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"SmartFins are based on the idea that the fin profile can adjust itself under the influence of the water pressure. We have successfully developed this idea further into a working design and the international patent was applied for in 2004."

"Typically the power of SmartFins can be compared to standard fins which are about 20% longer. E.g. a standard fin of 38 cm length can be replaced by the 32 cm SmartFin. In the table (below-right) this can also be seen in e.g. the combination with a sail area up to 7.8 m2 and a board volume up to 130 liter. A normal 32 cm could not do that.

Very important is also that a SmartFins performance remains optimal in a much wider range of wind conditions, boards and sails. This means that you don't have to change your fin as often as you used to do with ordinary fins. The 32 cm delivers the power of a 38 cm fin, but in strong wind you will keep the same control as if sailing with a standard 32 cm fin."

smartfins.nl/products.html


Anybody got any thoughts on this?

does it come in a weed version

Looks like they'd be worth a try. But an asymmetric 32 will have its lift centred closer to the hull than a symmetric 38 that provides the same lift. So the fin length board width balance for best trim may take a bit of adjustment. A 38 on a 64 wide board trims nicely in moderate wind. Maybe that's why they dwell on the overpowered handling of the 32.

Give one a go Lao and let us know, A$300 for the 32.

Pros:
Looks like a classy implementation of this concept. The assymetric form would provide more lift for the size and help to resist spinout. I think the notion of a wider wind range would be true. The quality of manufacture looks very good too.

Cons:
They dont claim higher performance which would be the goal of such a design exercise. I suspect that it has not yet been achieved, although the concept is capable of it if they can get the drag down. And perhaps in the category of nit-picking... it could have a high stress loading through the front stainless steel pin causing damage to the fin, or the fin box.

Overall one of the better fin developments in recent years I think.

They also miss out on flex which, as is being demonstrated by formula boards, is a good thing in the larger fin sizes.

I put that down to the swings and roundabouts of sines and coses. As a fin bends the lateral lift drops with the cos of the flex angle but the vertical lift goes up with the sign of the angle. Ie for a 6 degree flex the cos drops from 1.0 to 0.994 - down by only 0.006 but the sin of 6 degrees = 0.1.

That's 10% of your fin lift in the vertical direction - less work for the draggy hull to do. You've just got to be able to handle the flighty feeling.

There might be two interesting design strategies to watch - the tacking asymmetrics and the further development of flex. They soon should start to put a bit of the flex in the box rather than all in the blade I'm guessing.

ummm, won't the gap in the surface mean that cavitation will occur fairly easily?


I had an idea kinda like this a while ago, except that I had the foil as a single piece of elastic material without the gap that a mechanical joint would introduce.

I note "international patent applied for 2004" not patented design or patent No. Blah blah. So I suspect they were not granted a patent. Not that this makes much difference as the US Patent Office, unlike most others which require an idea to be "novel and involve an inventive step", will allow just about anything to be patented without it appears much of a search of the prior art. For example here is one the granted for fibre orientation of a windsurfer fin to control twist.

United States Patent 5,273,471
Molnar December 28, 1993

Fin for a windsurf board

Abstract
A fin (1) for a windsurf board is constructed from fibers (5) which have a preferred fiber direction with the majority of the fibers lying in this direction, the preferred direction extending at an acute angle (a) to the longitudinal axis (4) of the profile of the fin blade (2). This allows the fin to distort and to change the properties of its profile in a defined manner when loaded.

Didn't seem to matter that fin makers had already been doing this since??.

In the FAQs it says the camber is not progressive with load - it flicks across to the stops and stays put with increasing pressure.

It seems a little strange that they don't have any detailed pictures of the finished product on their website... Quality issues?

Great concept overall IMO.

Would love to see variable camber with speed/load and also that 0.5mm gap seems a bit rough.

From their web site..



A standard fin will not generate side force at an angle of attack of zero degrees (the blue line goes through the origin of the axes), whereas Smartfins does (about 15 kg). The slope of the curve is directly related to the fin area, therefore the curve of the 38cm fin is about 30% steeper than the 32cm Smartfin.

At a constant speed in choppy conditions the aoa will not be constant, but it will vary in a range as indicated by the arrows (typically 0.5-1 deg)

Hmm... I think people here are confused by geometric AoA and actual lift values.

How is geometric angle of attack defined?

