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Lift in windsurfing fins
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ittiandro



Joined: 22 Nov 2009
Posts: 290

PostPosted: Thu Mar 31, 2022 4:55 pm    Post subject: Lift in windsurfing fins Reply with quote

I am resurrecting an older thread on this topic.

I always thought that for the board to overcome the water resistance and the weight of the rig (and the sailor), there has to be a vertical upward force and this vertical force would be provided by the fin combined with the sail. .
The more physics-oriented explanations speak, however, of “ lateral” lift when referring to the fin. Is there a vertical lift, too, provided by by the fin as distinct from the lateral lift? If so, how is it created?

Also, I cannot understand how the lateral lift of the fin can be explained, as we often read, by analogy with the airplane’s wing : unlike the board’s fin, the airplane’s wing runs horizontally through the air, hence the lift provided by the pressure’s differential is truly vertical. How can the fin provide a vertical lift, if it is itself vertical?

Furthermore, the typical board’s fin, unlike the airplane’s wing, is symmetrical: how can a pressure differential arise between the two sides of the board's fin, if the curved distance from leading edge to trailing edge is the same?

Lastly, speaking of the lateral lift of the fin, I cannot picture how the water flow around the fin can create a pressure differential between the two sides, regardless of whether the fin is symmetrical or not.
Perhaps if I could see diagram showing how the water flow separates differently ( and creates a different pressure ) on each side of the fin. I would understand better.

Thanks for your help

Ittiandro
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dllee



Joined: 03 Jul 2009
Posts: 5194
Location: East Bay

PostPosted: Thu Mar 31, 2022 7:28 pm    Post subject: Reply with quote

Should we factor in the speed of the moving board?
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ittiandro



Joined: 22 Nov 2009
Posts: 290

PostPosted: Thu Mar 31, 2022 7:52 pm    Post subject: Reply with quote

dllee wrote:
Should we factor in the speed of the moving board?



In fact, this is exactly my question i
In other words, does speed provide lift to the board or does the fin itself provide this lift once the board moves ?

Ittiandro
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dllee



Joined: 03 Jul 2009
Posts: 5194
Location: East Bay

PostPosted: Thu Mar 31, 2022 8:45 pm    Post subject: Reply with quote

How much lift do you need?
David White, at 250 lbs., rides a 18" wide speed board, in the straps, in 30 mph wind and 5.5 sail.
A same weight rider in 15 mph wind would need a 85 cm wide biard, and a 8 meter sail.
Huge lift difference between 22 moh board speed and 45 mph board speed.
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mac



Joined: 07 Mar 1999
Posts: 17108
Location: Berkeley, California

PostPosted: Thu Mar 31, 2022 10:55 pm    Post subject: Reply with quote

I don't think the vertical lift comes from the fin--the fin, and the pressure differential from flow around it, allows you to drive it up or downwind, and keeps the board from sliding sideways. It's providing horizontal lift. As Doman posted, vertical lift comes from the speed of the board. In a straight line, the more speed, the less wetted surface area, the less wetted surface area the more speed. Plus you get a little lift from the sail and the angle of attack of the board with the water. The actual picture of forces is pretty complicated. There is water flow along the bottom of the board, up and down forces are balanced or you leave the water. You can add some later tracking force by railing the board,

When we were sailing formula boards, they had fins that were about 70 cm. I've seen video that shows those fins bending in pretty close to an L. So they were functioning to some degree like a foil--lifting the board. I certainly had puffs pick me up out of the water--at 210 pounds. Modeling the bend of a fin and all of the forces is pretty complex.
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isobars



Joined: 12 Dec 1999
Posts: 20713

PostPosted: Thu Mar 31, 2022 11:37 pm    Post subject: Reply with quote

A fin provides lateral lift because, like the whole board, it's crabbing. i.e., it ain't going the way it's pointed, even at top speed. It's pointed upwind of your path through the water, so the water hits the lee side of the fin (from the fin's perspective) and pushes it windward. This works fine until the crabbing angle exceeds the fin's limit (stall angle?) and it breaks loose/spins out.
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swchandler



Joined: 08 Nov 1993
Posts: 10348

PostPosted: Fri Apr 01, 2022 12:03 am    Post subject: Reply with quote

Rather than understand the scientific forces at work in fins, I concentrate on picking the right fin for the job. It's a much more practical focus that yields results.
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ittiandro



Joined: 22 Nov 2009
Posts: 290

PostPosted: Fri Apr 01, 2022 12:34 am    Post subject: Reply with quote

Thanks
Well formulated explanation, Mac.

As I understand, then , that the lateral “lift “ of the fin is nothing but the lateral resistance that the fin provides against the force of the wind in the sail that would push he board laterally downwind instead of forward..

At the same time, you seem to agree that the fin, too, provides vertical lift because it functions like a foil by bending. You must be right, but I never thought about it because when I look at my fin, it is so hard and stiff that it is hard to imagine that the water density can bend it, even less that it can bend it like an L….

