Fin design questionnaire for all you experts

General Discussion
1

Hi everybody,

I’m writing a story about fin design for a campus science magazine at UC Santa Cruz. What I want to do is to understand the hydrodynamics that make fins work. Then the next step will be to understand what the modifications to a basic fin accomplish. I’ve spoken with a variety of fin designers, salesmen, and surfers and received a range of answers with varying scientific plausibility. So I thought I’d ask you guys, who have created an actual (or virtual anyway) forum on the topic.

Please answer as many or as few of these questions as you like. Use as many words as you like, but don’t feel compelled to write theses… By the way, for simplicity, let’s limit the discussion to shortboards with thruster setup.

OK, here goes:

Years surfing?

Physics/Engineering background?

Have you ever swapped out the fins on a board and actually felt a difference?

Do you think a performance gap exists in surfing that a new fin design could fill?

How does lift help a surfboard?

In what direction does the lift vector point?

What is drive and where does it come from?

What is projection and how does it differ from drive?

What is the significance of foil?

Does a double-sided foil generate any lift?

On a single-foil fin, what is gained by making the trailing edge slightly concave?

When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? How does fin design, esp. shape, affect the change in speed?

Many thanks in advance for taking the time to contribute. Come to Santa Cruz with some graph paper and I’ll buy you a beer.

Hugh

p.s. If you want to be all scientific and double-blind about this, don’t read anyone’s answers until you’ve posted.

2

hey thats to many questions to answer and i probably dont have all the answers but if you click the search for box and type in anything to do with fins you will probably get some answers.

3

Years surfing? Many

Physics/Engineering background? 30 years as a Plumber, No formal training in either however.

Have you ever swapped out the fins on a board and actually felt a difference? Yes.

Do you think a performance gap exists in surfing that a new fin design could fill? That was proven many years ago when Nat Young got George Greenough to foil his fins when he won the world championship. Nothing has changed.

How does lift help a surfboard? I generates speed while turning, which allows the surfer seek any part of the wave face. The lift created buy rail fins has given surfers the ability to drive off of any part of the wave face and climp it very aggressively.

In what direction does the lift vector point? The exact lift vector is in constant flux as the the wave face and board attitude changes. A superior surfer is constantly adjusting this so called vector to accomodate the power in the wave.

What is drive and where does it come from? Many places: A surfer’s muscularity, attitude, balance and wave sense along with from the foil of the board and fins.

What is projection and how does it differ from drive? Without drive you cannot have projection. Drive can come from many places but projection can only come from the force of the wave and a shape to carry it on. Drive is the force. Projection is the extended path that the drive delivers one to.

What is the significance of foil? Everything: Foil is what determines how a surfboard and fin perform.

Does a double-sided foil generate any lift? Yes, The surface behind the vertical cord of the fin produces lift when it is pressurized during turning.

On a single-foil fin, what is gained by making the trailing edge slightly concave? The trailing edge of a fin is simply the line that is formed by it as you look at the fin profile. There is far too much going on fin dynamics to isolate this single element’s effect on fin performance. But all other things being equal the more concave you make the trailing edge the weaker you make the fin because you take more profile from the fin as you add more concave.

When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? From the surfer delivering his/her muscularity and balance changes to the board and fin foils along with wave wall steepness, the spinning motion of the breaking wave.

How does fin design, esp. shape, affect the change in speed?

This question is far to general to answer succinctly. One could write a book trying to answer it.

Mahalo, Rich

4

Years surfing? 26

Physics/Engineering background? Yes

Have you ever swapped out the fins on a board and actually felt a difference? HELL Yes…its profound

Do you think a performance gap exists in surfing that a new fin design could fill? Oh yes

How does lift help a surfboard? Lift reduces drag…pressure and frictional

In what direction does the lift vector point? ha…generally only one, perpendicular to the planeing surface…freebody diagram is the answer…you can make basic assumptions and simplify the diagram to get only one direction

What is drive and where does it come from? Halcyon’s answer works for me but I’d also add that one of the primary resources to the surfer/board is the bank of water under the board…multiple fins hold you in while youre banking…when I was a kid I could drive my finless skimboard quite well

What is projection and how does it differ from drive? this could have different definitions…the length or extension of your desired path

What is the significance of foil? Very…generally, it has been used to generate lift on wings…it also streamlines, helps deal with angle of attack, reduces wake and induced drag

Does a double-sided foil generate any lift? At zero AOA, generally, only if its assymetrical, symmetrical foils cancel lift vectors out…at other than zero AOA, just about anything, including a no foil section can create lift from pressure differential…a foil creates the differential P at zero AOA.

