The Flow a Fin Experiences

There’s something that never seems to make into discussions about fin design and maybe for good reasons. I know I’ve struggled with it for a long time.

Something changes

Consider, in waist to chest high water, let a wave pass over you. Is the flow the same on the leading face as it is on the back of the wave-form? No? How about half way through? No? So, somewhere between the leading face and the back of the wave-form something changed.

Penetration matters

It’s not unreasonable to assume that flow of water in a wave-form will vary with penetration into the wave-form. (When you really think about it, how could it not.) In particular, that the water particles moving on the face of the wave are moving differently that those immediately adjacent to them as you penetrate into the wave. That is, if you stuck a series of devices that measured both the magnitude and direction of flow along the penetration length of the fin, these quantities are likely to change as you move along the length of the fin. It’s also not unreasonable that the magnitude of the upward component of the flow along the fin would decrease as you penetrate deeper into the wave. If so, then surely this would have implications with respect to fin design.

Forces develop on the fin from the various flows it experiences. But if the flow is different with penetration into the wave face, than the fin itself will experience different flows along its penetration length, and hence different forces will be generated. That is, different forces would be generated from those if you assumed the fin was experiencing the same flow over the whole length of penetration.

So regardless how you’ve decided or believe how a fin is operating, how it functions is surely flow dependent, and therefore one might expect there would be some need to account for the differences in flows with respect to penetration (assuming the changes in the low field are sufficiently significant). For example, holding base length and rake constant, then maybe whatever the impact that one might count on for a given surface area has to be adjusted simply because the flow is changing with penetration depth.

But its not just the magnitude of the flow that would be changing.

There’s also the consideration that once you start moving transversely across the face of the wave, the flow takes on that transverse component -i.e. it will be a function of both the upward flow and transverse flow (at least). So the direction of the flow, when moving transversely also stands to change with depth.

… fun, and exciting, but maybe it’s delusion…

Modeling fins after dolphins, aircraft wings, etc. is very romantic and, I guess kind of exciting. It allows the designer to attribute all sorts of properties to the fin. But maybe it’s delusion. Maybe the flow dynamics on a wave are sufficiently different to warrant their own approach. (Dolphins can be seen playing in waves, but it’s very unlikely their brief time spent in heavily shoaling or braking waves would have had much of an impact on their evolution. And I’m fairly confident that the same is true for just about any other aquatic or semi-aquatic creature.)

Surfboard fins themselves have had a kind of Darwinian evolution however, and it quite likely that many of the more popular fin styles, in many ways already take this differential flow profile into account. Or perhaps it’s been discovered by the somewhat random trial and error that, the difference in flow as you penetrate into the wave may not be enough to have much of an impact.

If you survey the more popular surfboard fin designs, it’s apparent that they have a lot in common. So, for example, at least over the ranges of penetration depth that you encounter today on say on the more popular shortboard fins, perhaps whatever influence this differential flow might have has been accounted for in their design (as best they could be.)

kc

http://ebarrens.blogspot.com

I’d suggest that even though there is a wide range of fluid pressure and direction on any fin, it’s all within an accepted range and current fins are well designed to create a very average ‘average’ to make for a rideable experience across the spectrum.

However, I don’t think fins should be foiled from tip to base, mimic the outline of pectoral fins or have flex but surfers aren’t the type of people to try something different.

 So I suggest that it’s not the lack of scientific understanding, fin theory, R and D or independant commercial fin  producers, but it’s the consumer that’s holding fin design back.

 

I’m not suggesting that whatever the forces* are, that they are outside an ‘acceptable range’, just that the flow, which is generates those forces changes with penetration into the wave.

Also, there is a lack of scientific understanding of the exact nature of the flow field in a heavily shoaling or breaking wave. The general direction flow at the surface is fairly well understood, but that’s not the case as you penetrate into the wave. A search of the academic R/D and/or literature will reveal as much. As to whether there has been any commercial R/D along these lines, I wouldn’t know – at least they haven’t made it available**.

As for fin design being “held back”, I’m inclined to think it’s the opposite. There are identifiable standard approaches to fin design, but the full range of designs that are available or have at some point been available (from the big commercial producers to the one-offs) are all ‘over the map’.

