toe in & inside foil

General Discussion
1

Hi guys

I was studying my course of fluidummechanics and this got me thinking about two things:

*Toe-in

*Flat inside foil (no foil on the inside of sidefins)

Toe-in:

Why toe in? Sidefins are foiled (singlefoil) to create lift directing to the rails. But a toe-in angle will result in lift in the direction of the stringer! Result is that they cancel each other (partly) and result in extra drag!

So guys tell me what the point of this is, do we want lift to the out or inside? So do we want toe-in or singlefoil? we can’t want them both!

Where the streamline are close together the pressure is lower (bernoulli’s law) -> lift to where the streamlines are closer.

lift due to foil:

Effect of the angle of attack, the pressurefield (red is high pressure):

-> TOE IN CANCELS!!!

Flat inside

A lot of fins have a flat inside with a sharp edge. This works good as long as the angle of attack > 0°! When the angle becomse negative (what toe-in does by the way) we get separation from the beginning, this creates a region of very low pressure (the wake) at the flat side wich cancels the lift completely and creates a lot of drag (the low pressure is under and behind the fin).

Foiling the fins is to reduce the wake, the flat foil enhances it

So inside foil reduces the drag for sure!

2

Think of it as power steering. When going straight the vectors of the side fins cancel each other out. But, as you initiate a turn the fin you are engaging is getting closer to zero degrees angle of attack and the opposite side fin is gaining a greater angle of attack and less opposing lift vector. This disparity increases until the engaged fin is soley engaged and the opposing fin is out of the water. That’s why side fins whether foiled or flat are toed in.

If you’ve got inside convex foil, the leading edge is cleaner and the pressure differential is slighty less. Consequently, less drag when going straight and less turbulence making the foils more efficient at speed. So, as long as you are surfer waves with some power these fins work great.

But, fins perform two functions. The first we’ve been talking about is directional control. The second is kinetic propulsion via pumping. This is where concave and flat sided fins out perform convex fins. They grab more water and redirect it aft due to the turblence at the leading and trailing edges. They don’t shed the flow as cleanly off the inside surface especially the leading edge and consequently they grab more water when you pump them.

3

Okey, this makes perfectly sense!

But then again, getting a little bit of foil on the inside of the leading edge would have a positive influence (by keeping the body flat or concave), or not?

And for a longboard, since longboards arent pumped that hard, they would profit from double foiled fins, I believe.

Thanks, your explanation helps me a lot in understanding the flow!

4

It was also my understanding that toeing in a single foiled fin, or fin that has inside concave, shifts the low pressure region forward… or slighly ahead of the middle of the fin, creating a pulling effect on the trailing edge of the fin. Essentially, this creates lift both on the outside and ahead of the fin, reducing drag and adding speed.

5

You can fillet the leading edge of a flat sided fin and get less drag from the leading edge. But, as you do you reduce the amount of toe and the max girth of the outside foil the larger the radii of the fillet is. But, if you wish to experiment around with leading edge fillets and toe angles a system like 4-Way might really help you figure it all out.

6

Not to hijack, but, and this relates to the fin question, how much of surfing is done going straight and flat?

I’ve been wanting to bring this up for a while - due to some threads about concave, rails, rocker,etc…

My position is: Most of surfing takes place on the rear third on one side of the board or another, and while the rail to rail transition is huge - especially when coming out of a turn, and bringing it back around - the function of fin(s), rails, bottom contours, rocker, etc. are, to me, at their most critical point of inner relationship when working in the pocket and trying to make sections.

As for the question at hand - all that Tom said (Heck, he’s where I’ve gained a lot of my knowledge.) is “spot on.”

Anyway - I’d love to read what others think…

7

On a performance thruster not a lot of going straight except for the take off…that’s kind of important!

8

… drag is good.

Toe and cant may be doing a lot of things, at any given moment, but I believe your argument is correct – the symmetric nature of application, results in more drag. The good news is that drag is good, in fact, it’s that other big tool in the designers tool box, right next to planing surface presentation.

Consider the common method of setting toe angle – using board bottom length -i.e. drawing a line from lateral fin placement to the tip of the board.

… exception that proves the rule

Traditionally it is common practice to back off on toe at either end of the ‘conditions’ spectrum, -i.e. that is for classical fishes and for classical guns.

Fish, at least the classical fish (there are any number of fishy or fish hybrids now) came with twins, and installed with far less toe than would have been called for if using the board length method. In general you can still use board length method for guns, but that’s because board length in general is longer for guns. The point being that fish required a change in the ‘standard’ method, the exception that proves the rule, so to speak.

The longer the board the less drag you want. The basic governing reason for this, I believe is pretty simple. Mass isn’t generally thought of as resistance by most people, but in fact its definition is ‘the quantitative measurement of resistance to motion’. The more board, the more massive an object. Adding more drag to a more massive object is going to make acceleration all the more difficult – and acceleration is key in surfing. (There are exceptions, but they tend to be at the extremes.)

