The functions of Fins

General Discussion
1

Hi, this to some may be obvious, but I’m confused on the matter of fin functions and would be delighted with some feedback. I’m unclear on the function of fins. I imagine they have 3 primary functions…

  1. To stabilise a board and hold it in a wave. 

  2. To allow control of a board. 

  3. To create drive.

Ok, I think that maybe I can take it that my first two points are accurate, the third though, this is where I feel confused. So, do fins in fact create drive to help propel a board forward. If so, whats the mechanism by which this occurs? I’ve come across a lot on the web, a lot of dodgy explanations, but nothing really clarifies, so here follows a mix of what I’ve seen and my thoughts on it.

I’ve come across the idea that water passes around the bevelled side of fins more quickly than the flat side, causing low pressure which in terms of an airplane foil would create lift, this is the same with a surf foil I imagine. Since fins are generally not operating on a horizontal plane, indeed probably at most times an almost vertical plane (which varies as the board moves and turns on the wave), do they also create lift, on both sides of the board in terms of twin or thruster setup?

If this above is true, is lift the mechanism by which acceleration/drive is created, ie, back of the board is continually lifting (not leaving the wave as it continually lifts behind it) as the board is propelled through the water, first by paddling followed by the effect of gravity and momentum? In this sense the board is in a state of perpetual fall as long as there is wave to propel/power it?

I’ve noticed on Tom Wegeners site he mentions the fact that his large wooden fins, through their inherent bouyancy property, actually create additional lift in the tail of his boards, does this support the lift theory?

I’ve also wondered about the fact that water is compressed to a certain degree as it channels through the plane of fin space, as it releases, does this create drive and if so, is this great enough to compensate for the drag fins create so that there is a net gain?

Single fins…do they create drive? The fact that they present a larger plane on its reverse facing the oncoming wave, does it actually get pushed, if so, once again do we have a net gain in terms of drag produced?

Ok, so that’s what’s been on my mind, to someone in the know, I may be a million miles off, but it seems to me there is a lot of confusion in terms of what fins are actually doing and indeed what they may be capable of doing. I feel it’s an area of surfing really worth looking into in terms of possibilities for perfomance varitions.

Thanks,

LionFische.
2

If a fin is angled, toed-in, and the inside of the fin sees the water flow, the water creates forward thrust. See

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

Wegener is talking about vertical lift. Just like if you put something very buoyant underneath you, it “lifts” you up.

Drive REQUIRES an angle between the fin and the stringer. Single fins can create drive through fin flex, or by allowing the fin to rotate in toe-in at the finbox. But a stiff planar single fin will create no forward thrust.

Forget EVERYTHING YOU EVER LEARNED about fin foiling and its contributions to lift, those are really small higher-order intricacies in understanding fin function. Think about the fin as re-directing water so that it flows parallel to the fin, and the change in water momentum having reactive forces. For example, if water is flowing towards the rail, and the fin re-directs it to flow more towards the tail, then the reactive force points perpendicular to the fin orientation. If the fin is toed-in, a component of that is in the direction of motion ie: forward thrust.

3
Quote:

I’ve also wondered about the fact that water is compressed to a certain degree as it channels through the plane of fin space…

In most any design that does not involve extreme pressure or temperature, the compressibility of water is negligible.

4

hey Dave

Define “drive” as you’re using it. Toe-in isn’t definitively necessary for trimming drive or even pumping drive, as I understand the concept of drive to be a physical dynamic of the water/rail/bottom and water/fin interfaces which gives lateral progress down the line. Even pumping for speed. You don’t need toe for that. (You don’t even need fins–refer to the Jamie O video with no fins and various others on the non-thruster vid thread.) It’s the opposite of “sliding ass.”

If you’re using drive as a synonym for pumping off the rail fin face or the combined AOA arc dynamic between the center fin and rail fin, I think that’s a narrowing misnomer by dint of it describing an act that is possible given the presence of drive. I think drive is simply given by a surface or surfaces that are made to oppose water in trim, arcs, pumps, all. Again refer to the finless videos.

Toe-in enhances fin turn-in, on surfboard fins and race cars. It’s also a LOT like counter-steering a motorcycle.

BTW saw some Roy vids recently (Squidtail and another one) where he’s using big thick double-foiled non-toed vertical fins on a twin and single (with cans) and looks to be getting some really fast trim speeds on a high line, not just weight-speed off the drop. I was impressed. He actually did catch a guy from way back and the guy kicked out. Those crazy newfangled vertical fins are the way to go Roy. (I know he’ll be here soon)

