# A Modest Epiphany; Fins are Energy Transducers

Years ago I used to think that fins were mostly hold, control and drag. Fin drag being evil.

But now I realize its analogous to a race car.

The board is the chassis and suspension…fins are the engine.

The engine’s fuel is the conversion of energy; from wave, gravity and surfer.

Car engines have power to weight ratios. Fin setups have power to drag ratios. A powerfull fin setup can easily overcome drag. Materials have a significant affect on how power is utilized and how efficiently energy is converted.

So thats my little epiphany. I usually have a couple of them each year

Btw, the source of this is not from theorizing on mind or paper, its from in-water trials. When your revised board makes you go “WOW!” you know youre on to something. The epiphany may be modest, but the results in the water are not.

can you tell us about your revised board? maybe a pic or two? thx

Wow craftee,

You got me interested chipfish stylee!!

How do the fins and the board look??

Yunno, photo’s!!?

Thanks Soul

BINGO !

Transducers convert energy from one from to another. i.e. Mechanical energy to electrical energy (microphone) or electrical energy to mechanical energy (speaker).

IMHO, the primary function of fins is to provide directional control. They do this because they provide a path of least resistance.

A surfer can transfer their weight and change the direction of the path of least resistance, and turn.

I don’t understand how this is energy transduction.

They (help) convert gravitational energy and wave energy and surfer energy into lateral energy, vertical energy, and stoke.

Viola!

AMEN

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They (help) convert gravitational energy and wave energy and surfer energy into lateral energy, vertical energy, and stoke.

Viola!

Exactly, they do this by providing a path of least resistance, not by acting as a transducer.

Kind of like how skateboard wheels only spin on one axis, providing a path of least resistance, when combined with trucks the angle of this axis can be changed providing directional control. If the wheels could spin on every axis it would not be possible to have this directional control.

Okay, then your job is to reconcile the below with your analogy:

Overview

Technically, lift is the sum of all the fluid dynamic forces on a body perpendicular to the direction of the external flow approaching that body. The mathematical equations describing lift have been well established since the Wright Brothers experimentally determined a reasonably precise value for the “Smeaton coefficient” more than 100 years ago,[2] but the practical explanation of what those equations mean is still controversial, with persistent misinformation and pervasive misunderstanding.[3]

Sometimes the term dynamic lift or dynamic lifting force is used for the perpendicular force resulting from motion of the body in the fluid, as in an aerodyne, in contrast to the static lifting force resulting from buoyancy, as in an aerostat. Lift is commonly associated with the wing of an aircraft. However there are many other examples of lift such as propellers on both aircraft and boats, rotors on helicopters, sails and keels on sailboats, hydrofoils, wings on auto racing cars, and wind turbines. While the common meaning of the term “lift” suggests an upward action, the lift force is not necessarily directed up with respect to gravity.

Physical explanation

Lift is generated when an object turns a fluid away from its direction of flow. When the object and fluid move relative to each other, the object turns the fluid flow in a direction perpendicular to that flow, and the force required to do this creates an equal and opposite force that is lift. The object may be moving through a stationary fluid, or the fluid may be flowing past a stationary object—the effect is the same in both cases.

http://en.wikipedia.org/wiki/Lift_%28force%29

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Okay, then your job is to reconcile the below with your analogy:

Overview

Technically, lift is the sum of all the fluid dynamic forces on a body perpendicular to the direction of the external flow approaching that body. The mathematical equations describing lift have been well established since the Wright Brothers experimentally determined a reasonably precise value for the “Smeaton coefficient” more than 100 years ago,[2] but the practical explanation of what those equations mean is still controversial, with persistent misinformation and pervasive misunderstanding.[3]

Sometimes the term dynamic lift or dynamic lifting force is used for the perpendicular force resulting from motion of the body in the fluid, as in an aerodyne, in contrast to the static lifting force resulting from buoyancy, as in an aerostat. Lift is commonly associated with the wing of an aircraft. However there are many other examples of lift such as propellers on both aircraft and boats, rotors on helicopters, sails and keels on sailboats, hydrofoils, wings on auto racing cars, and wind turbines. While the common meaning of the term “lift” suggests an upward action, the lift force is not necessarily directed up with respect to gravity.

Physical explanation

Lift is generated when an object turns a fluid away from its direction of flow. When the object and fluid move relative to each other, the object turns the fluid flow in a direction perpendicular to that flow, and the force required to do this creates an equal and opposite force that is lift. The object may be moving through a stationary fluid, or the fluid may be flowing past a stationary object—the effect is the same in both cases.

http://en.wikipedia.org/wiki/Lift_%28force%29

What I am talking about is the primary function of the fin. While I do think that it is possible for fins to create “lift”, I don’t understand how this lift, which is perpendicular to the forward motion of the board (because fins are placed parallel, or close to parallel, to the direction of travel) contributes significantly to the forward motion of the surfer and surfboard.

Craftree said that the “fins are the engine”, but I think a better car/surfing analogy is that the fins are the wheels and suspension. They provide directional control. You can only travel in the direction that they spin.

It seems to me that the surfboards “engine” is gravity and the forward motion of the wave (which is transfered to the surfboard, moving the surfer and board along with the wave).

I by no means am trying to say that I am completely correct, as I am still trying to figure out how surfboards work, but this is what logic and my 20 years of surfing experience are telling me.

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on On On. … . s’it aliov ton aloiv !

Boy silly !

Fiddle.

2 planes of lift: the bottom and the fin’s lifting side–the bottom’s lift is dominant/primary, the fin’s lift helps the bottom (the rail, really) stay in its track(s). You have to understand that the lift of the bottom is primary and then you can see how the function of a foil (the fin, especially a rail fin) enhances your control of the bottom ad rail (AKA your surfboard).

