming from. I was taught bernoulli's theory and applied it practically in gaining my private pilots licence about 30 years ago so im set in my ways but academically minded people are priceless in that they have the ability to look deeply at established theories and question everything and usually do come up with alternate theories and sometimes prove the established theory to be wrong, advancing the collective knowledge of humans as a whole. But there is also another kind of person that contributes equally to technological advancement that I admire just as much, maybe even more, the person who just builds the damn thing, tries, fails, refines, tries, fails, refines etc, like a bulldog with a bone and eventually improves something with persistence alone. When I started windsurfing in 2001 someone said to me a fin is a fin what else could they do to it and I thought yes your right. Now in 2022 as I watch a foiler pass by nearly 1 meter in the air in perfect silence on a highly technologically advanced carbon foil, I can now see how wrong I was to just think that statement to be true. There is always room for improvement, in just about anything. Theoretical+practical= improvement!

These fellows aren't questioning Bernoulli or any of the associated tricks to solving F=ma in a fluid anymore than anyone questions F=ma itself. They're just trying different methods of getting the gist of it all across to a general audience. It's a bit of a chicken and egg problem to explain. Does the velocity cause the pressure drop or vice-versa?

I am intermediate sailor and have been using Fangy's 22, 24 and 28 fins for about 8 months and just love them. (Have a 20 cut down to 18 but so far not the conditions to use it). I have a pile of tribal weed speed fins (and other random fins) as well and for my level of ability and other gear the Fangy fins just work better. (less spin outs, never pick up weed or reeds and less likely to hit rays and sandbars, no detectible change in my speeds and due to the shallow draft open up much greater sailing areas). Note I am not knocking Chris's fins, just my experience so far. I use the 22 and 24 mostly and the 28 reserved for low wind days and gets me going where others can't.

The following questions have been on my mind for a while and comes about as I can use the FF22 with my big board (120 Litre, 75cm wide) and sails up to 8.6m. The only issue with this combo is spinouts in the gybe and I really have to dig the rail in to get around. My smaller boards 65cm wide, 97 litres and 57cm and 86 litre never spin out in gybes with the 22. According to Fangman this is because not much fin in the water when turning which makes sense and agrees with experience. (the 24 and 28 are fine when gybing on the big board)

Question#1 is whether 2 (or 3) smaller Fangy fins of roughly the same combined area in a thruster config would work? The concept being when starting and going straight you get about the same lift (and drag since same area?) but when gybing you get one fin fully engaged. You would get less draft which means shallower water and less likely to hit rays, sandbars and other hazards which are common where I seem to be sailing. Possibly you would get reduced weed drag but depends on the type of weed. I'm thinking Albany subsurface weed you might get less weed drag.

Question#2 Would a "V"tail Fangy fin design (to use a normal Tuttle/powerbox mounting) achieve the same goal? This idea came when I stuffed up cutting the powerbox shape on my first 22 and its bent to vertical...not by a lot but certainly on the piss as one might say. Fangman said it makes no difference which indeed it doesn't. So you'd have two fins each at some angle to vertical coming out of the fin box.

This config raises yet another question#3 whether this V config would help lift the board out of the water (sort of a half way place on the way to a foil) to reduce drag or just make the board nose dive? Then Q#4 maybe this would improve??? gybing as one fin would increase area/lift in the direction of turning forces while the other fin diminishes with the overall effect being constant lift where as a normal vertical one fin or thruster config would have reduced lift in turns. (the board rail of course compensates for this reduction in lift)

This leads to others questions like whether you can take the thruster config to the extreme and have maybe 10-20 Fangy shaped ridges in the bottom of the board so have a finless design...would never work in chop I'm guessing.

These ideas have probably been tried on 'normal' thrusters and/or fin shapes but would be interesting to see (or at least know what the brainiacs out there think) how the Fangy shape behaves in those configs.

My personal objective is not ultimate best speed but maximising sailing days by being able to expand operating envelope..i.e. lower tides, lower winds, less impacts/surprises....and no I don't want to get a foil.

