Dear Another Bill,
This is in response to your request for clarification, of the quote you have attributed to me. PLEASE re-read my above posts #24 and #28. The ''quote'' is not there, is it? I believe that statement was made by Mr. K.Casey. So, you may wish to do a little editing to your comments, and redirect your request for clarification to the appropriate party.
I'm very sorry.
you are correct.
I've edited my previous post to show that this was the comment i was refering to:
In other than a skidding ''radical'' maneuver, the relative flow is always parallel to the centerline of the board, as it moves forward over the water. The deflection of water toward the rail, is a shallow surface phenomenon, though it can contribute to a ''root stall'' at the base of the fin.
regards,
-bill
This image shows the flow. The explanation is sorta there, also. The board in the water is angled, so that the nose is out of the water, and the tail most deeply immersed. It “plows” through the water. The net effect of this “plowing” is that the water is parted. Water on either side of the centerline is pushed away towards the rail. The size of the “plow” effect depends on the angle of the hull - if the nose is REALLY up there is a lot of plowing going on.
When you turn on a rail. the board is still plowing. The center of the flow is no longer aligned with the centerline, but the plow effect still exists.
wrt the plow effect being smaller shallower or deeper…consider sitting 20 feet blow a surfboard. The plow effect is non-existent. As you move closer and closer to the hull, the plow effect will get bigger and bigger. Of course, in fluid mechanics you are taught there is an attachment layer that moves with the hull - this is true. But in the grand scheme of water flow it is negligent - this boundary layer is a quarter inch or so, whereas the plow effect exists for several inches below the hull. This matters, a LOT, because the forces on a rail fin are concentrated in the 2-3 inches closest to the hull (and not at all in the quarter inch closest to the hull because of the boundary layer).
hth, and here is a re-post of Bill Barnfield’s excellent diagram of what I call the “plow” effect.
[quote="$1"]
'' In other than a skidding ''radical'' maneuver, the relative flow is always parallel to the centerline of the board, as it moves forward over the water. The deflection of water toward the rail, is a shallow surface phenomenon, though it can contribute to a ''root stall'' at the base of the fin.''
Bill Thrailkill
[/quote]
Imagine a still pond, with no water movement, and you push a surfboard across the surface. There will be water ''moveing'' under the board as the board progresses across the surface. That movement is RELATIVE to the board, even though the water is motionless. It is always parallel to the centerline of the board, and opposit the direction of travel. This is the most significant ''flow'' impinging your fin/fins. Very close behind is the water movement up the face of the wave, as the wave advances toward shore. Those two forces, with a tug from gravity, are what is propelling the board/rider combination through and over the water on a wave. The condition described by Mr. Barnfield coexists with what I have described above. It will vary by degree throughout the ride, as maneuvers are executed. Is this helpful?
I think that a lot of great ideas have been presented in this thread. I enjoyed reading Bill B.'s earlier post regarding design modifications that were necessary to surf a single fin at Pipeline. One word that I kept seeing used over and over again was "energy." We all know that you cannot create energy, but you can convert potential energy into kinetic energy. Potential energy comes from all the calories in cheeseburgers and beer that you have stored up in your muscles. When you bend your legs, and push off the board your are forcing the board to do something that it doesn't want to do. Sometimes this movement results in your board going higer on the wave, so that you can then slide down again. What you have just done is work! You could aslo make a movement that puts you in a worse position than you started. However, the goal is to make the most efficient use of the energy that your muscles expend.
Let me expand on this concept of work. It takes work to move a stone from the ground to the top of a table. The path that that the stone travels does not matter in calculating the amount of work done. It would make the most sense to simply lift it straight up and put it on the table. You could also lift it a little, let it back down, shake it back and forth, and then bring it up to the table. The amount of work done in both cases is the same.
You can do a lot of inefficient things while surfing. You can wiggle your but, you can drag you hand on the wave, and you can make a sharp turn and spray water everywhere. In order to get the most speed, or to position yourself for getting tubed at Pipeline you need to be efficient with your movements. The design of your board and fins enables you to be more efficient.
