Macroscien said...

Could we have a fin with adjustable pitch (angle of attack) ?
By comparison to airplane's airfoil if we could set or even change " the angle of attack" probably we could use this "side-ways" lift to go nicely up the wind.
Everytime with sail flips (change tack) fin should flip also
At this moment our sailboards construction reminds advanced Raptor -with working vectoring trust but broken Aileron.

"Ailerons" on fins (or actually keels) are an old idea in sailing, having been used in the late '60s and called "trim tabs". Actually an old-style yacht with a rudder on the back of the keel is effectively the same.

I'm not equipped to go into the theory, but as I understand it "ailerons" don't greatly affect the angle of an entire sailing craft. That is determined by the drag of the rig and the sideforce. An "aileron" (or trim tab, or gybing centreboard) merely means that the angle of attack of the fin is not linked directly to the centreline of the hull or board.

Effectively, they only mean that the fin goes upwind at the same angle it would before, but the hull is at a different angle to the wind. That can have some good effects and some bad effects, depending on the hull shape.

This is very much a layman's version, but as I understand it having an "aileron" on a fin is like fitting a tighter leach on a sail - it's certainly not a simple key to being able to go upwind better because so many other factors are involved.

elmo said...

And the difference between sticking just your hand out and your whole arm? is a lot more surface area being exposed to the wind.

The bending of the fin does not really matter a huge amount in the equation, it's the energy going to the point of least resistance.

Cavitation and spin out are one in the same, an air pocket forms around the fin (I'm a master of both and have the bruises to prove it).

What continually throws out every bodies thinking is the foil shape, it looks like a wing so it should behave like a wing, unfortunately it wrong.

The aerofoil shape is there as a compromise.

The most efficient fin would be one as thin as a razor blade, unfortunately that would have almost zero mechanical strength. So we have to make the fin thicker so that we are able to withstand the lateral forces.

The aerofoil shape is used as it is the most efficient at moving through a media, you could just use a rectangular piece of G10 as a fin, it would work but it wouldn't be efficient.

So we compromise on a NACA profile which has as low as possible drag coefficient whilst still allowing us to have mechanical strength to oppose the sail.

The bending of the fin whilst spectacular to look at under water doesn't provide any lift even though the tip of the fin can be almost horizontal as the fin is symmetrical so the flow going over one side is the same as the flow going over the other side canceling each other out.

A softer fin helps reduce cavitation as the fin flex's sideways rather than being forces sideways through the water creating a low pressure zone cavitation on the windward side of the fin.


Sticking your hand out of a car window flat and then turning it is effectively what happens with Fin tip twist.

The base of the fin is nice and firmly mounted in your board (provided you've done up the fin screws) but the tip of the fin has now support apart from the mechanical strength of the fin, hence the fin bends like and exponential curve with no bend at the base to maximum bend towards the tip.

Now that is just lateral bend caused by us heavy baskets hanging of the rig.

Fin tip twist is where the strength of the fin is actually overcome by the flow of the water and start pushing the tip around so that when you are looking from the bottom of the fin the front to back line is no longer 12-6 O'clock but more 1-7 o'clock to the flow of water, it doesn't happen all the way down the fin.

This twisting increases the frontal profile which is subject to water resistance increasing lift. Carbon formula fins have this designed into them through different layup methods.

In my understanding, cavitation happens at approx 5oknts and the water boils at the tip of the foil. As windsurfers we experience ventilation (spinout) this is where air is sucked down the length of the fin so sudently there is heaps less friction and the fin pushes a pocket of air through the water IE the fin spins out.. these two are totally different.

Sucking down of air is NOT the major effect causing spinout, who says it is?
Both fins and sails don't have a sharp leading edge because we want the water or air to "stick" to the low pressure side and flow past smoothly. (Two thirds of lift comes from the low pressure side of fins and sails, RAF sails make the low pressure side work properly by providing a rounded outside surface).
If this top flow breaks up into eddies we have stall (fall out of the sky) or spinout (fall into the water).

oldie said...

