How important is lift in fins ?
As I understand it Lift sucks the fin into the face due to Asymetery in the foil.
So how important is it really ?
Will anyone notice the difference between side fins like FCS H2’s and True foil side fins, forgetting about angle of attack for a moment ?
i.e If I put 3 FCS H2’s center fins on my board, would I notice a difference between the standard 3 fin H2 set.
Hey B,
Not very.
Suck/hold from foil lift is somewhat overrated. Fin/board suck/hold is drag and pressure dominant.
hmm, some might notice, others wont.
Swap you front fins, put the left on the right and viceversa.
Everything works…just to different degrees.
Your lift inquiry relates to foils, foils have good function to within a small range of AOA…at large AOAs flow seperation (ventilation) occurs which causes severe drag. Lucky for us, when we are trying to go faster, the angles are not very severe (and they can be very severe during moderate to sharp turns) so in the end it all works out.
Bernoilli and Newton are both correct. Try not to get too ‘dragged’ down by fin foil pseudo-science…there’s lots of noise out there…but what’s out there is what works…although some prefer fat foils and those arent very common commercially. If you want a worthy experiment, go with some home made fatties…thickness 10-13% of base chord length…whatever that means 
Good answer craftee.
It really depends a lot on how sensitive you are to the effects of different fins. Some people feel it, some don’t.
The whole surfing experience is the combination of pressures and how hard each of us push them, so the fin pressure combined with board pressure gives us the experience we want.
A perfect example is the thruster fin, general shape and size. The small surface area, depth, and shape make it easy to break free and feel loose, which is why so many people love them compared to stiff singles with big fins, and loose twins with no rear stabiliser. The small fin still provides lift by being positioned nearer the rail.
The improvements on the latest fins will get better, making more people surf better. But the fine changes in lift characteristics will only felt by those sensitive enough to feel it.
Good fins and foils always make a board feel and go better. I like thicker ones too.
The more speed you need to generate the more lift you need.
Different conditions require different needs when it comes to lift. Conditions vary greatly and so do fins but some fins will surf on a wide variety boards in wide variety of conditions and do fairly well. Some fins are very board-specific and some are very condition specific. If this all sounds like a bunch of generalization it’s because answers to general questions as the one above must be addressed in a similar fashion.
I’ll risk saying one thing more specific here:
On smaller waves more fin lift works better as long as there isn’t too much drag generated by the fin profile.
Finally – the faster the wave pushes the board the less fin lift is necessary because fin/fins with a greater lift coefficient will produce an unwieldy (touchy – grabby) ride.
No Worries, Rich
The larger and more powerful the waves, the less lift and release. The higher the speed, the greater the need for all elements of design to function in synergy… fin(s), rails, rocker, thickness profile and template.
Thanks guys, You validated what I was thinking. I was trying to explain the reality of fins to a guy that was confused by the pr on the FCS website
I’m currently doing a paper on the fluid dynamics of fins and their related drags. comparing a thruster setup to single fin.
Quick software processing, and one thing that really stood out is how little the foil mattered…
If you think of the generic lift of a plane wing, theres a few things to consider:
1.) plane wing surface area is much much larger than the fin.
2.) actual thickness of foil is more in the 10’s of cms rather than less than 2cm…
3.) air fluid passing over wing has a much much greater relative velocity compared to water over the fin.
4.) ocean wave fluid is much more turbulent than high altitude air streams.
Never the less im learning shit loads and its all interesting!
Cheers
L
Interesting Lavz.
I do think foils matter…but not the way that it is commonly argued or marketed. One basic fact is that at the AOAs the surfboard fins have to deal with, airplanes would have a much harder time flying, if at all.
Btw, in your formal analysis, what are the effects of fluid density. It has to be dramatic because of the pressures involved.
Cheers
“”““Quick software processing, and one thing that really stood out is how little the foil mattered…””“” ???
What could this possibly mean?
“”““1.) plane wing surface area is much much larger than the fin. 3.) air fluid passing over wing has a much much greater relative velocity compared to water over the fin.””“”
How much less viscous than air is water?
When we consider the difference it the two fluids and compares the flow patterns off of a surfboard fin and a jet airplane wing we find the these flow patterns on the jet at Mach 2 are comparable to a surfboard at 25 knots. So the foil doesn’t matter? Hmmm? All my testing leads me to a contrary conclusion.
“”““4.) ocean wave fluid is much more turbulent than high altitude air streams.””“”
Needs to be restated.
I’ve got lots to learn ~~
No Worries, Rich
Let me make my humild question: do you know any thread about basics of fins dynamics, I’d like to study this subject more, so I can understand it in more variables, not in part like this discussion. I’ve made a search (of course) but only found spare subjects like this one.
Thank you.
Joni
Joni,
Try this: http://www.vacantisw.com/foildesign.htm
I’ll defer to Tom at Daumtooling (he molds fins to most precise tolerances) & Blakestah (PhD and inventor of the rotating fin system). These guys have far more technical education than I do. Much of my knowledge is intuitively and practically driven. Oh yeah, that’s right, I watch the fishes and birds intensly and ride surfboards and fins with a variety of configurations.
No Worries, Rich
P.S. The little guy attached may not look like much but he can get through the water at an amazing rate.
I once found a small paperback book titled ‘Shape and Flow’. It gave an excellent explanation in text, photos and diagrams, on how different shaped objects pass through fluids of different densities, boundary layers, turbulence etc. If you find it you might get some insight.