The bottom line is that you need a setup that will push against the wave in just the right way when you move your body. Different waves, and riding styles require different configurations to make the most efficient use of your precious muscle energy. The great thing about surfing is that everyone has a different riding style and no wave is the same. There is no best configuration. Along the lines of what Blakstah said above... you just have to keep trying new modifications until you get something that you like.
Aloha guys
Sorry it looks like I have been ignoring your posts. I haven’t, in fact I had two windows open and was writing responses in both of them when … BAM!!! the power went off here in Haleiwa! No big deal, it has been doing that a couple times a week for decades. Problem this time was that my APS UPS backup for my computer died awhile back but I was in Oregon for the last month and since I got back, neither Costco or Sams Club had any. In fact, I just researched Best Buys stock earlier today so I could go get one there tonight. But then the power went out and beat me to the punch. So, I am off to Best Buy and will try to find time to reply in the next couple of days.
But one thing I would like you guys to consider and specifically comment on is how the riders weight effects the water flow. Blakestah called this the “plow” effect. I have been trying to get people to recognize this for years but because what is happening is in not visible it get easily ignored and left totally out of the discussions. Everyone (or almost it seems) gets fixated on the spray or flow they see in photos and then completely misses the fact that there has to be an equal amount of water flowing off the other side of the board. Otherwise the riders weight wouldn’t be supported in a balanced way. Just because that water flow on the other side isn’t as easy to see, doesn’t mean it doesn’t exist!
If one omits the above from their view and theories of water flows they won’t be able to perceive which way they are really going and how it may be interacting with their fins. So, help me out guys. If I am wrong I want to know and I want to understand where and why.
Bill T’s analogy about the board in a calm pond is a good start but without the riders weight pushing down on the board the water flow would indicate it is parallel with the bottom of the board. Add the riders weight and its downward force and you get the plowing effect that causes the water to be flowing upward also. The upward and rearward flowing water can’t totally disperse parallel out the back of the board so a good portion of it has to disperse out sideways toward the rails with a rearward drift. This creates a high pressure on the inside of the Canted side fins and also better aligns the water flow with the Toe In. I think Crafty’s little experiment a while back came to a similar conclusion. Are me, Blakestah and Crafty the only ones who see this? Or are we totally crazy and way off base? Honestly, it seems so obvious to me but maybe I am missing something? Please help.
This conversation’s way out of my league , but this vid that Srpato posted a link to a few weeks back
YouTube - the skeleton boy is a nice study in the plow effect Bill B mentions , as well as mesmerising ya with the wrap and release of water on the rails.
To all Bills.
I'd already wriiten the following response to bill T before reading bill Bs recent post.
so what follows is not meant to ignore the BB perspective, its just my poor typing skills.
To Bill T:
I believe what you are describing is the dissipation of the potential flow field around the board.
Forgive me for putting words in your mouth, as you obviously have this much better sorted than me.
If we return to your “still pond” analogy, the water does indeed 'attempt' to remain still.
However, when board and water collide, no two elements can occupy the same space.
So, each particle of water in the path the path of the board quickly moves out of the way and just as quickly returns to its original location as the board passes. ( assuming frictionless conditions).
So, now the fuzzy part.
Each water particle that moves, must displace other water particles that were not originally in the path of the board. (incompressible flow)
However, the relative displacement of water particles quickly dissipates as the distance from the board increases.
You believe (quite logically) that the potential flow field of the board dissipates in a distance much less than the length of one’s fin.
Yes/ No?
To BB:
I'm not sure if the above is completely in sync with your explanation, however i dont believe it gets in the way either.
To be honest, i think you are simply proposing another "free stream" vector to be added to the forward velocity of the board.
you have suggested an UP vector to create the "plowing" flow seen in your drawing . I suspect the wave rising vertically past a rider going left is enough evidence that such a behavior exists
My apologies to anyone that has suggested as much, before now.
I can be a bit slow at times.
Regards,
-another bill
Are me, Blakestah and Crafty the only ones who see this? Or are we totally crazy and way off base?
