shouldn't fins have symmetrical cross sections?

[pmcaero]

On my BIC Minimal, after breaking a side fin, I noticed that its cross section is in the shape of a non-symmetrical airfoil. This means it generates a side force as it moves through the water. I already changed the setup to a twin by moving the center fin, but this seems will give me some side-force and turning moment.
So, I can only ride it as a single fin,or buy new fins? Bummer!

The other issue seems to be that, the way the tri-finis laid out, if you lean on one side, causing one fin to be partly out of the water, the other fin will work against your turn , since the pivot point, you, is in front of the fins.

[jaffa1949]

OK BIC minimals and multi fin systems, centre fins usually have a symmetrical foil, the side bites and others outside of the centre fin usually have a single sided foil with the symmetry being provided by the opposing fin on the other side. There are also fins with altered foils and different cant and toe in and changes in the fin at various points in the depthof the fin, there are also difference in rake and response as to where you put the centre fin in a long fin box.
BICs and a lot of other generic type boards put in just a basic set, refinement is not really needed.
For further fin information try this .
viewtopic.php?f=9&t=16523

There are no real shoulds other than the board floating a little, in surfboard design, only what floats your boat at the stage you are at.
here are boards that are asymmetrically for different breaks and a whole lot of interesting stuff elsewhere.

[waikikikichan]

Who knows, you might like the feels of the current set-up. I had a friend who said she had the best session ever. Later we found out she had put her 9" fin on her longboard facing the wrong way in a rush by mistake. I had friends "sabotage" my board, surfed for hours and didn't notice until I rinsed off the board at the showers.

free4.JPG (90.71 KiB) Viewed 1426 times

[pmcaero]

ok, well, the only advantage I can see to side fin lift in the outer direction is that it would create a stabilizing force when you dig a rail, pushing it back up.
for turning, when you are standing up, the fins are fairly close to the pivot point (you) to not make much of a difference when loaded in any way.
now, in my situation, the paddle out will be annoying because the asymmetric lift force will create a more significant turning moment.
I'll try to glue the sheared fin tabs back on and reinstall.
Has anyone successfully glued the tabs back on?

[jaffa1949]

The tabs on the fin are designed as break points so they break before excessive damaging force is applied to the structure of the board. Buy a new fin or get rid of the side bites

A lot of people would only notice the increase in paddling drag, with multi fins, and very few would notice a side drag in paddling out. So if you are aware of that go the single!

Thought a little more about this answer
Multi-tailed planes and even the main wings are not symmetrical, as the curved surface is about lift and most aircraft wings now have an element of flex as well.
BTW you are not the pivot point, you are not even in a wetted area, changing position and applying weighted forces to different parts of the board rather than just leaning drives a harder turn, pivoting on the fins much like high speed jet turns, there is also the interactive contact of the rail with the wave face. Certainly the fin not in the wave face is not engaged but heavier pressure on the rear foot over the engaged fin and rotational direction with the front foot makes the turn up the face much more acute. Vice is versa for a top turn, pros use both the hold of the fins and also that moment of release as part of their functional turning skills
Roy Stewart could give you a thorough briefing on foil and fin characteristics at the engineering level.

For most surfers, I would suggest, it is about what works best for them.

[pmcaero]

Hi Jaffa
Thank you for the thoughtful response. In order to determine a rotation moment, all forces times their moment arms are summed up around the center of gravity of the object. (I'm using a simplification here, see below). Of course, your board is not level and, like you well pointed out, the fin can also interact with the wave's face, which will give it some bite.
All I know is aircraft so I am looking at the board as an aircraft, and through that assumption I stand by my statement that a fin with some camber will not be much more effective than a symmetric fin for turning. No matter what the surfer tries, an asymmetrical fin foil won't generate a significant turning moment unless the surfer is way forward.
Here's how I'm looking at the problem, my background is mostly in aerodynamics and fully immersed bodies, so I might be missing something, I just look at the board as an aircraft on it's side in this case.

http://en.wikipedia.org/wiki/Longitudin ... _stability

[waikikikichan]

Surfing isn't so much aerodynamics, and not even very much hydrodynamics. It's riding the waves. Duke Kahanamoku said " ride the wave, not the board". I had a professor of hydrodynamics and water laminar flow tell me all about surfboards and designs. But when I saw him out in the ocean, he couldn't surf, ( couldn't even sit on the board ). Learn the ocean, learn the wave and it wont matter how many fins or what shape they are.
Study this photo of Taylor Knox riding a Campbell brother's Bonzer. See how the center fin and two of the side fins are clearly out of the water. How can he still be holding traction with only the two little wings engaged ? He is turning by using the rail.

