Possibly everything we can think about was already invented and tested.Anyway, I will be interested to know if somebody invented and tested already twin blade fin ?
Our fins have this aerodynamic, shape, significant thickness to withstand lateral forces.
But what would happen if instead of one thick thin we use two ultra this blades at the similar distance - few mm apart? Two withstand lateral forces our two parallel blades and bridge at the end and maybe few places in the middle.Ideally, we want our ultra thin blades as far apart as practically possible ( to avoid drag inducted in Tesla turbine-like mechanism). I think 5 mm should be sufficient.
Obviously, our high-speed fin will be useless if clogged by weed, but in ideal condition should be quite resistant to cavitation and drag.
So if not already done and tested our first prototype could be shaped to existing thins but only from cut steel sheet joined by almost horizontal bridges strategically.The simplest could be made of two blades coming from a box at such angle that starts at top 1 cm will join at the bottom together. But beauty for perfected design should lay in precise parallel surfaces.Obviously, such fin works also as perfect mincer, so I would not use a such without proper boots and kevlar pants.

I think the thickness is needed to create a curved profile and generate lift... you would not use a flat steel blade as a keel or rudder on a boat for the same reasons

Select to expand quote

powersloshin said..
I think the thickness is needed to create a curved profile and generate lift... you would not use a flat steel blade as a keel or rudder on a boat for the same reasons

The is the famous scientific / engineering dilemma if we need airfoil shape and thickness to create lift. (?)
That all comes to this basic: Do we need thick airfoil ( wing in an airplane ? ) where one surface is longer and another shorter,or this is a purely technical issue to provide rigidity to the wing.Let's imagine: If we shape our wing from the ftat thin sheet - that represent the only upper surface,( and assume that is rigid and will not bend) - how efficient it can be? Specifically in a dense environment like water....Intuitively we could say that wings and propeller are built thick for hundreds of years, so it must be. But there could be interesting to see if one day our research in windsurfing wings could bring hollow wings and propellers to life.

Select to expand quote

Macroscien said..

powersloshin said..
I think the thickness is needed to create a curved profile and generate lift... you would not use a flat steel blade as a keel or rudder on a boat for the same reasons

The is the famous scientific / engineering dilemma if we need airfoil shape and thickness to create lift. (?)
That all comes to this basic: Do we need thick airfoil ( wing in an airplane ? ) where one surface is longer and another shorter,or this is a purely technical issue to provide rigidity to the wing.Let's imagine: If we shape our wing from the ftat thin sheet - that represent the only upper surface,( and assume that is rigid and will not bend) - how efficient it can be? Specifically in a dense environment like water....Intuitively we could say that wings and propeller are built thick for hundreds of years, so it must be. But there could be interesting to see if one day our research in windsurfing wings could bring hollow wings and propellers to life.

A flat surface does produce lift - but it has terrible efficiency. In fact, a flat plate is how we baseline-model all foils. So there is nothing here that we dont already know (sort of).

The reason we dont put two foils side-by-side is because water (for the most part) is incompressible, and the speed of sound + density of water defines how fast the pressure-wave is translated from a foils boundary layer, outwards. Basically... one foil detrimentally interferes with another. This same idea was discussed here a few years ago - we worked out that the foils would need to be separated by at least 1m - for non-interference.

It might be possible to have the interference patterns of both foils, to be reinforcing (vs detrimental), but as a community, we dont the testing facilities available to us.

Not thought this through at all but could you shape each fin so the interference wave from one cancels out the interference wave from the other?

We do that in electronics for soundwaves:

= no sound.

en.wikipedia.org/wiki/Biplane

In a biplane aircraft, two wings are placed one above the other. Each provides part of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere and thus interfere with each other's behaviour. For example, in a wing of aspect ratio 6, and a wing separation distance of one chord length, the biplane configuration will only produce about 20 percent more lift than a single wing of the same planform.[2]

Select to expand quote

mathew said..


The reason we dont put two foils side-by-side is because water (for the most part) is incompressible, and the speed of sound + density of water defines how fast the pressure-wave is translated from a foils boundary layer, outwards. Basically... one foil detrimentally interferes with another. This same idea was discussed here a few years ago - we worked out that the foils would need to be separated by at least 1m - for non-interference.