In most scientific circles, theories need to be proven. My little $5 experiment proved to me things that I never knew existed, and my own theories going in were totally disproven. Thanks for the plug, hope your doing good Bill. Im somewhat surprised others have not repeated the test btw. It would put a lot of these arguments to rest.
Btw, I really like the term "plow". In plowing, water gets out of the way in the least resistant way. As simple as that sounds, scientifically/mathematically it is extremely complex and difficult to predict because of off-the-scale dynamics.
Speaking of dynamics...here's one....there are high pressure forces acting on the inside of the fin that vary not only in magnitude but also in direction. Moderate-to-hard on rail, the canted inside rail fin and adjacent board surface is creating a venturi-like vee channel affect that doesnt not exist when not on rail...dynamic thrust when you need it most and, off-rail, upward lift when you need it most...Woa!
So youre thinking on a water particle scale?
Supercomputers have trouble with this....why do you think you can sort it out using your brain?
K.I.S.S.
[quote="$1"]
To Bill T:
So, now the fuzzy part.
Each water particle that moves, must displace other water particles that were not originally in the path of the board. (incompressible flow)
However, the relative displacement of water particles quickly dissipates as the distance from the board increases.
You believe (quite logically) that the potential flow field of the board dissipates in a distance much less than the length of one’s fin.
Yes/ No?
[/quote]
If you believe that a Butterfly flapping its wings in China, can cause a tornado in Kansas, then you have to believe that displaced water particles keep on displacing other particles endlessly. Perhaps they do. So what? My commentary was to address the most significant condition affecting the function of the fin, or fins. The rest are the ''dancing angels'' I mentioned. Your question about the ''potential flow field'' of the board, and the distance over which it dissapates, is not significant, IMO. To the extent that it is present, it is overshadowed and dominated by the ''relative flow'' of water impinging the fin, and its magnatude. I paid attention to the macro, not the micro. I believe the character of the dissplaced water particles is that of a pressure wave. The short answer to your question is yes, from my perspective.
[quote="$1"]
........Bill T's analogy about the board in a calm pond is a good start but without the riders weight pushing down on the board the water flow would indicate it is parallel with the bottom of the board. Add the riders weight and its downward force and you get the plowing effect that causes the water to be flowing upward also. The upward and rearward flowing water can't totally disperse parallel out the back of the board so a good portion of it has to disperse out sideways toward the rails with a rearward drift. This creates a high pressure on the inside of the Canted side fins and also better aligns the water flow with the Toe In. I think Crafty's little experiment a while back came to a similar conclusion. Are me, Blakestah and Crafty the only ones who see this? Or are we totally crazy and way off base? Honestly, it seems so obvious to me but maybe I am missing something? Please help.
[/quote]
To BB,
Here’s the rub.
If we can stick with Bill T’s analogy of the still pond for a while:
Still pond.
Flat board , very thin
No rider weight
Forward direction of travel.
Given the conditions above,
I think we all like Bill Ts perspective that the streamlines past the board will be straight and parallel to the stringer.
If we include BB’s criteria and add a rider of significant weight the board must be pitched at some angle of attack in order to provide enough lift to keep the board surfing/planing.
The heavier the rider the more the required AOA will be.
However , the board is still traveling straight ahead, despite the fact that it is pitched at some AOA.
I believe you are asking how far away from the board will the potential flow (or pressure field) continue to influence the neighboring fluid..
I don’t know, but I doubt it is very far.
If I had to bet I would say closer to the board than the fin tips (per Bill Ts theory).
An extreme example would be a flat plate in cross flow moving through the still pond.
Not aligned with the flow , but having max projected area as it moves forward.
(the same as a very very heavy surfer sinking without moving forward)
The stagnation point would be in the middle of the plate and the streamlines would point radially in all 360 deg.
How far upstream of the plate would you be able to influence the water flow at any particular moment?
The devil is definitely in the details.
Again, I don’t have a feel for these numbers, but I think I can look up some references at work that may provide a ball park to land in.