5

LionFische,

  The reason you're having difficulty with the concept of creating drive is because the first two (I call them) fin affects are achieved by flow and angle of attack. The third fin affect occurs when the surfer uses kinetics to pump or swim his board through areas that he is lacking flow. To pump or swim a board a concave or flat inside surface acts like a swimmer cupping his hand to get more power out of his stroke coming out of the hole. But, those same characteristics create turbulence at higher flow rates and angles of attack. So, a concave or flat inside surface works well for #3 but is adverse for fin affects number 1 & 2. Flat is not as bad as concave. So, it's kind of the middle of the road solution. If you're surfing weak waves and won't be in trying to control a lot of power concave fins will give you the most benift. If you're looking for a set of fins that will work well across the broadest range, flat inside surfaces are will allow you to pump when you have to without setting up too much turbulence in more powerful situations. But, if you want to have great control with minimum drag in powerful conditions, convex inside surfaces work best.
6
Quote:

Forget EVERYTHING YOU EVER LEARNED about fin foiling and its contributions to lift, those are really small higher-order intricacies in understanding fin function. Think about the fin as re-directing water so that it flows parallel to the fin, and the change in water momentum having reactive forces. For example, if water is flowing towards the rail, and the fin re-directs it to flow more towards the tail, then the reactive force points perpendicular to the fin orientation. If the fin is toed-in, a component of that is in the direction of motion ie: forward thrust.

And, if I may, is exactly how sails help move (with the assitance of the keel) sailboats forward.

7

I’ll stimulate this conversation a bit- what do you guys think happens if you foil a single fin setup so that it is shaped like a side fin? (asymmetrically… or in technical terms- add camber to the foil) … -Carl

8
Quote:

I’ll stimulate this conversation a bit- what do you guys think happens if you foil a single fin setup so that it is shaped like a side fin? (asymmetrically… or in technical terms- add camber to the foil) … -Carl

Very little if you align the cambered (asymmetic) fin so that the zero lift angle lies parallel to the longitudinal axis of the board. For typical fins, this corresponds to a rotation of around 3 to 4 degrees negative AOA (for the asymmetric fin) relative to the symmetric fin.

As far as second order effects, the lift coefficient at stall will be slightly greater, and the AOA corresponding to minimum drag will be shifted to occur at a lift coefficient of about 0.2 to 0.4 (vs 0.0 for the symmetric fin). Of course the performance will be reduced (relative to the symmetric foil) if the foil is operated at a negative AOA that is less than the zero lift angle.

9

I just spent about an hour and one-half responding in detail to your post here (and the discussion at you referenced web site). Did a spell check prior to posting and IE locked up. I don’t have the time right now to redo the whole thing. If you want to talk about it over the phone, give me a call.

mtb

10

Trick question. It depends upon the amount and efficiency of the foil and probably the speed of the board through the water. If you do it right I would expect the async lift to overcome the async drag and the tail would be pulled towards the side with more convex foil. I wouldn’t expect a huge effect though.

11
Quote:

Drive REQUIRES an angle between the fin and the stringer. Single fins can create drive through fin flex, or by allowing the fin to rotate in toe-in at the finbox. But a stiff planar single fin will create no forward thrust.

Not true Blakestah, it is the angle between the fin and the rail which counts, not the angle between fin and stringer. . . single fins can create drive if there is an angle between the fin and the rail, (which there always is)

:slight_smile:

12

I’m no expert, but after talking to people, and reading about the subject (and experience surfing) here’s my opinion. Fin size, shape, quantity, and positioning play the biggest roles in the performance of a board. You can make a magic board with fins that haven’t cant nor toe-in.

I don’t think that fins can produce significant drive like a sail on a boat. The angle of toe-in is so small its like sailing straight into the wind. Drive is generated on a surfbord by turning, and positioning the board higher on the face of the wave. Fins can either make this turning motion too fast or too slow.

When I hear about people talking about fins generating drive it makes me think of how we come home from a day of sailing when the wind completely dies. We start rocking the boat back and fourth while skulling the rudder. Note: If you don’t skull the rudder, then you won’t get any forward momentum. This procedure requires precise rythm. If you turn the rudder at the wrong time, the boat will lose speed. Turning the rudder is akin to adding toe-in to a surfboard fin. It seems that by toe’ing-in the fins on a surfboard to oppose each other, the forwad thrust created by one fin would be canceled out by the other. In addition, the rocking method doesn’t get a boat going very fast. You would never do this technique when there is wind, because you add resistance to the water, and slow yourself down.

13

swied,

Exactly! Rocking and pumping a sailboat is refered to a kinetics. If you time the rocking and pumping correctly at hull speed in waves you can break onto a plane at a lower windspeed than normal as well. And, like you said you need to time the angle of the rudder with the rock to get your forward momentum when there’s no wind to get you to the dock. If you’ll notice it’s in the concave direction of the roll. That’s the cupping I’m talking about. But, you wouldn’t push that much rudder in when you’re flat water planing because you have all the wind speed you need. That’s exactly why convex 80/20 style fins work best when you’re controlling all the power you could possibly want. They minimize leading and trailing edge turbulence while redirecting flow.

On another note: toe does have a lot to do with control. Cant is much more subtle. But, toe is pretty noticable. Take a look at your own avatar. How much more engaged do you think you’re backhand fin is than your front side fin. Pumping is rolling from one fully engaged fin to the other while timing the swing or AOA at the same time. You can gain forward momentum with this timing in lots of different scenarios above dead slow. But, when you are trying to control the amount of power you are dealing with the last thing you want is concave surfaces.