Not sure that would be the reason? Most fluid flow models make the assumption of incompressibility if they can, so if interference was dependant on the velocity of sound they wouldn't work. The compressibility of the water is in the equation for the speed of sound somewhere.

A fin going at 40 knots only moves forward 13mm in the time the sound wave travels 1 metre.

Read up Busemann biplane. Only works at a certain velocity. Researched a bit for hypersonic planes but no one has figured out how to get around the "choke" effect as you try to get up to speed where destructive interference cancels out wave drag

Select to expand quote

Ian K said..
en.wikipedia.org/wiki/Biplane

In a biplane aircraft, two wings are placed one above the other. Each provides part of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere and thus interfere with each other's behaviour. For example, in a wing of aspect ratio 6, and a wing separation distance of one chord length, the biplane configuration will only produce about 20 percent more lift than a single wing of the same planform.[2]

Biplanes need two wings for a simpler answer than "optimised lift". The materials available back in those days, simply couldn't build a structure that was strong enough, for the required length/span. Once the materials/construction got better, we lost the other wing -> getting both a lighter lifting body, and reduced drag.

Select to expand quote

Ian K said..

mathew said..


The reason we dont put two foils side-by-side is because water (for the most part) is incompressible, and the speed of sound + density of water defines how fast the pressure-wave is translated from a foils boundary layer, outwards. Basically... one foil detrimentally interferes with another. This same idea was discussed here a few years ago - we worked out that the foils would need to be separated by at least 1m - for non-interference.

Not sure that would be the reason? Most fluid flow models make the assumption of incompressibility if they can, so if interference was dependant on the velocity of sound they wouldn't work. The compressibility of the water is in the equation for the speed of sound somewhere.

A fin going at 40 knots only moves forward 13mm in the time the sound wave travels 1 metre.

I should clarify that as "1m" as a vaguely minimum distance for the leading edge of the fin. As you point out, scale of measurement is important.... say the fin span is 10cm, then the other fin would need to be approx 8m away for no interference at all.

Importantly - water is actually compressible, but we usually dont care about that precision.... But when talking pressure-waves from one foil impacting on another foil, we need to adjust the model.

A fin designer I talked to has modeled the interactions between fins in multi-fin setups (thrusters and quads), and used it to design better wave fins. The interactions between fins that were about 20 cm apart were still large enough to warrant design changes; I'd guess that interactions of closer fins would be much larger.

Select to expand quote

Ian K said..
en.wikipedia.org/wiki/Biplane

In a biplane aircraft, two wings are placed one above the other. Each provides part of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere and thus interfere with each other's behaviour. For example, in a wing of aspect ratio 6, and a wing separation distance of one chord length, the biplane configuration will only produce about 20 percent more lift than a single wing of the same planform.[2]

That's only part of the story, and while the fins do interfere with other. A multi fin setup still has the potential to have less induced drag or at least the same induced drag...

If you ignore structural considerations, which you can't on an aircraft but you might be able with the strength of a carbon windsurf fin. You might be onto something but you would not be the first. You still have the drag associated with the connection to the board and viscous drag over the added surface area of the fins.

And macro your right all of this has been thought about in a different context about 100 years ago. By prandtl and munk.

All the theory is IMHO amazingly complicated, what is more interesting is how people also invent, or come up with similar solutions without any complicated math or theory.

The real problem is more likely the cost and complexity. A single fin is still the simplest solution IMHO.

Select to expand quote

mathew said..

Ian K said..

mathew said..


The reason we dont put two foils side-by-side is because water (for the most part) is incompressible, and the speed of sound + density of water defines how fast the pressure-wave is translated from a foils boundary layer, outwards. Basically... one foil detrimentally interferes with another. This same idea was discussed here a few years ago - we worked out that the foils would need to be separated by at least 1m - for non-interference.

Not sure that would be the reason? Most fluid flow models make the assumption of incompressibility if they can, so if interference was dependant on the velocity of sound they wouldn't work. The compressibility of the water is in the equation for the speed of sound somewhere.

A fin going at 40 knots only moves forward 13mm in the time the sound wave travels 1 metre.