Sucking down of air is NOT the major effect causing spinout, who says it is?
Both fins and sails don't have a sharp leading edge because we want the water or air to "stick" to the low pressure side and flow past smoothly. (Two thirds of lift comes from the low pressure side of fins and sails, RAF sails make the low pressure side work properly by providing a rounded outside surface).
If this top flow breaks up into eddies we have stall (fall out of the sky) or spinout (fall into the water).

as fin provide substantial drag , injecting air should lower it

Elmo said
"What continually throws out every bodies thinking is the foil shape, it looks like a wing so it should behave like a wing, unfortunately it wrong.
The aerofoil shape is there as a compromise.
The most efficient fin would be one as thin as a razor blade, unfortunately that would have almost zero mechanical strength. So we have to make the fin thicker so that we are able to withstand the lateral forces.
The aerofoil shape is used as it is the most efficient at moving through a media, you could just use a rectangular piece of G10 as a fin, it would work but it wouldn't be efficient.
So we compromise on a NACA profile which has as low as possible drag coefficient whilst still allowing us to have mechanical strength to oppose the sail."
Point us to some learned writings about this, please.

Why, then, does a fin not behave as a wing? Is it because air is compressible but water is not?
It does seem to behave like a wing in that a low speed fin is best with a bulge that is fat and forward.
While a high speed fin is thin.

Macroscien said...

Could we have a fin with adjustable pitch (angle of attack) ?
By comparison to airplane's airfoil if we could set or even change " the angle of attack" probably we could use this "side-ways" lift to go nicely up the wind.
Everytime with sail flips (change tack) fin should flip also
At this moment our sailboards construction reminds advanced Raptor -with working vectoring trust but broken Aileron.

There is already a fin like this available but i carn,t remember its name. German i think with a full length hinge. Wouldn,t send over a freebie and i wouldn,t part with 180 euros for an experemental design

2009 Junior Slalom Champion used a cammed fin
pol75gutek.blogspot.com/
Let him use a fin about 4 cm shorter.
He said (as a paid promoter, maybe)
" A few weeks ago I signed sponsoring contract with new fins company - Smart Fins. These fins are quite revolutionary.
It has different and changing profile - depends on the tack you are sailing. Company says that they look it up on fishes flippers and planes wings, which are also not symmetrical.
Because of this idea, we got about 20%
shorter fins then normal. I sailed my 7.6 sail
with 32cm fin today!"

www.smartfins.com/spinout.html
Maybe they are still a goer...

Newton's third law. That is all.

This seems like a familiar discussion: can we compare a wing with a fin??

I often read the same arguments which should explain why we cannot compare a wing with a fin. Unfortunately these arguments are not true.

Argument 1:
Wings and fins cannot be compared because of the air being compressible and water not.

This argument is not completely correct. The assumption that air is compressible does not hold when you look at wings traveling in sub sonic conditions. In these conditions air behaves like an incompressible fluid. Sounds strange, but this is very well accepted in the scientific world.

Argument 2:
Wings and fins cannot be compared because the lift generated by a wing is in a normal direction to the planform area (90 deg to the wing span and 90 deg to the wing chord), and the lift produced by the fin is parallel to the fin span.

This argument is based on a misinterpretation of the definition of fin lift, and there for it is not correct. The correct definition of fin lift is that it acts horizontal (90 deg to the fin span and 90 deg to the fin chord). So very similar the a situation of a wing traveling through air. The drawing from the earlier posted thesis gives a nice picture of the forces acting our windsurfing gear.

Argument 3:
Wings have asymmetric foils, and fins are symmetric. There for they work in a different way.

The first part is true, sometimes. As mentioned earlier both wings and fins can be symmetric and asymmetric. Note that when you place a symmetric foil under a certain angle in a free stream (AoA) you actually create a asymmetric flow field around to the foil. Besides this, the shape of the foil does not explain how lift is generated, it only has a certain influence on the amount of lift generated.