I've got one board with a thruster config and happy to donate my 3d printer/time to printing out moulds if Fangman wants to come up with crazy designs. Attached is my design effort.

The trouble with twins and thrusters, is they need to be towed in, for the board to turn, this creates drag. but the fins can be heavily asymmetric, which reduces drag. I've no idea how to work that out.
If you have the time and patience to experiment go ahead. I started a similar idea to see if I could get into the really shallow areas at fangys. But the board I was experimenting on was too small. You need lots of flotation, so the tail doesn't sink into the mud. I still have the fins, if you want to have a go with them.

What is fun, with wide boards, short fins and flat water, is the slide gybe. It's just like planting your foot in a rearwheel drive on a gravel bend, the tail hangs out and you slide around. But you need good control, over cook it and you end up in the drink.

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Flex2 said..
I am intermediate sailor and have been using Fangy's 22, 24 and 28 fins for about 8 months and just love them. (Have a 20 cut down to 18 but so far not the conditions to use it). I have a pile of tribal weed speed fins (and other random fins) as well and for my level of ability and other gear the Fangy fins just work better. (less spin outs, never pick up weed or reeds and less likely to hit rays and sandbars, no detectible change in my speeds and due to the shallow draft open up much greater sailing areas). Note I am not knocking Chris's fins, just my experience so far. I use the 22 and 24 mostly and the 28 reserved for low wind days and gets me going where others can't.

The following questions have been on my mind for a while and comes about as I can use the FF22 with my big board (120 Litre, 75cm wide) and sails up to 8.6m. The only issue with this combo is spinouts in the gybe and I really have to dig the rail in to get around. My smaller boards 65cm wide, 97 litres and 57cm and 86 litre never spin out in gybes with the 22. According to Fangman this is because not much fin in the water when turning which makes sense and agrees with experience. (the 24 and 28 are fine when gybing on the big board)

Question#1 is whether 2 (or 3) smaller Fangy fins of roughly the same combined area in a thruster config would work? The concept being when starting and going straight you get about the same lift (and drag since same area?) but when gybing you get one fin fully engaged. You would get less draft which means shallower water and less likely to hit rays, sandbars and other hazards which are common where I seem to be sailing. Possibly you would get reduced weed drag but depends on the type of weed. I'm thinking Albany subsurface weed you might get less weed drag.

Question#2 Would a "V"tail Fangy fin design (to use a normal Tuttle/powerbox mounting) achieve the same goal? This idea came when I stuffed up cutting the powerbox shape on my first 22 and its bent to vertical...not by a lot but certainly on the piss as one might say. Fangman said it makes no difference which indeed it doesn't. So you'd have two fins each at some angle to vertical coming out of the fin box.

This config raises yet another question#3 whether this V config would help lift the board out of the water (sort of a half way place on the way to a foil) to reduce drag or just make the board nose dive? Then Q#4 maybe this would improve??? gybing as one fin would increase area/lift in the direction of turning forces while the other fin diminishes with the overall effect being constant lift where as a normal vertical one fin or thruster config would have reduced lift in turns. (the board rail of course compensates for this reduction in lift)

This leads to others questions like whether you can take the thruster config to the extreme and have maybe 10-20 Fangy shaped ridges in the bottom of the board so have a finless design...would never work in chop I'm guessing.

These ideas have probably been tried on 'normal' thrusters and/or fin shapes but would be interesting to see (or at least know what the brainiacs out there think) how the Fangy shape behaves in those configs.

My personal objective is not ultimate best speed but maximising sailing days by being able to expand operating envelope..i.e. lower tides, lower winds, less impacts/surprises....and no I don't want to get a foil.

I've got one board with a thruster config and happy to donate my 3d printer/time to printing out moulds if Fangman wants to come up with crazy designs. Attached is my design effort.