I should clarify that as "1m" as a vaguely minimum distance for the leading edge of the fin. As you point out, scale of measurement is important.... say the fin span is 10cm, then the other fin would need to be approx 8m away for no interference at all.

Importantly - water is actually compressible, but we usually dont care about that precision.... But when talking pressure-waves from one foil impacting on another foil, we need to adjust the model.

I don't know the details of models used but am guessing that an assumption of incompressibility would be made to simplify the mathematics. If you can assume incompressibility you then can have the maths transmit pressure changes, across the relatively short distances involved, instantaneously without greatly affecting the accuracy.

Basically I'm thinking that the reason two side by side foils do or do not interact is due to something other than the time taken for the pressure generated by one to get to the other. Models using the incompressibility assumption will have the pressure perturbation of one fin instantaneously transmitted to the other regardless of the separation.


www.flow3d.com/resources/cfd-101/physical-phenomena/the-incompressibility-assumption/

"
Comparing with expression (1), we see that the factor multiplying (u/c) must also be much less than one.

Physically, this condition says that the distance traveled by a sound wave in the time interval t must be much larger than the distance l, so that the propagation of pressure signals in the fluid can be considered nearly instantaneous compared to the time interval over which the flow changes significantly. "

I think that we could learn a lot if in the experiment we could attach a microphone and listen - record sound coming off the fin.This wide spectrum graph should tell us a lot what is going one around the fin: if there is any cavitation, laminar or turbulent flow, fin resonating etc. I remember that one day I was struggling to have any reasonable speeds. I discover that bolts are a bit loose and fin could move a bit sideways. Simple tightening increased speed instantly by 3 knots. I am guessing the main role played here vibration of that find while approaching speed.
That resonance took enormous energy....
Possibly every other fin , even properly installed have some specific resonating vibrations. Once activated - means possibly -top speed for given fin. Only enormous gust could overcome now that barrier and suddenly the same fine could later achieve even higher speed if resonating have been extinguished ( if that possible or not I don't know -
It could also happen that once vibrating fin can only vibrate even more...)

I suspect the proposed double blade fin conjoined at the end may well eliminate vibration at our speed scale.

Select to expand quote

Macroscien said..
I think that we could learn a lot if in the experiment we could attach a microphone and listen - record sound coming off the fin.This wide spectrum graph should tell us a lot what is going one around the fin: if there is any cavitation, laminar or turbulent flow, fin resonating etc. I remember that one day I was struggling to have any reasonable speeds. I discover that bolts are a bit loose and fin could move a bit sideways. Simple tightening increased speed instantly by 3 knots. I am guessing the main role played here vibration of that find while approaching speed.
That resonance took enormous energy....
Possibly every other fin , even properly installed have some specific resonating vibrations. Once activated - means possibly -top speed for given fin. Only enormous gust could overcome now that barrier and suddenly the same fine could later achieve even higher speed if resonating have been extinguished ( if that possible or not I don't know -
It could also happen that once vibrating fin can only vibrate even more...)

I suspect the proposed double blade fin conjoined at the end may well eliminate vibration at our speed scale.

You haven't thought about real world implementation of this test.... how will you filter out the far-more-dominating effect of the wind and water-splashing ?

Select to expand quote

Ian K said..

I don't know the details of models used but am guessing that an assumption of incompressibility would be made to simplify the mathematics. If you can assume incompressibility you then can have the maths transmit pressure changes, across the relatively short distances involved, instantaneously without greatly affecting the accuracy.

Basically I'm thinking that the reason two side by side foils do or do not interact is due to something other than the time taken for the pressure generated by one to get to the other. Models using the incompressibility assumption will have the pressure perturbation of one fin instantaneously transmitted to the other regardless of the separation.

I believe you can use prandtl's biplane equation to predict the reduction in induced drag, this equation does consider the interaction between the foils. One text I researched recently showed a 25% reduction in induced drag could be achieved when the ratio between the span and the distance apart was 20%, (identical span and loading), even better with a trifin...

The only trouble in taking this approach is that it is very easy, for the total drag to be increased by other effects, e.g. greater friction drag, cavitation, ventilation, effects from the board itself, difficult or impossible to gybe etc. Hence single fin, is the simplest and probably the best...