I would argue that we can perfectly compare our fins with the wings of an airplanes, but only if we look at planes traveling at subsonic speeds where air behaves like an incompressible fluid. A very critical note here will be to take account for the differences of density and viscosity of the two fluids. To do this you must have a good understanding of the Reynolds number and the physics involved.

Than we still have the question about how the lift is actually generated. I think this is best explained by Bernoulli's principle. Before you start the discussion about whether Bernoulli's or Newton's method is the correct one, please keep in mind that both gentlemen are actually doing the same thing. They both start at the same point, and they arrive at the same point, but take a different path. See the next url: www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html (i think this was already posted in this thread)

I prefer to use Bernoulli's method because it seems less complicated then Newton's method and for me it makes it more easy to visualize what is going on.
So when we look at Bernoulli's method and its application to wing lift we can identify some key features. First of all, the whole theory is based on the principle of conservation of energy.

Nice quote from Wikipedia:
"Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy and potential energy remain constant. Thus an increase in the speed of the fluid occurs proportionately with an increase in both its dynamic pressure and kinetic energy, and a decrease in its static pressure and potential energy."

Second we need to 'see' this in the application of a wing/fin. For this I think it is most easy when you understand the phenomenon 'boundary layer' (See: en.wikipedia.org/wiki/Boundary_layer).
When a fluid flows along a surface (wall) we see that the velocity of the fluid layer which is in direct contact with the wall is 0. When we increase the distance from the wall we see that the velocity is gentle increasing, until a certain maximum is reached. This transition region is called the boundary layer.

Knowing that the flow near the wall is zero and that the path of the flow is influenced by the presence of an object (wing or fin in this case), it is not hard to imagine that there are small localized velocity differences near the object. Following Bernoulli's method, these velocity differences come together with localized pressure differences. If you take the static pressure component which acts on the wall and you multiply this with the area on which this pressure acts you get a force. Now you can calulate a big number of localized forces acting on the wing/fin, and because of the direction of this force is always normal to the surface. Now we have a set of localized forces and we know in what direction they act. Now it is easy to sum all these forces with the direction components to come to a total force acting on the wing/fin, caused by the flow.

Does this make sense?

MichielR's summary is no doubt unassailable (joke?).
Therefore we will assist Elmo's descent into depravity by making him accept all the ramifications.
OK on the pipe with a boundary layer where the water flow is zero. But Bernouli would have us seeing a fin dragging along a water layer with it at 30 knots. Must be only one molecule thick, this endangered layer
Sharpening the front of a wing will make it too critical to different angles of attack at low speeds, accepted. But a fin? With a good breeze we just bear off and we are planing without using the dam thing at all. After that it runs at 1 or 2 degrees angle, so they say.
In the early days of motorcar streamlining (before downforce became important), it was found that a very long tapered tail was needed to improve on a sharp cutoff at the rear ("Camm Tail").
Apply both those facts to a fin and Voila! we have a so called "cavitating fin" which seems to work quite well in a non cavitating environment.
(see SailRocket )

Yes, Starboard have a flexi CENTREBOARD on their new 295 "one design raceboard" (there are now about 20 "one designs, take your choice).

By now y'all have digested Paul's adventures with the SailRocket and realised that windsurfing will not be the same after his revelations.
(OK, nothing will change while the gurus and advertising machine keep the lid on.)

But, here is a summary:
His fin has no surface directly above it, so no fin bolt holes to block up to prevent air being "drawn down", instead he had up to 4 "fences" to prevent this.
Then, with a sharp "cavitating" fin in non-cavitation mode, he needed NONE.
(We may end up with slotted fins to avoid blood in the water, however.)

Check where spinout begins (from the SMARTFIN site or wherever)- right where the blunt leading edge of the fin hits the water head on and wipes away the Benthingo effect.

A worthy addition to my rig with the slotted sail....