And what is the advantage of generating lift at zero angle of attack? ( apart from not having the board crab through the water, if that's a problem)

And someone at last confirms my estimate of 30 kg of fin lift for a windsurfer. I'll get one.

"And someone at last confirms my estimate of 30 kg of fin lift for a windsurfer. I'll get one."

Hey Ian 30kg is a good benchmark- its what I've been using for years. Of course it depends on if you're a porker or Porka, or if you are going upwind, downwind, or across the wind... The lift component of the fin gets less as we go off the wind as the body/rig balance puts more of the overall force forward- balanced by the drag. The lateral component is reduced so the fin doesnt produce as much lift.

One advantage of cambered fins is that they generally have a higher maximim lift and a higher maximum lift/drag ratio. But the catch is you need to be running it at high lift coefficients to get there. This means you have to use less surface area for the same lift and angle, to get the lower drag.

But if you need the fin to be a certain length (to balance the torque applied through our feet on the rail of the board) the only way you can get an advantage over a symmetric fin is to go shorter in the chord. Which then means for the same proifile you have to make it thinner by that amount too... Then we get into the realm of structural limitations- not helped by having a big stainless pin jammed in there.

So Asymetric/cambered fins are great in really small sizes for speedsailing in square, or rough conditions, or just cruising around at low speeds where you can get the lift coefficient up. Not sure I'd want to be loading one of those smartfins up in heavy chop... I cant see it staying in one peice unless they have found a secret source of unobtanium to make it from.

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slowboat said...

" found a secret source of unobtanium to make it from.

That must be really expensive stuff, probably cost the same as a board for 1 fin Just what we need our sport........ to get more expensive

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hardie said...

slowboat said...

" found a secret source of unobtanium to make it from.

That must be really expensive stuff, probably cost the same as a board for 1 fin Just what we need our sport........ to get more expensive

Think that's a horribly big underestimate of the cost of unobtanium.
Last time I checked, the total resources of the multiverse couldn't buy it.
(Nebs had a similar project in mind)

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decrepit said...

(Nebs had a similar project in mind)

Just wait till I get a shed! MWA HA HA HA HA HAAARRRR!!!!

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decrepit said...

Think that's a horribly big underestimate of the cost of unobtanium.
Last time I checked, the total resources of the multiverse couldn't buy it.
(Nebs had a similar project in mind)

I have a dream.......
That one day.....
While digging in my yard......
I will find the elusiveanium.....
And rather than have it mined for untold wealth......
I will have it used for making fins for free!!!!!

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hardie said...


I have a dream.......
That one day.....
While digging in my yard......

I have this vision of you actually digging! .... that alone is elusivanium

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yoyo said...

I note "international patent applied for 2004" not patented design or patent No. Blah blah. So I suspect they were not granted a patent.

Is an "international patent" even worth bothering with these days? I would have thought getting patents in the individual countries most likely to ignore "international patents" and knock-off your designs would be more effective (if at all)?

Although I guess doing that would still require you to have the financial backing and local knowledge of the legal systems to ensure your patents were respected.

Patents in general are another tool used by large organisations (ie those that can afford to staff a legal department) to knock out small innovative companies. He with the biggest lawyer firepower wins.

These smaller companies often get royally shafted when a larger one comes in with a patent infringement claim. Even if there is clearly no infringement the threat of legal action is enough to cripple a small business without the means to pay for solid defence. Take the case of Microsoft settling out of court with Tom Tom- who use a linux OS. The case against TomTom hase been analysed by industry experts as extremely weak, but it was more cost effective for Tom Tom to just pay MS rather than blow all their cash in legal bills and drawn out court battles.

Patents- fine if you have access to enough lawyer swill to keep the legal machine fed in a drawn out battle. And better for acquiring or squashing small hard-working innovative companies.

If you are a small company with a patent, you have to do all the work to make sure its not being violated, and then be prepared to take legal action (with associated costs) to defend it.

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slowboat said...

30kg is a good benchmark- its what I've been using for years.

Thanks Slowboat, I'll lock it in. Although it sounds like a speed sailor may want a fin designed for minimum drag at 50 knots when giving say only 15 kg of lift? Don't worry about the details - trade secret.

Also sounds like we shouldn't be using a fin designed for 50 knots if we're still a fair way off cracking 40 knots?


One question. What sort of lift-to-drag ratio does the freeride/slalom fin optimised for 30 kg of lift at 30 knots have? I can't find an easily read graph anywhere.