Even after absorbing that into my own ideas, the variables are still wide and varied, and to apply them to everyones everyday board is still very subjective and personal.
As most of the fin fanatics here will agree, the best way to learn, after gathering data and ideas, is still trial and error. The best fins I ever used had a combination of science applied to experience.
Sorry, I don’t know the magic formula yet, and if anyone claims they do I would be sceptical, most of all the fin company marketing hype.
Here’s another link:
http://en.wikipedia.org/wiki/Foil_(fluid_mechanics)
Finally – the faster the wave pushes the board the less fin lift is necessary No Worries, Rich
Hi Rich, probably just typo on your part but waves don’t push boards (except when riding the soup)
Sorry to be a pest.
Roy
Ok, thanks Rich and Greg, I agree that trial and error is the best way to comprehend the fin design role. It disturbs me so much that I’ve begun to ride singles just to have few “variables” to deal with and build up my knowledge on it little steps at a time, I’m really bad in physics. Anyway this trial practice make me more sensitive to board or fins changes, I begun to know, better saying: to realize, subtle aspects of board riding.
My main concerns nowadays are:
1- Outline: how does it affects turning, turbulence and resultant drag;
2- Foil: how does it affects turning, AOA, and drag.
I do spent much time guessing about the different kinds of outline, how did they evolve historically, what’s the role of rake, etc.
I’ll check those links you told me Rich and, Greg, I feel so sorry, but we don’t have many surfing design books in english here, indeed ANY design books. Maybe one guy who have lived many years in Hawaii may tell me about this paper you told. After my search, I’ll bother you again with some beginners questions, if you may and if my english skills permit.
BTW, if I’m saying too much bulls**t, please correct me. Newbie thing.
Thank you guys for your attention and advice.
Joni
Hey Bloke,
No offense taken mate, You’re right ~ bad choice of words on my part. moves along the wave face would have been a better way of putting it.
No Worries, Rich
Cool Rich, just me splitting hairs again I suppose, and trying to resist a mad frenzy of tunnel posts in response to the keywords ‘lift’ and ‘fin’.
Hey DJ craftee and Halcyon
OK some points to clear up first, sorry I rushed into the last post really quickly cause of the excitement of some of the stuff I’m looking at in my paper.
Anyhoo…
I ran the water at 6m/s pass the fin, straight on, and had a quick look at the image for total pressure distribution over the fin looking from the top, so seeing the cross section of the fin, with one side flat and one side foiled (this is a side fin btw).
What I found was there was two low pressures, one each side of the fin, about half way on either side… so in the whole 10 mins this thing took me, and not really thinking about what I saw, It just seemed to me that on the top (foiled) side, the low pressure should be out of line with a low pressure on the bottom of the fin.
I will do more simulations, but again, they are just simulations because I have no water tank facilities at my uni.
Also Halc, if I got my conversion right, 25 knots is around 12 m/s or 46 km/hr… is that a bit fast for a fin? What I wanted to ask you guys or anyone who would have a rough estimate, is how fast do you think the speed of the water is passing the fin?
From a paper I’ve read it claimed " 5 m/s for a one metre wave, and 9m/s for a 3 metre wave", so I took a rough guess of 6… Does anyone know ?
And I guess I used plane wings as an example, because to me I think there is definately similarities in theory, but i think they should be treated differently in analysis, so when we use common wing examples, it might not always be the same for a fin… Who knows maybe at the end of the paper I’ll change my opinion.
Yer craftee, I’ve got about another 2 months to finish the paper, so by the end I’ll have a lot more info. I don’t claim its all right, just one opinion from an undergrad.
Cheers!
Ant
Surfboard fins are too complicated for anyone to understand so we should all give up now. Besides all the templates that are ever going to be made are already made so why bother even talking about it?
But the one area that really needs to be explored scientifically of course is FIN COLOR and the effect FIN COLOR has on performance, because performance is the most important thing! And pros are the only high performance surfers and they surf with very colorful fins. So since their fins have practically no foil and they are all pretty much the same template they must be deriving their performance advantage from the FIN COLOR!
This is the most important issue concerning fins! Especially to the buying public. They don’t want to hear jack about template or foil. Rake or AOA or toe or setback or cant or positioning or moveability. Everyone knows that is all witchcraft and pseudo science. What they want is real performance they can see! And everyone knows performance is only related to FIN COLOR. That is all that reeeeally matters.
I know a sneaky old kneeboarder who made an art out of meaasuring his speeds. 17-20 knots is ballpark for an avid surfer up to overhead conditions. That would be maximum speed in an entire session.
I think the trickiest thing about fin templates, foils, and flex, is that the fin aspect ratio is so short, typically close to 1, that classic foil theory is a really bad approximation. Yet, it is applied nearly universally. Anytime you turn there will be a huge tip vortice that will have a far larger impact than moving the foiling wide point from 25 to 35%.
You can minimize that vortex, but there are negatives to that too, you lose hold. You get more hold from a fin tip than from a fin base, and more drive from the base. So, to increase hold, you enlarge the tip until your hold is satisfactory, then you lengthen the base chord until drive is satisfactory, and all of a sudden your fin template looks like a G5 anyway!
A lot of smart people have done a lot of fin testing to give us the empirical database we have today. There is a reason for the typicaly fin rake, thickness, aspect ratio, foil, and tip, and although it came from a dolphin initially, it was refined by surfers who felt it under their feet. I was making a fin or two a week and reading NASA papers on low aspect ratio wings for a while. The papers sort of suggested directions to take things, but the testing under the feet is the only way to get a truly satisfying answer.