Elmo, there is always some room for discussion about the interpretation of a generalized model subjected to specific situation like fin performance.

“OK on the pipe with a boundary layer where the water flow is zero. But Bernouli would have us seeing a fin dragging along a water layer with it at 30 knots. Must be only one molecule thick, this endangered layer “

Boundary layers are not just limited to flows through pipes. It is applicable for all wall bounded flows. Dragging a fin through the water at 30 knots gives the same effect as fixing the fin in a 30 knot water stream. The velocities in the boundary layer are referenced to the wall surface. So with a 0 velocity layer at the fin surface, this actually means that this thin layer of water sticks to the fin and travels along with us. The thickness of this layer is about 0.001 to 0.00001 mm for a fin, depending on the speed. But still the existence of this small boundary layer plays a important role in how the flow field around the fin will be developed.

About the flex and twist. I did some research to find out how the water flow effects the flex and twist of a fin. I used a system of coupled CFD solver and FEA solver. First the CFD solver solves the flow field around a initial undeformed 3D model of a fin, and at the end of the flow analysis the local static pressures are transferred to the FEA solver. The FEA solver then solves the deformation of the fin's 3D body, taking into account the material properties of G10. When the FEA solver is finished, it sends the deformed model back to the CFD solver which again solves the flow field, but now for the deformed model. This process is repeated until a steady state is reached, where there is almost no difference in deformation of the 3D model at the start and end of the cycle.
Results of this kind of analysis gives a pretty accurate picture of the actual deformation under the load of a fluid flow, and also a good picture of the actual flow field around the deformed body.

I can share the results of one case to give you some more insight into the effect of the twist and flex. The case I used was a fin of 29 cm with 195 cm2 area and a straight leading edge at a rake of 15 deg, and the velocity was about 50 kph. The section used is a NACA0010, and in an additional case I used a NACA0008 section (other parameters are kept the same).

The results:
NACA0010 rigid
AoA: 2 degree
Lift: 300 newton
Drag: 53 newton
Vertical lift: 62 newton
Tip deflection: 0 mm

NACA0010 flexible
AoA: 2 degree
Lift: 283 newton
Drag: 42 newton
Vertical lift: 72 newton
Tip deflection: 12 mm

NACA0008 flexible
AoA: 2 degree
Lift: 251 newton
Drag: 42 newton
Vertical lift: 64.5 newton
Tip deflection: 19.4 mm

Because of the fin tip is being twisted the local AoA at the tip section is smaller as it is closer to the base. This explains partly both the reduced lift and reduced drag. A second part influencing the lift is because of the bending a small part of the lift is projected upwards. There for we see a little increase in vertical lift when comparing the rigid and flexible case. With a difference of +/- 16% this effect is not negligible, but it is not very big either.

As you can see the vertical lift is small in all three cases. Only 60-70 N which is approximately 6 – 7 kg, where we need more like 90-100+ kg the support the weight of our windsurfing gear plus rider. I would say that the fin does not contribute that much to the overall vertical lift. I think the board is mainly responsible for this part.

Of course this is only a small fin, and the vertical lift generated by a bigger fin would be much bigger. But a bigger fin finds itself under a bigger board to, which generates also more board lift. I do think that with bigger gear the fin takes a bigger part of the vertical lift, still I think the board lift is much more than the vertical fin lift. But I need to run some big fins through the analysis before I really can make a statement like this.

Don't forget the main fin lift is off set to the board centre line, therefore applies a twisting force to the board.
This twist is trying to lift the rider up, (and sink the opposite rail) so may feel as though it's a total lift?

Yes, and there's also what is actually happening and what we think is happening. The tricks the brain plays in presenting the world to us.

(Just see PeterMac's post on the stationary Earth)

MichielR said...
................................ It makes the discussion much more easy to understand when we are talking about the same thing.

I think there's a greater probability that the sun will gobble the earth up before anyone starts talking about the same thing in a forum

PS - Very interesting reading and excellent bit of research too Michiel