opening a can of worms

[boardsurfr]

Good question. No way you'll get anything close to agreement in the answers. One reason is that different people have different ideas what lift is. Another one is that how much lift is generated depends a lot on sailing style and equipment (all of it, not just the fin). Many windsurfers sail for years without ever experiencing vertical lift from fins.

When describing flow over foils, the term "lift" describes the force perpendicular to the foil. With some fins pointing straight down, that force would just balance the force of the sail, and not have a vertical component. Angle of attack is essential to produce the lift.

But fins can also produce a significant amount of vertical lift. This is easiest to understand to formula fins. They bend a lot (check https://www.youtube.com/watch?v=OEhYC9IYOSU), so the lift perpendicular to the fin has a vertical component. But if you keep watching the video above, you'll see the same bending in slalom fins. Slalom sailors often increase the vertical lift by pushing the leeward edge of the board down.

In freeride boards, fins are usually shorter and often stiffer, and the boards are sailed at much lower speeds. That's even more the case in multi-fin wave boards. So fins bend less, and the upward component of the lift is much smaller - it may not be noticeable at all. If the sailor is an intermediate who digs the windward rail in, that will negate the smaller fin bend, and reduce vertical lift to roughly nothing.

But even some fins that do not bend at all can produce vertical lift - they just need to be angled enough. In Australia, speed surfers often sail in extremely shallow spots (think 1 foot deep, sometimes less), and they want a lot of weed growth to minimize chop. which means they often have to sail weed fins with very high rake - 50 degrees or more. Simple Newtonian mechanics say that water hitting the front edge will impart an upward force. No point in calling me "too simple" here - there are several "Delta" fins out there that actually have a lot of vertical lift. One is the "FangyFin", an fin cast from aluminum (to better survive in weed that has shells attached). By design, it has a thicker front edge specifically to generate more lift. It's got a bunch of other clever features, too, like the Lessacher cutout at the back and a fillet (like turbines and airplane wings). Australian speed surfer love it because it works.

The Delta XT from Maui Ultra Fins is another example. The first time I sailed the XT 24 on my 99 l slalom board, I was absolutely amazed how much vertical lift it generated. This is quite different from the original MUF Delta, which created no noticeable vertical lift (and was much more spinout prone and overall inferior).

For nice illustrations of lift generated by various foil shapes, check this article: http://www.foilingweek.com/wp-content/uploads/2015/04/The-foil-alphabet-Fred-Monsonnec-04-04-2015.pdf. You can see the similarity between the bent formula and slalom fins and the C foil, J foil, or S foil.

[bericw]

Interesting post, boardsurfr, and all others too...

Link...

https://www.google.com/url?sa=t&source=web&rct=j&url=http://eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf&ved=2ahUKEwi2me-Sx-zbAhXH5IMKHaJpCuIQFjAAegQIBRAB&usg=AOvVaw2UnL7FZRJQ-f3oqYHsIfWO

for the pdf of the reference I provided in this old thread,

http://www.iwindsurf.com/forums/viewtopic.php?p=253345

where I commented, in part:

"...Viscous force and lift force are not exclusive. To repeat my prior post, both lift force, the force normal to the fin surface, and drag force, the force parallel to the fin surface, have both pressure and viscous force terms. Please reconsider the references if this is not agreed with."

"...the body of knowledge relative to the understanding of non-complex rheology fluid flow around an object is quite vast and is a good place to start if one wishes to determine whether lift or drag forces are dominant on a windsurf fin. One would certainly hope it is the former; much better designs are obvious for the latter! 

A google search of "ratio of drag to lift symmetric foil resisting sideways motion" readily yielded the thesis of Simon Barry Fagg, Bournemouth University, 1997, which is replete with interesting references. Moving beyond the statements that describe that the primary role of a modern planing hull windsurfing fin is to provide a lifting force to counteract the sideways force of the sail as well as provide directional control and stability, the magnitude of the lift force on a fin on a planing windsurf board is referenced to vary between 300 and 600 N, while the drag force on the fin, 1/6 of the total drag on the system, varies in magnitude from 20 to 50 N. Of course, due to lack of time and funding, I have not verified the thesis was accepted nor the validity of the references. I remain confident however, that an object on a windsurfing board that is dominated by drag is not wanted and, in keeping with my prior note of the lateral reinforcement present on windsurfing board fin boxes, that significant lift, i.e. normal forces, are present on a windsurfing fin."

So to anwer the question the poster of this topic, the fluid dynamics (What is lift? It is the force normal, i.e., perpendicular, to a surface, reagardless of that surface's orientation to gravity.) in the fluid of interest are well defined and understood, and the supplied reference link and the references therein may be of interest. One may also see any text similar to Engineering Fluid Mechanics, John A. Roberson, Clayton T. Crowe, any edition, for definitions of and expressions for lift and drag. Or, more immediately,

https://en.m.wikipedia.org/wiki/Lift_(force)

All I really know is that windsurfing is fun! Have a great day!

[J64TWB]

Wow. Definetly my #8, “way to complicated for my small mind”.

I’m beginning to think of this whole fin lift thing differently. It’s no longer a fin in my mind, it’s a board and fin as one solid entity. One acts different without the other. This single solid entity is permanently attached to a pivoting entity (rig), which complicates things more. This in turn is attached to a movable entity (human) to create a beautiful, extremely complex, sometimes chaotic ride over a liquid.