I just bought a new 44 cm fin for my Bic 160 L board to replace the original 38 cm fin which I find too small for light winds subplaning . With it, I felt like drifting sideways more often than not and upwind, too, was not very easy, even with a removable screw-on centerboard.

I didn’t try it on yet, but I am curious to see if it gives me more bite in the water, with a 7 m. sail. I weigh 90 kg.

Thanks

Ittiandro
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cgoudie1



Joined: 10 Apr 2006
Posts: 2517
Location: Killer Sturgeon Cove

PostPosted: Fri Apr 01, 2022 12:04 pm    Post subject: Reply with quote

Presuming you are planing, almost zero vertical lift (to the entire rig) from the fin (no matter how
big it is). Whenever fin lift is discussed (rationally) it's working to suck
the tail of the board back under you to counter the horizontal component
of the sail force, You are calling that lateral resistance.

The only time a fin might contribute to vertical lift is while using a
daggerboard and "railing up" sometimes the torque moment can flip
the board over (with you on it) at some point there's some vertical component there.

If you want a useful vertical component, get a foil (I seem to recall you
posting about fins with small foils a year or 2 ago. That only
causes turbulence and a likelihood for spinout, unless they're designed
as a foil (i remember some pretty bad fins from the late 80s with little
horizontal foillettes that were not based on hydro dynamics.

If you want to plane up earlier, get a bigger fin to go with a bigger sail,
If you want to go faster, get a narrower tail. If you want stability
in displacement mode, get a large surface area low profile fin (that looks like
a meat cleaver).

.02

-Craig




ittiandro wrote:
Thanks
Well formulated explanation, Mac.

As I understand, then , that the lateral “lift “ of the fin is nothing but the lateral resistance that the fin provides against the force of the wind in the sail that would push he board laterally downwind instead of forward..

At the same time, you seem to agree that the fin, too, provides vertical lift because it functions like a foil by bending. You must be right, but I never thought about it because when I look at my fin, it is so hard and stiff that it is hard to imagine that the water density can bend it, even less that it can bend it like an L….

I just bought a new 44 cm fin for my Bic 160 L board to replace the original 38 cm fin which I find too small for light winds subplaning . With it, I felt like drifting sideways more often than not and upwind, too, was not very easy, even with a removable screw-on centerboard.

I didn’t try it on yet, but I am curious to see if it gives me more bite in the water, with a 7 m. sail. I weigh 90 kg.

Thanks

Ittiandro
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ozzimark



Joined: 23 Dec 2020
Posts: 11

PostPosted: Fri Apr 01, 2022 12:11 pm    Post subject: Reply with quote

I know it's so easy and convenient to measure, but I wish we could move away from using the length of the fin as it's primary metric of size; there's a lot more to it that really matters!

To your original question about "lift", we generally use the term to describe the lateral force that prevents you from just sliding sideways through the water.

It's ok to think of it as a wing rotated 90° and in the water, because that's exactly what it is! For any wing or foil profile, even just a flat plate of metal, there is some amount of lift generated when the Angle of Attack (AoA) is increased. There is also some amount of drag. Everyone knows this as lift/drag (L/D) from their Airplanes 101 book they read as a little kid. You also know that there is a maximum AoA before a wing "stalls", or starts producing less lift. The reasons are complex, we don't need to understand here.

As a simplistic approximation: there is some forward velocity of the fin moving forward through the water (hopefully!!), and a sideways loading force that is the reaction of wind against the sail. The combination of these two generate some sideways velocity, and a resulting Angle of Attack. We usually want the fin to behave the same way regardless of which side of the fin is pushing on the water, so they're almost always a symmetrical wing profile. The guys doing speed runs on a starboard tack only might choose an cambered (asymmetric) shape to get better L/D and higher speed. That asymmetric fin will suck on a port tack though.

In general, you want the fin to be at the highest L/D point at the speed you're traveling and the load that's being applied. That's a simplified explanation on why bigger sails require bigger fins - lower speed and higher load! Too big a fin will be too much drag as it's not on the optimal point of the L/D curve, and too small a fin will stall and spin-out more easily.

Keeping things relatively "static", you also have the effect of the board trim on the fin. The lift is perpendicular to the fin surface, so as the windward rail of the board is tilted down, the fin tip moves leeward, and the lift vector starts pointing down, pulling the board more firmly against the water. Conversely, tilting the board the opposite direction (leeward down) causes the fin to generate some upward lift, pushing the board out of the water.

Fin flex is a really interesting thing. It bends in a couple ways, depending on the shape. Straight formula-style fins are simple - the tip just flexes in response to the lift forces. A swept-back fin will also twist a bit as it flexes, causing a change in AoA over the length of the fin. This twist will make the fin feel a little "softer" as it absorbs some of the sudden force changes you might see with chop. There are some really interesting videos showing just how much the bigger fins do flex! https://www.youtube.com/watch?v=OEhYC9IYOSU

As you can imagine, those longer fins do contribute some amount of lift in the vertical direction from the flex, which generally helps pop the board out of the water to get planing!
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