14

…without any technical aspect

I put a number

4.: Fins create lot of drag if you for ex in a thruster only stand up and leave the rest to the board in comparison with standing up and move the board to avoid that “anchor” (that you see a lot when some surfers seems to ride in slow motion…) so you “loose” the shape and it came alive

in this way all the other design aspect (for ex.: last 1/3 rocker +bottom) will start to do the homework

15
Quote:

When I hear about people talking about fins generating drive it makes me think of how we come home from a day of sailing when the wind completely dies. We start rocking the boat back and fourth while skulling the rudder. Note: If you don’t skull the rudder, then you won’t get any forward momentum. This procedure requires precise rythm. If you turn the rudder at the wrong time, the boat will lose speed. Turning the rudder is akin to adding toe-in to a surfboard fin. It seems that by toe’ing-in the fins on a surfboard to oppose each other, the forwad thrust created by one fin would be canceled out by the other. In addition, the rocking method doesn’t get a boat going very fast. You would never do this technique when there is wind, because you add resistance to the water, and slow yourself down.

Interesting comparison Swied, I hope that you will excuse me for nit picking but rocking, pumping, and skulling are all effective techniques which are used even in strong winds by top sailors, though they are outlawed.

16

I’m a Blue Collar man, I ride waves less than head high , I have a big respect for guys like Blakestah and Tomatdaum. I like the opinoins of Roy Stewart because he helps me to think outside of the norm. I love threads like this one. Here’s how it was explained to me…

Single fins are for trim and glide…think long board…the rider sets the rail and moves forward or backward on the board to increase or decrease speed.

Thrusters (tri fin)are for pumping…think short board… the rider pumps the board to create speed (drive) and does hard turns to decrease speed.

You don’t pump a single fin , you don’t glide on a thruster.

In my Blue collar world fins do not create drive … the surfer creates drive…

Lift is another subject. I still have not made up my mind on that one…

I’m riding board number 10…starting to understand more and more about lift,drag,drive.flex ect…

share the stoke

Ray

17

I’ll go back to the beginning. Going back in time, the original intent of putting a fin(s) on a board were:

  1. To allow DIRECTIONAL control of a board.

  2. To LATERALLY stabilise a board and hold it in a wave.

(these two are essentially the same)

Thats it.

Any additional benefits, such as drive and turnability, are just bonuses. Please note that much of those add’l bennies came with multiple fin steups, where those extra fins are NOT parallel to the stringer.

If you really want to find the truth for yourself (instead of only reading educated opinions), using a board with removable fins, go out for a surf with NO fins…then ONE fin…then TWO fins…then THREE fins.

Combine that experimental knowledge with a basic understanding of Angle of Attack (hint: the flow is NOT parallel to the stringer) and youre confusion will likely end…or at least, you’ll be less confused.

‘In God we trust, all others must show data’

Right now, I prefer five fins.

18

My belief - drive (rebound) is created when you push against the board, the fin surface resists moving sideways or drifting as water is in its path and you now have a platform to rebound from into the next push (rebound). The larger the fin (base) the more the drive (rebound). If I push hard and hold it, drawing out my turn, a larger fin will prevent sliding better and more energy will be transferred to forward movement. Toe in - more gives you a tighter turn( tighter steering radius) but you loose forward speed as you have more resistance to forward movement. Cant - more gives you more lift, which may or may not be useful. Smaller waves - lift may help. Larger waves - lift may hinder. I believe the above are the major forces regarding fins, fin size being #1, toe in #2 and cant #3. There may be minor but I think they are negligible compared to the above.

19

craftee,

If drive isn’t what you get when you add those two together (EDIT: plus gravity, which gives you lateral progress down the line), what is it?

Is it pumping? No, pumping is pumping (EDIT: against gravity and/or inertia.)

Trim is pure drive, pumping is available from drive, turning is arcing use of drive. You can get drive without fins too.

If that’s all wrong, please define what you mean by drive.

EDIT: I think drive is rail/fin purchase/penetration, giving opposition, giving control surfaces, such that you’re stabilized against gravity and/or centrifugal force as you deflect across the wave in various directions, trim and otherwise, and that fins allow you to more or less control the release of the tailward rail out of that purchase; one question I have is whether fins ever become the majority control surface back there versus the tail edge release

When someone said you have to push against the water to get drive, well, gravity does that for you and centrifugal force too.

I’m not out to get a theory freight train restarted, but imprecision about this one term is real problematic for me.

20

when you say “…You don’t pump a single fin , you don’t glide on a thruster.”

not always, cause I surfed in a 6 3 thruster oriented shape but with a single fin an stabilizer in line (behind) for 3 years.

and I tell yo that this config is fast and you can pump exactly like a thruster and obtain great loose and speed for maneuvers.

well yes, I surf normally between chest and overhead surf

steep waves