Crudely, the kind of resistance (the kind attributed to fins, that is) that we are dealing with here is the kind that is proportion to the velocity of the flow squared – a parabolic relationship between flow and drag. For example increase the flow by 50% results in an increase in drag by 125% The relationship to the degree of toe and drag is more a simple proportionality - it sort of sets the operating baseline.

Drag is good. Its common to believe that that ‘drag basically means slower’ and I would agree, but in surfing you want to stay on the wave, or with the wave. You don’t want to simply always slip down the face. You can accomplish this but digging your rail or tail, but it amounts to the same thing -i.e. by digging a rail or tail, you are basically using your rail and bottom as fin.

A little extra drag via the fins was a big design plus when it was introduced, it tended to change the way people surfed – it was subtle, but noticeable. Curiously, with modern super light longboard, quads, etc. really do make a lot of sense.

Drag lets you stay connected, when staying connected is important, but sometimes it’s convenient to ‘disconnect’, so balance is paramount.

Nice diagrams.

kc

9

There is no lift if you place a symetrically foiled fin along the line of the stringer (while going straight). An asymetrically foiled fin will cause lift to a board that is going straight ahead unless you toe it in. If your toe-in angle is just right, then the lift of an asymetrically foiled fin will be zero. I wonding if it would help if you knew this zero lift angle for the fins you planned to use. Would it improve performance if you could dial-in this setting?

10

Im a bit confused where the term turbulence is being used… arent we talking separated flow, not turbulent flow? Laminar vs turbulent, attached vs separated, not quite interchangable?

To me, the advantage of toe-in is that the surfer now has easier control and more range for the net lifting vector of his/her fins. Instead of trying to alter the lift magnitude/direction by, lets say, sliding the tail to change the angle of attack of the fins, with toe in this is easily accomplished with a transition from surfing off one rail to surfing off the other.

11

I believe this angle is typically on the order of a few degrees (3-5°), which is right around what toe in is typically. Right? I just did a few basic trig calculations but i prob screwed them up…

Another important consideration in my mind is that even when surfing dead straight with symmetric flow on both sides of the stringer, the flow off the tail is not necessarily straight. There has to be some element of transverse flow due to the force of the surfer on the tail and the water that releases off the rails. Thus toed in fins may be encountering flow that is much closer to 0° angle of attack than we think. just my humble opinion.

12

I don’t know if it could be done. Would be nice, (So I guess there’s at least two of us now…)

I’ve never surfed a side-by-side set up. My first guess would be that it introduces more drag. The question is then, is that bad? One of my pet bits of nonsense is that ‘drag can be good’ - the ‘to a point’ being left out for effect. Perhaps some surfers find a side-by-side introduces just right amount of additional drag so they don’t have to sit on their tail all the time, or as much, which is likely to change the feel of a board. But, I really don’t know.

Longboards are notorious for ‘taking off’ at the wrong time and similarly ‘not taking off’ at the wrong time. That is, they’re not great at acceleration… well, modern longboards are better at it, but even so. Not that this is going to come as any great insight, but given that each surfer tends to have to deal with both ‘conditions’ (the kind of wave he surfs) and his own style, it’s difficult to say how universally beneficial a side-by-side set up would be, that’s assuming my guess about drag is correct. Obviously, if it works for you, there’s no need to take it any further… (unless you’re one of those ‘inquiring mind’ types in which case I’d recommend electroshock therapy - I’ve got Tuesdays and Thursdays booked at my doctors office, but maybe he can fit you in mid week or something.)

kc

ps

You think forklifts are strange? How about that advert for the body shaper or body corset, as in underbust ‘lift’ kind of thing that shows up from time to time when you open Swaylock’s home page? Or I’m I alone in seeing this? I’m guessing this is just some sort of Google kirk in how they decide what’s appropriate for a given market, or… it confirms my worst fears about Swaylock… he realized there’s no money in providing such a forum fto surfers and is slowly taking the site ‘porno’… can’t blame him, I probably would.

13

I refered to turbulence with regard to the leading edge of flat sided fins.

14

… segmented foil?

By the way, I realize this may be off topic a bit, but compare your ‘sphere-air-…’ picture to the one below. Obviously, I just removed most of the foil and left two symmetric sections. By the way, I leave it to you to put in the center fin, or turn the whole thing into quads, etc. This is basically my ‘segmented foil’ approach to what is going on under the board.

Do I believe that the flow really follows the streamlines I’ve left in the picture? No. But continuity suggests something is going on there. Does it impact the functioning of the board? I don’t know. I suspect however if the fins are close enough (the distance between the segments) it might – and whether or not the interaction would be beneficial, or under what conditions it might, is also an open question for me.