[drowningbitbybit]

Learn the ocean, learn the wave and it wont matter how many fins or what shape they are.

The most sensible thing I've read about fins for a long time.

[jaffa1949]

I though I'd give a Zen moment rule of thumb about surfing and why it is so beloved by those that get hooked.( I've been hooked since 1958. So last century )

Like Zen and martial arts, if you think about it in the action, you miss the moment.
Surfing is lots of fine neurons in the finest highest multifunctional random adaptive computer.
They if you let them, function faster than you can generate a cognitive thought.
GO SURFING let the SURF TRAIN you in the ways of the sea

Now some history, before boards there were bodies, by feeling the forces with your body and engaging proprioreceptors to link in one synapse to fine muscle nerves, shape your body to ride across the wave, like a dolphin for endorphins.
Next comes the board no fins.
Engage the rail to ride the wave, feel the subtle changes through your body shift your centre of gravity and load and unload pressure through your body to guide your board.
Receive endorphins and release of worry in pleasure.
Put fins on board, increase range of area on wave that can be ridden.
Repeat endorphins.

Surfboards are not aircraft, there is no constant force to drive them to a speed where they generate lift. The positioning of a surfboard on a wave is not the straight line application of Bernoulli's Principle.
As in lift generated by pushing something forward causing air to move over a curved surface.

Surfing is about, above all, sensing and utilising the subtle vectors between gravity drawing you down and generating speed, and the wave drawing you up as it water rises to break.
All this controlled in the moment, by single synapse body memory and response ( which you can enhance by training).

It is a dance in salute to energy generated by the sun

If you want to examine the forces in surfing, I am happy to recommend a book sold here through the shop ( I hope). The Surfboard Book by Sean McCagh
It was written by and engineer who surfs and he has had conversations with many shapers and their theories and chronicals them.
A Motto I gained from much experience Think Less Surf More

[pmcaero]

wow Jaffa, I thing YOU should write a book (and if you did already, please send me a link)

I finally went in today with the twin setup, the center replacing an outer fin, and I did not notice any adverse effects.
Granted, it was not clean conditions.

I'll definitely look that book up, The Surfboard Book.
thanks!

[jaffa1949]

wow Jaffa, I thing YOU should write a book (and if you did already, please send me a link)

I've written several books of children's poems and others based on Australian animals.
I'm writing a fantasy novel, and a couple of dissertations on Neurolinguistic Programing as part of my training.
I write some scripts for stage plays and theatre
No links yet, to share I'm also starting to storyboard videos i want to shoot and I am working to improve my craft there, as I am very much a beginner.
I freelance some surf articles

[waikikikichan]

pmcaero - " my background is mostly in aerodynamics ". When I was a kid, I remembered this Future Fighter plane. It was based onthe F-16 but with forward swept wings.

aageneral_dynamics_F-16_SFW_swept_forward_wing_1_big.jpg (48 KiB) Viewed 1341 times

Said it could turn a tighter arc than any other craft. What is your take on that design ?

aa158cwwwopy.jpg (37.83 KiB) Viewed 1341 times

this is a newer F-17 sfw

Do you think that could translate into surfboard fin design ?

halo.jpg (45.17 KiB) Viewed 1341 times

This the Donald Takayama Halo fin. Although not forward swept, it has a straight leading edge with the curve at the back. Kind of reverse of standard fins. That is one of the only fins I haven't been able to try out.

[jaffa1949]

I found a link on Aerofoils , they are not surfboard fin foils although there is some awareness of them out there in the surfing world

https://www.flyingfoam.com/content/what ... ld-i-use-0

The plane of the foils on the plane is plainly different, to the plane of the foil of the fins on board a board, the fin is more a tail plane, and other dynamics like rails and tails and weight placement before, during and after turns have their part to play.

I thought about it quite a lot since my last post,and one thing aircraft don't have is a heavier mass with comparatively huge body leverage, outside the boards centre of gravity applying force and mass to effect a turn. Aircraft effect turns by changing their foil surfaces, usually at the trailing edge quite a difference in technique.
A simple lean turn is highly ineffective and once the centre of gravity shifts beyond a certain point a wipeout ensues.
Highly effective turns require the skill of shifting weight and applying muscle power and varying throughout the turn.
Turning of course is in response to the wave and if the skill is there (like a martial artist) faster than thought.