At speed, the angle of attack is quite small so the induced drag is also quite small with form drag and surface drag dominating.

Select to expand quote

yoyo said..
At speed, the angle of attack is quite small so the induced drag is also quite small with form drag and surface drag dominating.

That's a good point left out. The corollary is in the speed range where spin outs might occur, or at least the buffer between a spin out is low, the ratio between induced drag and the rest will be much more significant.

For the real speedsters out there is spin out ever an issue at the top end?

Select to expand quote

racerX said..

yoyo said..
At speed, the angle of attack is quite small so the induced drag is also quite small with form drag and surface drag dominating.

That's a good point left out. The corollary is in the speed range where spin outs might occur, or at least the buffer between a spin out is low, the ratio between induced drag and the rest will be much more significant.

For the real speedsters out there is spin out ever an issue at the top end?

I don't know what the real speedsters think, but for me it is very rare these days to have spin out under load in the 40's. Even with the 16.5cm and 18cm deep fins I use most now (Tribal assy).

The two conditions most likely to encounter spin out for me, are when rattling over 'seriously large rolling chop' when the course is too broad, and the fin is coming half out of the water between waves, or when slowing down at the end of a run when you back off the power and stand up.

I think the reasons for the first are obvious, but the second has always puzzled me a bit. Granted, the water is usually quite rough at the end of the speed course at Sandy Point when it is epic strength and angle, but it seems strange to me that the fin, which was holding well under full load, suddenly gets scary when you back off. I can only surmise that it may be to do with the board wobbling or bouncing more without the mast-foot and foot pressure to stabilise it.

One other thing related is that when I recently went back to a high aspect assy. fin that I had done some of my best speeds on, and had not used for a few years, (TM48) I was surprised and reminded of how much more prone to spin out it was at low speeds when just starting a run. I think one big advantage of the lower aspect speed fins is how forgiving they are in this aspect and how much pressure can be absorbed at low, accelerating speeds.

Select to expand quote

sailquik said..

One other thing related is that when I recently went back to a high aspect assy. fin that I had done some of my best speeds on, and had not used for a few years, (TM48) I was surprised and reminded of how much more prone to spin out it was at low speeds when just starting a run. I think one big advantage of the lower aspect speed fins is how forgiving they are in this aspect and how much pressure can be absorbed at low, accelerating speeds.

Thanks,

My guess is when you a slowing down, sheeting out, weight on the back of the board etc, you change the moments, such that the board rounds up slightly and the fin stalls. I think something similar happens to me when I find myself with a couple of extra m2 too much sail, and I am too chicken to put the hammer down...

A low aspect ratio fin (or a swept one) should stall at a greater angle of attack, and more gently than a similarly loaded high aspect ratio fin, so that last statement makes sense.

Speaking of twin blade fins..........

Select to expand quote

Te Hau said..
Speaking of twin blade fins..........

double tuttle or a tuttletuttle?

Select to expand quote

BSN101 said..

Te Hau said..
Speaking of twin blade fins..........

double tuttle or a tuttletuttle?

Std tuttle, short fins

Select to expand quote

Te Hau said..

BSN101 said..

Te Hau said..
Speaking of twin blade fins..........

double tuttle or a tuttletuttle?

Std tuttle, short fins

Wow didnt see them as that small!!

Any idea how it went? or how old is the pix

Select to expand quote

BSN101 said..

Te Hau said..

BSN101 said..

Te Hau said..
Speaking of twin blade fins..........

double tuttle or a tuttletuttle?

Std tuttle, short fins

Wow didnt see them as that small!!

Any idea how it went? or how old is the pix

The pic is 10 years old.
Bruce Nicholson of Big World Composites in Auckland NZ, made it.
Bruce was trying many different fin ideas around this time.
He called this the BWF 2win fin.
Chris Lockwood tried it and he said
"In short this fin turns well , goes high upwind and has good speed off the
wind. While the fin does have to be "sailed" it is controllable and spin out
is pretty much self correcting.
Still buzzing
Chris"
There used to be a large crew of keen speed sailors in Auckland but the wind seemed to disappear around this time and they all took up other activities and we have seen no high speeds from there since.