As for ‘surfers aren’t the type of people to try something different’, I’m inclined to think it’s pretty much the opposite. I would agree however, that ‘manufacturers’ are reluctant to commit to putting out a line of fins that they have little reason to believe will be profitable, or that ‘retailers’ are reluctant to stock all but the more popular designs. But both make these choices with understandably good/sound reasons. As it is also understandable for some kid who never seems have a bar of wax in the parking lot, is reluctant to pop for a set of new fins based on some anonymous (or otherwise) testimony or hoaky theory (… ‘patent pending’ on the hoaky theory, of course.)

In the end, whether a fin works for ‘you’ is completely subjective. It’s the occasional argument as to why they ‘work’ which are virtually all ‘after the fact’ and ad hoc, which whips up the credulous, that drives me nuts. That’s not to say certain basic principles haven’t emerged – the interesting question is to why, as in the ‘physically why’, not as in ‘because people like them’, as that much is already known. And there’s always the plus that knowing ‘physically why’ will lead to more interesting and less random experimentation.

-----------------------//-------------------------------

  • The term pressure tends to be reserved for what is called normal stress -i.e. the stress that is perpendicular to a surface, as in the ‘atmospheric pressure’, or ‘hydrostatic pressure’ (but, in general, it need not be static.) Stress a point on a surface has two components, the normal stress -i.e. ‘pressure’, and the shear or tangential stress, which is the stress component tangential to the surface. The final stress at a point on a surface is just referred to as the ‘stress’ at a point. (Note, whether it’s normal or tangential, the units of stress are the same -i.e. both are force per unit area.)

** For example, a while back some research was made on available Swaylocks on the lift/drag of various shortboard fins (for some, but not an exhaustive range of different foils, rakes, total surface areas, etc…) The tests were perform under the conditions of constant flow along the full penetration length of the fin. If the flow field is not constant with penetration than it’s questionable how applicable the results could be.  

I see it a bit differently (I think). I think that the water is moving mostly UP. That means that the water directly below the water on the surface is moving very similar to the water above. The water below is pushing up on the water above. 

When you add a surfboard (in trim for reference) the hull is curving the flow (creating lift, countering the riders weight). The water closest to the hull is curved more than the deeper water (which is moving more up). All this means, in my opinion, that the same fin experiences varying flow directions. The part of the fin closest to the hull experiences more horizontal flow (because it has already been turned by the hull), and the part of the fin furthest from the hull expericnecs more vertical flow (because it hasn’t been turned as much). 

In my opinion, this is part of the reason why surfboard fins work better with some cant, some tip flex, and why they have a tapering design, with a curved leading edge. 

The paper referenced below, is pretty neat. You don’t have to get the strange variables, they’re just normalized so that a comparative study can be made. The plots kind are fairly intuitive.

http://rsta.royalsocietypublishing.org/content/roypta/370/1964/1572.full.pdf

Sadly, the closest it comes to modeling a breaking wave is modeling a modestly shoaling one - but maybe, it’s enough to imagine what is happening as a wave continues to shoal. (Remember the actual velocity field is continuous. The field shows up as bands because cranking out a more refined picture would be expensive (computationally) and not really necessary when you think about it.) 

Not sure about your interpretation. That is, I’m inclined to believe the flow farthest from the hull is moving the slowest vertically. (Which is at least consistent with the paper above.) As for the transverse flow, I think it’s pretty much the same along the penetration depth.  Therefore, this kind of difference in vertically flows means that the ‘total’ flow the fin is experiencing, as you move down the fin (penetrate into the wave) changes in direction with penetration depth.   Anyway, if true, is kind of wild. 

I don’t doubt that a lot of the fins on the market work, and some work really well (I’ve got my favorites!), it’s just ‘why’ part that’s kind of interesting (and I don’t mean, the ‘testimonial why’ explanation.)  Like I mentioned in my response to Surffoils, fins (like just about all of surfboard design) have had an organic evolution (“survival of the fittest”), but maybe it’s time to get a better picture/idea of what it is they are doing - or at least what it is they are experiencing during certain conditions, like trimming in the barrel, or just trimming in general. 

 

Just briefly going over the paper you referenced, it doesn’t seem counter to my theory, since it is modeling waves moving at a constant depth.