Ian, 25-30kg would be close when loaded but my view is you'd be wasting your money.
I'm sure their Force vs AoA graph isn't to scale but it would indicate the numbers don't favour them. As for the geometric AoA , well here is an explanation...

Geometric AOA is referenced to the airfoil chord line. At zero geometric AOA the chord line is pointed directly in line with the flight path. The problem with defining AOA this way is that the lift is not the same for different airfoils at zero AOA. A symmetric airfoil will have zero lift at zero geometric AOA. A cambered airfoil will have some amount of lift at zero geometric AoA.
A more sensible AOA is what I'll call aerodynamic AOA. Aerodynamic AOA is referenced to the zero lift line. For a symmetric airfoil the zero lift line and the chord line are the same. On a cambered airfoil they are not the same. A cambered airfoil that has zero lift at -1 degree geometric AOA has 1 degree aerodynamic AOA when it is at 0 degrees geometric AOA.

If you look at the graph they provided you will see that (by extrapolation) zero lift of the Smartfin is about minus 1-1.5 degrees. So for 25 kg of lift their foil has to work at about 2.5 degrees to the flow/direction of travel. The symetrical foil gets the same lift with 1.25 degrees. I note they don't show drag or L/D numbers but there is no point having 20% more lift if it come with a 50% drag increase.

Thanks Yoyo, so the slopes of the 2 curves is the important thing to take from those graphs. Fair difference there. What does that tell us?

This one summarises what you get for a given side load (30kg) and a given fin size and a given profile as speed changes.

Fin is 29cm x 70mm chord (0.02m^2 area)
Section/profile is a 10% NACA64A010

Speed is in kts.
The increasing one is the drag (kg)
The decreasing one is the angle of attack (deg)

Thanks Chris,

Ian note the AoA is only 1 degree at 30 knots.

Yes nice graph Chris. Also noted that for 30 kg lift the drag at 30 knots is only 1.3 kg. That's a good lift to drag ratio.

The angle of attack times the velocity squared is also very constant from 20 to 45 knots it's ~ 1000. Is this an experimental or a theoretical curve?

The other curve of interest, ( too many questions I know) is to hold velocity and lift constant and plot the drag of different sized but geometrically similar fins.

the curves are all theoretical, and based on the polars generated by xfoil- which shows good agreement with tank tests, however I think the drag is a little higher than predicted in that graph at higher speeds, since it assumes plenty of laminar flow at those angles (ie below 1 deg AOA its well into the drag bucket)- whether we get that much is still a matter of speculation.

When you run a lower surface area for the same load its the same as shifting the speed axis to the left. The drag at a given speed goes down, but the AOA increases. As expected, and pretty much what we find in practice. The effects of Re changes are pretty small unless you make huge changes to the chord length.

Thanks again Slowboat, good to see curves labeled in real units.

So the graph you posted doesn't include the drag due to pressure distribution over the foil and angle of attack - which I gather is sometimes referred to as induced drag?

Doesn't appear to because at X = 15 knots my rough estimate of induced drag would be 30kg * sin 4.4 degrees = 2.3 kg, way above the 0.7kg on the graph.

Or is that a way too rough way of estimating induced drag ? It should be valid for flat plates, but maybe not useful for complex surfaces and variable pressure distributions?

The graph only considers 2D profiles, and does not consider induced drag.

At <1 degree AOA the induced drag is very small compared to the overall. The L/D of that section with most normal looking planforms varies between 16 and 25 at those angles (depending on where you decide to put the transition point).

The code I wrote to generate that graph is for comparing different profiles. I used the XFLR5 source code as a base and hacked it to death so I could get some more directly useful information out of it rather than constantly trying to interpret smaller subsets of information as the original program presents. The code is useful (thanks mr xflr5 author), but tedious to work with.

The planform (outline) and the profiles are practically independant in terms of optimising for induced drag with symmetric fins.

Ian, XFOIL is a great piece of 2D CFD code which has been around for a while and well verified in several studies both for aerodynamics and hydrodynamics. I use this as the basis for all of my profile evaluations. There are a couple of easier to use wrappers for it- Profili, and XFLR5 (free).

Your method might be good to get the drag within an order of magnitude, but its (quite) a bit oversimplified/off the mark when we consider streamlined shapes.

really off topic here but similar can someone design a board like these so it would be easy to lug around

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Ian K said...

Thanks again Slowboat, good to see curves labeled in real units.

Amen!

This is gold Slowboat, thanks for sharing