The reason I’ve approached it in this manner is an attempt to be wholistic in my approach to fins. I’m not a big believer in looking at the operation of a single foiled fin and extrapolating some of the more secondary effects. (See below on the comments regarding transients.) Admittedly, the new language is not likely to be of much use to the industry in general – just to geeks like me.

… experience is king.

You might think that the answer is out there -i.e. journeymen have already discovered it in some form or another. The problem is that most surfboards, short or long really don’t vary all that much in the tail section, and hence the distance between lateral fins. An inch or two but that’s really not enough to get a feel for the relative importance of the governing parameters. I believe at some point, bottom planing surface becomes the dominant factor in design – so maybe the journeymen did answer the question – its not an issue given what we are working with.

… transients?

Also, the point has been made that surfboards don’t really go straight a lot, which surely can be the case, especially on small to mid sized waves. But if they aren’t going straight a lot then what the foil is doing becomes an open question. The flow around foils takes some time to set up so the foil can operate like a foil. Those transient moments, those times between one characterizing flow and another tend to produce a lot turbulence –i.e. throw off a lot of energy. Which tends to support this whole ‘drag’ view of toe, and fins in general.

kc

[img_assist|nid=1042740|title=.|desc=|link=none|align=center|width=529|height=229]

By the way, the following exercise would be interesting, though I'm not sure how productive it would be - construct an animation for 'twins', Obviously, it wouldn't be true to the phenomenon, but it might useful in visualizing the interaction. But, I know, that's a hell of lot easier said then done, even my quick little effort is really off the mark -i.e. gives the wrong impression regarding angle of attack, etc.

[img_assist|nid=1042742|title=.|desc=|link=none|align=left|width=295|height=584]

15

So, I’m reading all this good stuff about fins,foils,angles,and a lot about lift,lift,lift. I get to the bottom of the page and what do I find ? An advertisement for Fork Lifts !!! Now that is real lift.!! I like the color graf showing the high low presure of a fin as it turns,it makes it more understandable to a simple mind (ME). Is it possible to do that with a Thrailkill side by side center fin. I really cant figure out what is happening with that set-up ? Mahalo- Wood_Ogre

16

I don’t think that this is necessarily true. When going straight (down the line), the fin(s) is positioned so that the relative water flow is at a high angle of attack, which creates lift.

And as durbs has alluded to, a few posts down, the toe in actually reduces the angle of attack, and reduces the lift.

My opinion on toe-in, is that it allows the surfer to maximize the surface area of the fin that is presented to the flow, thereby increasing lift, while turning up the wave, and to minimize the surface area presented to the flow, decreasing lift, while turning down the wave (toward the beach).

Without toe-in, the maximum lift is created while in trim, and as you turn up the wave (when you want to maximize lift), the surfer is actually reducing the surface area of the fin that is presented to the flow, and decreasing lift.

17

Quite a bit there to chew on. Just when I thought I had a working model straight in my head.

I think we’re talking in much of this thread as through fins were straight along their z-axis. Surfers decided a long time ago that a curved profile is important. I contend that foiling in the tip acts to pull the tail down in the water. This seems contrary to the lift ideas for toed-in fins going straight.

Also the outside foiling of side fins pull the fin into the water when it is on edge through a turn. Cant allows this effect to be traded for directional control.

18

…seems that all of you have valid opinions

but in my opinion, is more like the ex. Tomatdaum put about the contemporary thruster

I mean, depends on the design and most important, we dont have enough speed (in 80 % or so of the scenarios) to achieve all the stuff atributed to the fins.

more important is how to “manage” the water passing (with interaction between rocker, bottom, outline and fins)

the height, flex or stiffness, area and tilt of the fin is more important than the foiling and how much toe in in average slowly conditions

19

I’ve watched and participated in a few technical discussions on fins and their performance. I’ve learned lots from Tom and many others I won’t list here. I am compelled to make a call here with the risk of offending.

“the height, flex or stiffness, area and tilt of the fin is more important than
the foiling” is a quote from this thread. Bull Shit! Such a gross statement simplistic and very short in scope. If this was true I may as well cut the plugs, forget grinding and stick them on the board.

I have been working with fluid dymamics for a good long time in many ways. I can say without a shadow of a doubt that I can put two fins of exactly the same template thickness, flexibility, size, toe and cant in the same place with two completely different foils and create a huge difference in board performance because I’ve done it. It’s been part of my learning experience on several occasions.

We simply cannot isolate one feature a fin physics regardless of how insignificant it may seem as every nuance plays into the performance picture. Strange as it may seem some of the most subtle things have the greatest effect. Such is the world or water dynamics as it plays on the little part we call a fin.

Mahalo, Rich

20

Well stated, and on point. Like you, I found the effect of foiling to have a DRAMATIC impact on board performance.