Waves break (the ones that are of concern to surfers) where there is abrupt change in water depth (a shelf, reef, sandbar, point, rock formation) and the horizontal velocity of the wave is deflected upwards (which is why the wave form slows down). Open ocean waves are surface waves, which is why, I think, the upward velocity is less the deeper you go, but that changes as the depth of the water changes, and the horizontal velocity of the wave is redirected by the ocean surface in to vertical-ish flow.

I guess it is possible (although it is hard for me to visualize) that the deeper flow has less velocity, but I was talking about the angle of the flow.

That the velocity field would invert at some point is unlikely, it’s a continuous process.  That is, shoaling to breaking is a continuous process - the flow field changes continuously. It might occur quickly or slowly but it does so continuously.   And if you have some reference supporting you’re notion that ‘horizontal flow’ is directed upward during a breaking wave I would enjoy reading it.    

 

I didn’t say anything about the velocity field inverting.

The only evidence I have is observation and logic. Tell me this: If the horizontal movement of the wave isn’t redirected upward when it encounters the sea shore bottom then how do waves increase in height and slowdown (and eventually break) in the process?

Please read my thread “The Deceleration Wave-Form” .  Friction with the bottom does not redirect flow where the friction is occurring, it slows it down. 

In the case of a shoaling wave moving into shallower and shallower water, the motion of the forward water particles in contact with the bottom is continually slowed down by friction, but the communication with the upper layers is not instantaneous and the upper layers will start to overtake and over-ride the lower layers. This generally results in a distortion of the circlular paths of the water particles, but if continued enough this “overtaking” of the slower lower portions can lead to “breaking”.  Lips of waves are not really ‘thrown’, no more than you can say your upper body is ‘thrown’ when you trip on a stone or curb - it simply continues to move forward while your lower body is stopped.  That picture is an over simplification, but the overall process is fairly well understood and a more detailed explanation can be found in just about any introductory physical oceanography text.    

Actually given the results in the paper referenced, it’s hard to interpret what you suggested in your post in any other way other than an inversion of the velocity field - but yes, I agree, you did not explicity state as much? 

I look at a surfboard fin like just a tail on kite. A device to keep you pointed in the right direction and give you traction for turns. how much traction it gives the surfboad depends on size shape and placment. All this theory on water flow shouldn’t matter to much if when your having fun on a board, that fin(s) will be all over the place on the wave.
A curved leading edge is very prapractical where sea weed is a issue.

Sorry double post

I’m not technical about things like some of you guys but if the water volume and not just the surface is moving up the wave, then when you takeoff the water is moving parrallel to the hull and flowing from front to back for the fin too.

But when you turn along the wave you create an angle between the flow and the craft, the flow isn’t parrallel to the hull so now the flow is coming up beneath the board at an angle that is from the tip to the base of the fin. The steeper the wave the greater the angle.



With all that has been written here .

Almost no boards in the 90’s could be sold without having Nylon FCS fins , worst performing fins ever .

Why is this ? :slight_smile:

Cheap ?

But can someone explain why common theory is that fins move thru horizontal flow of water, when its clearly coming up the face at about 17 degrees ( when in trim)  and thus up the fin too ?  It must be just easier to accept something than to think about it. 

Forward speed is much greater than upflow of the wave.

We are dropping down sideways as well which adds to what your speaking of .

The base area is ahead of the tip area , think of that as the fin moves forward . At speed , flow is not going to go backwards, from the tip down and forward to the base which is forward of the tip area .

 

I meant that the flow is rising from a deeper level up towards the hull at an angle of 17 degrees.

 

 

Deepest is the tip located furtherst back .

Wide base furthest forward  

With the fin losing enough area as it moves back and deeper into the higher energy position .

Balancing the pressure on the whole fin as a whole design.

:slight_smile:

A rising flow that hits the hull at around 17 degrees creates a parrallel or laminar flow under the board with a thickness of about 1 1/2 inches which would explain why so many commercial fins have an identical base length and why low fins like Bonzer runners, Herbs “Superchargers” and Bills “Pre-fins”  are so effective for their size.

I think 

I was just told this fin is not efective and possibly commercial .  :-(

…and worse, it’s yellow!