I understand the concept of the water flowing over the curved part of the faster than the flat creating lift in an airplane because the lift is perpendicular to the wing. However, in a fin the it seems like this force would be pulling the board apart - not up - based on the orientation of the fin.
i had the same question. here’s a discussion on it:
http://www.swaylocks.com/forum/gforum.cgi?post=208991;guest=2963677#208991
sorry Dan but i have to throw this one in here just to have some balance for the upcoming debate…one of my favs…
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
but im sure others will throw in easier nuggets to digest…
lift is a bit complicated but a lot has to do with AOAs, fin cant and foils…
Two fins creating lift acting against each other. If the board won’t separate in two, the path of least resistence is up vs. the board being pulled down into the water.
I’ve always been confused about this, i’ve read the first link and became more confused…
I have a basic understanding of vectors and bernouli’s but from what i’m reading…i could be WAY off…is that when they say “lift”…it relates to the force that makes the fin go into the wave and/or forward (lateral force relative to the surfboard’s orientation) and not lift as in “flying” or the raising of the tail end (out of the water) as i’ve always thought of lift. Am I right in this thinking or did I just confuse more swaylockers?
Rio
headache-from reading threads like this and neutral axis…
I like that site - its my first source to get definitions for terms that I need to no more about. However the lift they are talking about shoud pull the board apart (or create suction as somebody just suggested). I had been thinking that toe-in was what created lift, but I never took into account cant. I guess that it would make the fin function like a plane with steeply angled wings.
I start teaching this stuff in 2 weeks. I tried to get a link to the book we use. You would laugh at how closely it follows what we talk about here.
When you describe side fins wanting to pull the board apart, you’re visuallizing the surfing flat and straight. This is the Angle of Attack where the fins are fighting each other and are inherently unstable. Thrusters want to turn when water flows along the length their their bottoms and fins this way. But, put a board on rail in a turn and the lift vector of the working fin pulls the rail and fin down into the face of the wave preventing your board from spinning out. The lift vector of the non-working fin disappears as the non-working fin lifts out of the flow of water.
ok you guys are starting to make sense to me now…
for the longest time…when I asked someone in the surfshop about fins (yeah i know), they would say oh this fin creates more lift…and i thought for the longest time that it would make the back end ‘lift’ off of the water and doing the physics in my head just didn’t make sense. So when they say lift they mean that its just a term used for the force acted on the fins and is different from what is actually happening…sort of.
Rio
So the lift that the fins are creating would feel like suction (or bite) to me when I was surfing (or at least turning). Since the fins are fighting each other when the board is going straight what would I be feeling from the board? Is toe-in intended to fight this unstability?
“Is toe-in intended to fight this unstability?”
Simple answer is; you are asking the wrong question.
Your analysis is still a little off. Not being negative, just want you to get there yourself. Using your own brainpower.
Think about this system a little more, first.
Then check the Swaylopedia for terms and definitions.
Here is the concept: tension < stability.
Quote:
So the lift that the fins are creating would feel like suction (or bite) to me when I was surfing (or at least turning). Since the fins are fighting each other when the board is going straight what would I be feeling from the board? Is toe-in intended to fight this unstability?
Each side fin directs water outward when the board goes straight. The water under the board is pushed out. Therefore, the tail is pulled down, large eddies are created, and there is a lot of DRAG…last week I rode two boards, same size make and model, one with thruster fins, and one in which the side fins were not toed-in when you go straight. There is an ENORMOUS EASILY DETECTABLE difference.
Even turning, there is a big difference. Until the outside rail fin lifts out of the water, it is blocking the turn. The only time a thruster side fin points the right way is when you are hard on a rail…therefore the most successful thruster surfers are hard on a rail as often as possible.
The great thing about a thruster is what the inside rail fin and rear fin do in a bottom turn…unparalleled in other fin configurations. The bad thing is what the rail fins do the rest of the time.
I completley agree Blakestah, thats why I’m still interested in your project…let me know how it’s going. Fins are there for control, not lift. They are not designed to create the “normal” type of lift. What I mean is it doesn’t use fluid dynamics to increase lift and reduce drag. Although it could, thats what a hydrofoil is. Hydrofoil surfing is much different because during normal surfing you’re eliminating an axis of motion, that is your not moving yourn nose down and tail up like it’s on a pivot point. A hydrofoil would incorporate the contol of pitch and roll, like flying an airplane, or jet. There are few fins that manipulate lift a little bit more like the FCS 3dtip(I think thats the name), it was a fin design that was bought by FCS i believe, im not sure of who designed it first hand. I know theres a website that shows a history of fin design…If you take five minutes to google it I’m sure you’ll find it or look in the archives. Another fin that uses lift to reduce drag is the star fin designed and ridden by Cheyne Horan. If he’s still around he probably has some opinions on this topic. But think of most fins as sucking the board down and providing control. It doesn’t “add” lift like an airfoil.
Particularly under pressure, it’s the lift of the fin that governs how nicely the rail enters and sets, the pressures on the fins, how they are set up, and how balanced they are with rider and board design.
Another good reason why it’s a difficult job finding the right fin, exactly the right fin, that feels right. So many boards and so many fins!!!
I believe fully efficient fins of the future will become smaller in area, simply because they will function much more efficiently with better lift qualities.
That’s lift relative to the foil, not lift relative to gravity, stance, wave, horizon, etc.
After trying many and varied wing things, I finally realised, that job was for the bottom of the board.
" Why do fins create lift ?? "
Because they CAN !!
…now, please go surfing.
ben
Tunnel fins are another horizontal area fin which can be used to provide lift as a means of reducing drag while surfing apparently ‘normally’.
And because it’s their job!!!
Hey Chip, I am having fun, well, as much fun as you can over a mile up and over a thousand miles away from the wavy matter.
to get to the other side.
with the rest of the chickens that crossed the road
…ambrose…
simple question
simple answer.
Wildy,
You're exactly right. Fins of the future will have the drag characteristics of smaller templates and the lift characteristics of larger fins. The Next Generation foils are a smaller template 4-3/8" X 4-1/2". Through new foil generation techniques, I've reduced the drag even further than it's smaller size would normally create. But, they hold like a much larger fin and redirect much more smoothly and predictably.
hey tom,
what New Gen fin would you recommend for a FCS Std Rusty replacement? That fin is so right for me and my equipment but your comments would lead me to believe there’s better…
always game to try something new and better…
The Rusty fin is a big fin with lots of tip area. I have not looked over the entire M range yet and I’m still waiting on feed back from our customers about what type of foil they want on a larger template. But, I’d say such a big template is best for medium to low flow rate applications and a very slight concave inside foil might be a good thing for that template. I just don’t know that anyone offers one yet.