I am gonna have to have a think...this may take some time. But just off the top, the toe-in effect on thrusters presents a significant drag problem.
I have seen a 'V' fin used here - actually, I think it was a three fin 'W'version. The rider told me that it was OK but very draggy. My guess is that the interference pressure wave between the three foils was an unhelpful contributor.
Decrepit's experiments with the shoal draft twin did seem to have some promise, but if I recall correctly, please correct me if I am wrong here Decrep, the lack of toe-in meant the board was rather recalcitrant when it came to gybing.
I think I recall seeing Legless use a twin fin set up with MUF Deltas to good effect, but I have no idea of the toe-in on those boards.

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boardsurfr..

As to why the Fangy fin has not been copied much, that's easy: it's made of aluminum. The flange at the bottom is one of the design aspects that makes it work so well, and that's hard to impossible to do in GFC or carbon. The Tectonic Weed Demon has copied one aspect of the design (the extended tip), and it's a very good fin, but it is nowhere close to the Fangy fin in performance. For me, the Fangy 22 was pretty much spinout proof in chop; with the Tectonics 21 and 23, I need to be much more careful even on flat water to avoid spinouts. The only problem I ever had with Fangy fins was that I did not bring them with me when leaving Oz .

Seriously boardsurfr if after making fins for over 35 years do you really think Dennis Parton from Tectonics Maui needs to copy someone else's fin, you are either taking too many drugs or not enough. Get off the internet and enjoy some windsurfing.

I certainly have no clue about all this but aircraft designs that have V tails seem to have the fins at 90deg to each other which would reduce/eliminate pressure wave interference between those surfaces. In a windsurf setup presumably the board pressure wave interacts with fin pressure wave the logical first choice angle for a V fin would be 60deg..60 deg between board and first fin, 60 deg to next fin. If toe in is issue for the thruster config can't you just bring them closer together to reduce the effect..yes would have to cut two fin boxes. Obviously thrusters exist because they work for their intended purpose but if the objective is shallow draft/less weed drag should optimise design as required.

The other question I forgot to add was whether a 'stretched' Fangy fin would work? i.e. stretch the horizontal length and shrink the vertical, increase the rake angle but keep the area the same. At some point they won't want to go around corners but even the FF28 turns easily so it appears one could stretch it to some extent?? The advantage of course is less draft and less likely to come off if you hit something due to increased rake. In other words stretch the beast FF28 to have the attack/rake at say 65-75 deg but its vertical depth now only 24cm.

Ya Decrepit, might be fun..something to chat about tomorrow/next day

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decrepit said..
What is fun, with wide boards, short fins and flat water, is the slide gybe. It's just like planting your foot in a rearwheel drive on a gravel bend, the tail hangs out and you slide around. But you need good control, over cook it and you end up in the drink.

Yep, short fins wide boards is just another skill set to master. I'm great at getting it to pop and tail slide when aiming for that great Alpha. Or cranking it too tight and killing the speed. Just gota remember rail, rail,rail engage the rail

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fangman said..>>>Decrepit's experiments with the shoal draft twin did seem to have some promise, but if I recall correctly, please correct me if I am wrong here Decrep, the lack of toe-in meant the board was rather recalcitrant when it came to gybing.
I think I recall seeing Legless use a twin fin set up with MUF Deltas to good effect, but I have no idea of the toe-in on those boards.

I experimented a bit Ross, no toe in, pointed at the nose and even a tri fin set up. Just ran out of enthusiasm, not only was the board too small, but I wasn't convinced I'd actually want to ride it at high speed in a few inches of water, if I did manage to succeed.

As someone said, Legless has been using twin Delta fins in the shallow weedy lakes for quite a while and he seems to like them a lot.

My understanding is that two short fins of the same area will have more drag than a single deeper fin for the same lift. This is why Mono-planes have almost completely replaced Bi-planes and Tri-planes in aeronautics.

But effects and priorities change when you get into shallow weedy water. Shorter, lower aspect fins seem to work better in weed, likely for at least these two reasons.
1. They are shallower and present less frontal length to drag trough the weed and need to deflect the weed downwards on the leading edge for less distance.
2. The weed parting around the fin creates a barrier to tip vortex/longitudinal flow to some extent like an end plate effect. Similar perhaps to an effect we might expect in the tip of the fin was running very, very close to the bottom, or indeed dragging the tip though a few mm of very soft mud.