It is possible to surf without a fin, in that the bottom can be made to deflect you laterally down the line, with a suction rail, a hard down rail, a concave bottom, there are several different entry configurations, but the fin(s) make it so much easier to keep track and moderate the rail’s release. Without the fin, your tailward rail will want to release right out of the wave face. Once that release is moderated with a fin and you “understand” it (physically speaking), you can play the board against the fin(s) to turn and whatnot.

Surfing is the combined dynamics of these deflections.

Interesting analogy. So how come finless craft like mats and Alaia are so fast and when properly turned and trimmed have amazing acceleration? No engine?

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2 planes of lift: the bottom and the fin’s lifting side–the bottom’s lift is dominant/primary, the fin’s lift helps the bottom (the rail, really) stay in its track(s). You have to understand that the lift of the bottom is primary and then you can see how the function of a foil (the fin, especially a rail fin) enhances your control of the bottom ad rail (AKA your surfboard).

It is possible to surf without a fin, in that the bottom can be made to deflect you laterally down the line, with a suction rail, a hard down rail, a concave bottom, there are several different entry configurations, but the fin(s) make it so much easier to keep track and moderate the rail’s release. Without the fin, your tailward rail will want to release right out of the wave face. Once that release is moderated with a fin and you “understand” it (physically speaking), you can play the board against the fin(s) to turn and whatnot.

Surfing is the combined dynamics of these deflections.

I agree, for the most part.

I think that the lift on the bottom that you are talking about is better described as buoyancy. Or are you talking about a different type of lift?

I understand how the asymmetrical foil of a fin can provide “lift” and suck your board into the face of the wave, giving better stability and control. But boards with fins with symmetrical foils work good too.

My primary point and dispute with the original post is that it is not the fins that are propelling your forward. They are, for the most part, allowing you to control the direction of your forward motion.

(if this seems pedantic, forgive the style)

The lift of the planing surfboard isn’t buoyancy. Lift requires a dynamic flow. Buoyancy is a static factor. I think I infer a Bernoullian concept of lift in your post, which is in error. In fact lifting foils are now understood to lift given a Newtonian deflection on the positive AOA side and (Coanda) attached flow resulting in downwash on the non-AOA side converging to redirect the flows “downward”, which is lift in the aggregate. It’s not path of least resistance to flow, it’s actually definitively a deflection of the flow. Same goes for the bottom once it’s planing. There’s wash off the bottom and off the fins. They’re doing work. There’s drag. The fin(s) enhance your ability to control the release of the edge (rail) of the bottom out of the surface of the wave.

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(if this seems pedantic, forgive the style)

The lift of the planing surfboard isn’t buoyancy. Lift requires a dynamic flow. Buoyancy is a static factor. I think I infer a Bernoullian concept of lift in your post, which is in error. In fact lifting foils are now understood to lift given a Newtonian deflection on the positive AOA side and (Coanda) attached flow resulting in downwash on the non-AOA side converging to redirect the flows “downward”, which is lift in the aggregate. It’s not path of least resistance to flow, it’s actually definitively a deflection of the flow. Same goes for the bottom once it’s planing. There’s wash off the bottom and off the fins. They’re doing work. There’s drag. The fin(s) enhance your ability to control the release of the edge (rail) of the bottom out of the surface of the wave.

It is not pedantic because I am still trying to understand this stuff…

I think you are trying to say that the fins change the direction of the flow of the water over the fin, and this creates lift. But what I don’t understand is how this translates to forward motion.

I understand that as the surfer modifies the balance of his weight on his surfboard and puts the board on the rail (for example in a bottom turn) there is added pressure on the inside surface of the rail fin. This is because the surfer has all of this momentum that was provided by gravity, and as you change the direction that the fins face the direction of this momentum is changed because the board wants to travel in the path of least resistance.

Maybe if you could explain how the lift created by the fin translates to forward momentum. And if you could explain if this fin lift is a greater force and momentum than the momentum provided by gravity. And do so in laymen terms (without Bernouilli, Newton, dynamic flows, coanda, etc.)

I like the analogy, however I think rails around the fins transfer energy also.

Hi Obproud,

A very simple explanation of a fin.

Take a board of wood,

try to push it sideways through the water,

you feel a lot of drag.

If you keep it angled a little bit you feel it wants to move sideways.

This is how a fin works,

it deflects the oncoming water to give sideways motion.

Same for the rails, they work like fins too.

And for Janklow: coandanewtontransducingdynimicallymovingtotheend…

Sorry bro, try to keep it simple.

Soul

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It is not pedantic because I am still trying to understand this stuff…

I think you are trying to say that the fins change the direction of the flow of the water over the fin, and this creates lift. But what I don’t understand is how this translates to forward motion.

You are one good bottom turn away from understanding.

http://www.blakestah.com/fins/truckexplain.html

It seems as if some people think Im being nominated for Nobel prize in physics for my thoughts. Sorry to dissappoint, but Im stating an OPINION on a open surfboard forum. I dont need to substantiate anything relating to my opinion and you are free to ignore it. But its cool that Bill and Surfercross like my opinion. Smart guys

But hey I’ll tell you guys what Ive done. Ive been riding a Griffin5 fins style setup with cheap plastic molded fins. I replaced the rear three with custom made G10. If you had blindfolded me and told me I was riding a new faster better board I would believe you. What I felt was a more efficient use of energy with very little loss, as if I had installed a more powerfull engine, turbocharged. And all this from simply switching fin materials. The best part for me is that this is just the tip of the iceberg. Stoked!

Btw, Im just a student.

Anyone who is seriously interested, should buy a board from Mr. Greg Griffin.

Griffinsurfboards.com

Onward…