On a single-foil fin, what is gained by making the trailing edge slightly concave? Beats me…could be marketing hype…there’s so much of it it could make your head spin

When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? How does fin design, esp. shape, affect the change in speed? Uh, gotta disagree with this one…it ‘feels’ like youre speeding up…basic energy conservation…potential energy is max at the top/start, converted to accel/velocity at the bottom…when you bottom turn G forces increase and you ‘feel’ it

Beer in the mail?

Basics Bible: http://www.vacantisw.com/foildesign.htm

5

Years surfing? …35…

Physics/Engineering background? …no formal degrees , they just complicate your natural intuitive thought processes , 25 years of board building and making custom fins …

Have you ever swapped out the fins on a board and actually felt a difference?..yep…

Do you think a performance gap exists in surfing that a new fin design could fill? there will always be gaps in gaps , every surfer can do with a fine tuned customised fin set up thats adapted for there specific need …

How does lift help a surfboard? …by giving your board hold , quick direction changes ,inceased sensitivity …

In what direction does the lift vector point? … as a general rule 90 degrees to foil , or perpendicular to the imaginary line that runs from the leading edge to the trailing edge for cambered foils …

What is drive and where does it come from?..drive comes from your side fins and tail fin working as a package , your side fin wants to pull one way when you lean on it , the tail fin resists that pull and translates some of that force into forward movement …

What is projection and how does it differ from drive? … drive and projection are closely related , probably two words describing the same thing , rocker and outline can create drive as well , but just speaking fins , if you set your tail fin in such a way as to create more resistence to the board turning freely (moving tail fin back , or making it bigger )then that resistence creates a stiffness but also a corresponding increase in drive …

What is the significance of foil? … no foil no lift , some lift can be achieved without foil , but will lack control …

Does a double-sided foil generate any lift? … yes , any foil will get lift from angle of attack …

On a single-foil fin, what is gained by making the trailing edge slightly concave? … you reduce the pressure difference on either side of the foil , thus desensitizing the fin , bad for small waves good for big waves …

When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? How does fin design, esp. shape, affect the change in speed? …you turn the gravitational or rider energy into something you can control and use to change direction …without fins there is no bottom turn . fins add control , fins also add drag , well foiled fins give good control with minimal drag …if your traveling in an arc and then that arc is tightened , it gives the sensation of an increase in speed , with out making it to complicated . if you get a pole , tie a rope to it , then a weight to the end of the rope , now swing the weight so the rope wraps around the pole and the rope shortens , see how the weight appears to be speeding up , its air speed doesnt change , it goes the same speed over a shorter distance or arc …

with good foils you dont lose speed through a turn when you have angle of attack ,so you keep your speed through a tighter arc , if you could measure the strain on the rope as the weight moved around the pole you would get an increase in g force , thats what were feeling as increased speed in bottom turns …we dont physically speed up , even tho it feels like we do , when we add rider input to the free energy from the wave , we then generate extra speed and go faster than the wave …better foils make that easier…

regards

BERT

6
Quote:
Years surfing?

30

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Physics/Engineering background?

yes

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Have you ever swapped out the fins on a board and actually felt a difference?

many times

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Do you think a performance gap exists in surfing that a new fin design could fill?

I believe new fin designs can improve performance.

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How does lift help a surfboard?

This is somewhat ambiguous. Lift, in the hydrodynamic sense, is a vector perpendicular to flow. Fins can create lift that is vertical or horizontal relative to the surfboard bottom.

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In what direction does the lift vector point?

Lift is perpendicular to flow.

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What is drive and where does it come from?

I believe the term drive is somewhat poorly defined, generally referring to the ability of a board to take the speed going into a bottom turn and convert it to speed down the line, or back into the wave.

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What is projection and how does it differ from drive?

Projection is the path drive takes you on.

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What is the significance of foil?

Foils are studied for lift and drag coefficients at different angles of attack. Its about getting more drive and projection with less drag.

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Does a double-sided foil generate any lift?

ALL foils generate lift at significant enough angles of attack. A good double foiled fin will generate lift at any angle of attack.

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On a single-foil fin, what is gained by making the trailing edge slightly concave?

Other than thinking about fins in terms of lift and drag, you can also think about them as water re-directors. A concave rear fin edge will re-direct water MORE than a flat single foiled fin. It gains a few degrees effective angle of attack. It stalls more easily at negative angles of attack. It probably is optimized at different angles of attack than a fin without a concave rear edge.

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When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? How does fin design, esp. shape, affect the change in speed?

This is the magic question. Except, you don’t really speed up, the speed is supplied by gravity, and the success at converting “speed towards shore” into “speed through a turn” is the major design criteria for fins in performance surfing. The radius of turning allowed by fins is the other variable. As in, if you really know the answer to this question, there are a few million surfers interested in the answer.

