How do you figure out the AOA of a fin on a surfboard on a wave?
Very difficult.
My rules of thumb are"
at typical speed, AOAs are proportional to turning arcs…the sharper the turn (rail digs), the higher the AOAs.
The faster you go, the lower the AOAs. AOAs at Jaws are very low, less than 15 degrees.
Ive estimated a typical modern short board bottom turns AOAs in the 10-25 degrees range. Sharp top turns can be as high as 90 degrees and beyond!
Very touchy and difficult subject and no one has the definitive answer. No one has actually objectively measured this using an actual surfboard…there’s no money to be made in knowing the answer. Maybe a very wealthy and charital bloke can fund the research…any takers?
EDIT: you can also have high AOA in tight trim high on wave face.
Don’t forget - There is no water moving up the face, the energy is moving through the water, as are you. Not until the wave starts breaking does water “move.” I’ve written about my own experience regarding stalling at high AOA, which always felt to me like “getting stuck”/or the board wouldn’t turn any "sharper,’ and the vast improvement I felt using a MVG.
I used to think that too, but now I am convinced that the water does move up the wave face. On some waves it moves faster, on some it moves slower.
If you track a water particle it moves in a circular motion. Water is “sticky” the particles push and pull each other along, I think viscosity is the term. So when the wave energy approaches the shallow shore the energy is directed upwards and the water particles pull each vertical and back toward the crest. The particles approaching the crest of the wave are actually pulling the shoreword particles up the face of the wave.
Ah ha - I would agree there is water moving vertically up in space, and would agree on a molecular level water attracts to itself, but at the speed a board moves through the water, I’d argue it doesn’t make difference.
My classic example would be: Watch something floating in the surf, it will maintain it’s place on the water surface, sans currents, floating up and over a wave even as the wave come close to breaking, and it is not until the wave crests, and the energy pushes ahead of the water as it slows over the bottom at the point of 5/8 the waves hight, or some number close to that.
I would still assert the moving of the board through the water, and the change in the AOA while turning, is so significant it renders the molecular movement of the water irrelevant.
Even at an extreme example like Teahupoo or Pipeline?
And, I think, the water is moving faster toward the crest than it is at the trough (base). And the more vertical the wave the faster it moves toward the crest.
I agree that relative to the board moving through the water, the water moving up the wave has a smaller vector force. But is it really insignificant?
Good questions - But I still stand by my understanding of the science. And if you had video of something floating at Teahupoo it would still maintain its relative position on the water surface. I still maintain it is the energy wave which move, changes speed, and distorts as the wave feels the bottom. But I would concede, given the speed with which the changes take place at spot like Teahupoo it may be more significant, but I still don’t think there would be a significant difference in effect on the fins action relative to AOA.
What is an MVG? Also, it is my understanding and experience that fin performance is limited by the stalling AoA. So is one of the goals of fin design to maximize this stall angle (I would think so)?
Good questions - But I still stand by my understanding of the science. And if you had video of something floating at Teahupoo it would still maintain its relative position on the water surface.
It may maintain its horizontal position relative to the water surface but the vertical position increases… I think.
I think if you do a search you’ll find at least one picture, and some where you will find an explanation I gave.
In a nut shell - MVG = Multi Vortex Generator. Small finlets in front of the center fin, based on research of the phenomenal high speed turning ability of tuna, which create small vortices that wrap/attach to the fin, and this enhanced flow around the center fin reduces incidence of flow separation, and it is the flow separation which leads to the “stalling” (Not quite so detrimental as it is in aircraft) at high AOA.
Agreed, but as soon as the wave passes the “float” returns to it’s original position. In fact, the motion is circular, reflecting the motion of the energy wave itself.
I actually just did a research project on that (Im an aerospace engineering student), based on humpback whale flippers. I didnt know that tuna also used that same feature/idea. I was wondering if that had been applied to fin design at this point. Pretty cool stuff.
A guy named Frank Fish did a lot of good research on it (with regards to humpback whales), and now he has a company that is applying the idea to wind turbines and pumps making them more efficient.
Thanks for the heads up on the MVG. gonna look into that when i get some more time
Also, it is my understanding and experience that fin performance is limited by the stalling AoA. So is one of the goals of fin design to maximize this stall angle (I would think so)?
If you analyze the fin only, then yes these generalizations are true.
But if you look at your surfboards fins and their setup as a SYSTEM, the ill affects of fin stall can be overcome by more positive affects.
Fin stall creates drag, but well setup fins also create thrust. They are basically opposing forces. Some fin setups create more thrust than others. One can create a system that creates so much thrust, that stall drag becomes proportionately insignificant. So what you feel is lotsa thrust, and very little drag.
All this assumes you want to go real fast…not everyone does.
Good point. Also I think a positive of stall was mentioned in the thread I sighted earlier in this thread. I guess in some situations (heavy fast waves) having your outside fin stall can help to control the rotation of the board, and keep the rail in the wave.