Perhaps the efficiency loss of having two short fins, or one larger, short, low aspect fin, can be offset by having to deflect less mass of weed down the leading edge if the fin so far, -re. 1 above?

Perhaps the efficiency loss of the biplane configuration can be at least partly offset by this second possible 'endplate' effect above?

It strikes me that this would be a good PhD topic for someone to sort out for us interested by-standers.

Thin sails/wings v's thick wings.

From the illustrations in the first lecture, it appears that thin wings like a windsurfing sail generate more curvature of the streamlines on both sides. They appear to generate more lift than a thick wing with a flat bottom surface.

So why would anyone attempt to build a sail/wing with a thick profile and a flat high pressure side, if there is no need for it for mechanical strength

That is; why try to build a double skin sail

Ok,OK, I can see the possible advantage of the double skinned luff pocket to fair the flow around the mast to some extent - but is there any advantage in taking the width of this past a certain point to that end????

And do RDM masts possibly reduce the value of a double skinned luff? Or even mean the a narrower luff would be more efficient with an RDM mast???

So many questions.........

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fangman said..

Basher said.., I just feel it's important to tell the truth about stuff on a forum where newbies come to pick up ideas, and to learn.

Imagine what newbies, who sail in the aforementioned conditions, could learn if we had the benefit of your design. "We see further when we stand on the shoulders of giants" - c'mon, be the that 'giant', use all that top-level foil experience and help us all improve by posting your design solution.

The problem is all his designs are written with quill pen ink on parchment and cant be brought out from the museum.

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PhilUK said..

fangman said..

Basher said.., I just feel it's important to tell the truth about stuff on a forum where newbies come to pick up ideas, and to learn.

Imagine what newbies, who sail in the aforementioned conditions, could learn if we had the benefit of your design. "We see further when we stand on the shoulders of giants" - c'mon, be the that 'giant', use all that top-level foil experience and help us all improve by posting your design solution.

The problem is all his designs are written with quill pen ink on parchment and cant be brought out from the museum.

I'll just re mention my original point which is that fat fins are slow. Most windsurfers know that. It's the truth. As with airplanes, the fuller-figured foil is only really good for take off.

For sure, when windsurfing in shallow water and in weedy conditions you have to come up with less than perfect solutions.

In the meantime, I'm enjoying the nerdy conversations about foil lift from camber and foil lift from deflection. But of course that discussion has been going on for years.
What we do have nowadays is a way of measuring our board speed, so practical testing what works is now much easier - without us needing a smoke tank or a week-long regatta as a testing platform.

If you've got old fins lying around it's a fun thing to reshape them and the to see if you can improve them, or else in order to adapt an old fin to a new use.

If your old fin is slow because it's too thick, by all means thin it out.
If your old fin is slippery and fast, any further thinning will adversely affect it's character. However, you might still use it...in more wind.

Basher....You need to understand that these Delta fins have a root chord length around 250mm, most slaom fins are around 100mm. So if you took an 8% profile off a high end slalom fin it would still come in at at around 20mm thick if you applied it to a Delta. So he is not using a stupidly thick profile, its just the root of the fin being so long. Ok you then say..why not make it a lower chord percent, say 4% of chord at the root. That would make the thickness of the fin at the root closer to a standard slalom fin. However, it wont work, you will spend most of your day going sideways as the stall angle drops too much. I can show you simulation if you want.
So Fangy has designed a fin fit for purpose. It allows you to sail in shallow, very weedy water and still has sufficient grip to allow you to sail through the chop to access the flat weedy spots. No you would not use it at Luderitz or go slalom racing on it but that is not the point...it is designed to a specific set of requirements and from all accounts does the job very well.
And by the way, thinner not always faster....go too thin and as I said the stall angle drops too much and the fin is too vunerable to spin out, gets very technical to use. You dont go fast when you are spinning out.

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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.