7
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Physics/Engineering background? …no formal degrees , they just complicate your natural intuitive thought processes

OUCH! …that hurt Bert, one does not have to have an engineering degree to be an engineer…its just a formalized label…how bout “intuitively gifted technologist”…is that better?

Hmmmm…the long line of innovative creative formal ‘engineers’ is mighty long…

Some are button pushers…some are innovators…leaders vs followers…it parallels the general population…I’d say 90% of the general population are sheep (the general employers like that)…they become the button pushing engineers…Bill Gates dropped out of Harvard but he was also training to be another formalized label…he could of easily been a visionary with a formal label…the degree does not make the person.

b4 earning my degree I was doing mechanical/technical gearhead bluecollar work…the degree didnt make my mind worse, it made it better…what one labels as complex, another labels as robust, indisputible, mathematical proof.

We all have our strengths and weaknesses…Im glad you have enjoyed/capitalized on your strength…I think you would have made a wonderful engineer…but then again you already are

8

Years surfing? 28 years on boards, many more body surfing and mats.

Physics/Engineering background? 6 years head composite technician and workshop manager in a prototyping workshop for specialised aeronautical engineering company. Did not finish high school, still graduating the university of life.

Have you ever swapped out the fins on a board and actually felt a difference? Many times with many fins on many boards.

Do you think a performance gap exists in surfing that a new fin design could fill? Yes.

How does lift help a surfboard? Keeps it afloat while in motion. The variation in lift due to the variation in board and fin shape accomodates the variation in surfer size, weight and ability.

In what direction does the lift vector point? Opposite to the direction of force applied to create that lift. The lift vector infinitely changes due to variation of angles of attack of a surfboard combined with surfer weight and ability, not to mention the wave size and shape.

What is drive and where does it come from? The lift pressure under the board combined with gravity pressure of the rider combined with the motion of the wave. A well designed board and fins will take advantage of all three and accellerate while keeping controlled contact with the water due to good board and fin design.

What is projection and how does it differ from drive? Projection is the speed and direction the drive gives you.

What is the significance of foil? Plenty. The abilty to take advantage of speed and pressure with no loss of control.

Does a double-sided foil generate any lift? Yes, even in a straight line.

On a single-foil fin, what is gained by making the trailing edge slightly concave? Creates lift under pressure, on any fin.

When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? Pressure combination of rider gravity, water and wave motion. The three forces combined force mass forward. Basic physics. How much speed you actually gain depends greatly on board and fin design, and of course rider abilty.

How does fin design, esp. shape, affect the change in speed? Fin shape affects the amount of lateral fin resistance under pressure, the foil then contributing to the smooth flow of water allowing speed.

The company I worked for employed dozens of aeronautcal engineers who spent years studying etc, but only two or three actually designed the planes, and even then they didn’t design the airfoils. Nearly all were interested by my surfboards and fins, and nearly all their opinions confirmed what I had learned through trial and error over the years. The fine tuning I learned from them did make a difference, and made me realise that the many small and subtle intricacies, often overlooked, all contribute to better performance.

9

Years surfing? 37

Physics/Engineering background? 2 years undergrad engineering at Loyola Marymount, Finished with an Econ major UCSD and Math minor while running a surf shop. Worked with Rusty on board design development in early 80’s. Ran a race prep boatyard in the 90’s and work with most of the top naval architechs in sailboat racing. Now I design custom plastic injection molds to make Red X fins, and many other surf and medical related parts

Have you ever swapped out the fins on a board and actually felt a difference? Now that’s a pretty silly question. Not only swapping. But, just moving them by as little as a 1/16" can make noticable differences.

Do you think a performance gap exists in surfing that a new fin design could fill? Surfboard design is constantly evolving. Some periods create more dramatic changes than others. Fins can be used to compliment certain design elements or molify them. Lower drag with greater control is the direction I’m focusing on right now.

How does lift help a surfboard? Surfboard lift assists in planing which yields speed. Fin lift pulls the rail into the wave and assists in control.

In what direction does the lift vector point? Which lift vector? There are actually a matrix of lift vectors that work together to make a surfboard work and the are constantly changing. But, there are averages and I discussed their direction in the previuos question.

What is drive and where does it come from? Drive comes from the ability to redirect the momemtum of surfer and craft relative to the flow of water around them with the most efficiency.

What is projection and how does it differ from drive? Projection is the follow through after drive They go hand in hand. With out good drive it’s pretty damn hard to project.

What is the significance of foil? Foil is surface curvature that is utilized to influence flow.

Does a double-sided foil generate any lift? There are time when double sided foils are lift neutral. But, if you change the AOA lift is generated.

On a single-foil fin, what is gained by making the trailing edge slightly concave? I’m waiting for someone to give me a plausible explaination for that as well.

When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? Actually, the sensation of increased speed is actually the torque of redirecting the flow you generated during your drop. Fin lift pulls the rail into the wave and assists in control.

How does fin design, esp. shape, affect the change in speed?There are different foil curvatures that optimum for lower flow rates and greater changes in AOA. These tend to be fatter and fuller with more assymetry side to side and thickness forward. The other extreme are intended for high flow rates and smaller changes in AOA. These are shallower curves, max curve further aft and thinner foils.

10

years surfing:3 but studying hydrodynamic design since i was ten so six years of gobbling down design articles and bodyboardinbg for life

engineering: middle school physics :confused: o and a n ight in civil air patrol wher i read their packet on fluid dynmaics

have i ever swapped fins on a board and felt a differerence yes a stiffer fin in my long board egg makes it rided more like a short board and a flexy fin makes it feel more like a long board.

does a performance gap exist yeah

lift helps a surfboard by vcresating a drivew my canting a fin in then the drive is directed a bit more foward and opposing. vertical lift a hyrdofloil is interesting and i would like to read more . the fcs red tips wsith the tiny hydro foil was my idea whren i first learned of the hydro foils being riddden at jaws.

huh?

drive is a lift force created by the fins based on bernoulli(spelling) principle read the lift statement

foil affects the flow of the water and the way the water flows off it

a double foil generates lift(look at a fighter jet wing if you doubt this.

while i have yet to test it i believe based on what i have read is that it reduces turbulence allowing for greater speed bbeing generated and thde lift force being directed forward.

the speed comes from the a) the drop b) the water pressing againstthe board and the board acting being pressed back up the wave with the water

fin design affects all aspects of the way a board handles smaller surface area the board can slide a bit more more surface area more resistance rake determines the way the board flows in a turn

11

sorry meecrafty , man im racking up the dumb comments this week …

seems if its not design related , i only open my mouth to change feet …

regards

BERT

12

Years surfing? 27

Physics/Engineering background? No degrees but 15 years experience building boards with some of the best

Have you ever swapped out the fins on a board and actually felt a difference? This is an ongoing learning experience for me, but the differences are not only noticible but quite suprising. I’ve learned it’s best to simplify it when talking about the pros and cons. More rake, more hold and control. Less rake more pivot. Low drag foils for high speeds, fuller foils for more lift at lower speeds.

Do you think a performance gap exists in surfing that a new fin design could fill? I think that different level surfers could benefit from different foils, but first it would have to be clearly identifiable as to which foil you need for your level, or even your “spot”. Stay tuned…

How does lift help a surfboard? Without lift a surfboard would ride “dead”.

In what direction does the lift vector point? This is fairly vague question. Agreed that this a constantly changing medium we work with, this could be a discussion all on it’s own. But something that seems to never be discussed is as you double a fins respective lift, you quadruple it’s drag. Some call it control.

What is drive and where does it come from? Drive is a combination of several different factors. Your weight falling down the face, your rail engaging transfering gravity into lateral motion, the fin/s engaging to follow you through this transition, your body weight shifts, as well as efficiency of rocker, fin foil and shape, as well as the speed the wave moves towards the beach. Probably left out a few.

What is projection and how does it differ from drive? See above

What is the significance of foil? Foil is critical. Full foils add control but also drag. Not what you want at 50 mph. See reply to question 3

Does a double-sided foil generate any lift? Yes. The lift of a double foiled fin can be increased by how it placed on the board. More angle would increase lift on a flat plane. Of course no board rides flat always, so measuring the lift on boards is extremely difficult. The wave is constantly changing.

On a single-foil fin, what is gained by making the trailing edge slightly concave? It creates drag on the water flow on the flat side of the fin. This can also be called control. Most surfers think they need to go as fast as possible, but in reality there are certain situations where you may want to sacrifice a bit of speed for a bit more control(like 6 to 8 foot cloudbreak). However, this concave may actually cause you difficulties by overcontrolling your board once a certain hull speed is achieved. This speed is the part that is still undocumented. I’m guessing it’s around 10 to 15 foot faces. After that the foil may get a bit “erratic”. My experience has been that in surf where i’m trying to generate speed, a flat side fin seems more efficient. In surf where I have speed to burn, and need some control, the concaves or vectors may have an application. Jury is still out.

When you drop in on a wave and start your bottom turn, you speed up. Where does that speed come from? How does fin design, esp. shape, affect the change in speed? Again, you weight combined with gravity, fin foil, rail foil, and rocker all play a part.