Regards,
-another bill
Sorry guys,
I’ve been neglectful in contributing more to the discussion. My business partner passed away last month and I’ve had my hands full with all that goes along with that.
In catching up a couple of items I noticed:
[quote="$1"]
So youre thinking on a water particle scale?
Supercomputers have trouble with this....why do you think you can sort it out using your brain?
K.I.S.S.
[/quote]
No, not realy.
I suspect all i've done is alienate Bill T with a poor description of Potental Flow.
no need to beat a dead horse as he appears to understand the problem better than me.
however, w/o the benefit of experience and intuition, computers have a lot to contribute.
equations defining the conservation of mass and momentem have been around for a long time.
to date no one has proven them wrong, we just cant solve them directly
its only recently that the 'common folk' have access to computers and codes fast enough the solve the more interesting problems.
just the same , Mr Hoerner was able to do a pretty good job of determining 3-d flow fields by just graphically adding up all of the pieces.
how you gonna know?
no worries,
-bill
I'm not going to quote you, as the following isn't a dissection of what you've posted, but my take on what you've suggested or described in your post. My interpretation my be way off... if it is please let me know... but I have to admit, this is a fun one if it is a misinterpretation.
... shedding flow
You have suggested that under certain conditions the flow will tend to move off towards the rails once is has impacted that the bottom, or perhaps interacted with flow that has impacted the bottom. That from the interaction of this shedding flow with the toed/canted fin, there can be a component of the lift generated having a direction similar to that of the motion of the surfer/surfboard as a whole. More precisely, because the incident flow experienced by the surfboard's bottom surface is slightly different than that experience by the fins due to this 'shedding', the lift produced from the fins interacting with this shedding flow will have a component in the direction of the motion of the surfboard.
In fact, following this argument, the presence of cant would suggest that the lift produced under these conditions would have a component directed upwards, at least in the direction towards the deck of the board.
You've also seem to have suggested that this shedding, at least the direction of some of it, tends to depend on that upward component of the flow in the face of the wave, and the displacement water by the board/surfer. That is, the water has to go somewhere and not all of it is going to proceed in a straight line off to the other edge of the board once it has impacted the board. At least the more upward flow that the bottom surface experiences the more shedding there would likely be.
If fact, if I've interpreted you post correctly, the deeper a board seems to ride in the water the more likely this degree of shedding from any upward flow, will occur and hence, the more effective toe/cant can be, if you can get the toe/cant right.
That's definitely an argument, albeit one addressed solely to the operation of fins without consideration of their respective drag, but more so, to the system of surfer/surfboard as a whole. Nevertheless, perhaps when a surfer finds himself in a position that encourages this kind of shedding, this kind of mechanism may be possible, make that likely.
...that is, lets say it is.
What is interesting, is that what you've suggested can operate as a sort of negative feed-back system. As conditions slow and displacement increases, this kind of shedding will increase and provide both a component of force in the direction upward towards the deck and one in the direction of motion. The one in the direction of motion tending to counter balance the increase in drag of the surfboard as a whole due to the increase in displacement (which produces increased shedding), and the other assisting to raise the board out of the water (reducing displacement and hence shedding.)
As the board is raised out the water, the shedding is likely reduced and the effects on whole reduced. But once the flow moving under the board is not shedding to any significant degree, the toed/canted configuration reverses its net effect and produces a net down and back force. What you then have is a dynamic feed-back system. During conditions where the board is experiencing non-shedding flows, say when its speed is sufficient to minimize displacement, the fins produce a net force of down and back which ultimately encourages more displacement and the kind of shedding you've suggested, but as displacement and shedding increases the toe/canted configuration operate to minimize that down and back force, if not reversing it. Ultimately a dynamic balance or more appropriately tension is produced, allowing for a tighter connection with or sensitivity to changes in the flow, at least ideally, given my take.
.. so what's up with toed/canted multi-fin configurations?
I'm guessing this may not be exactly what you had in mind. But it is kind of a nice interpretation. I have listened to people speak of the “thrust” they have experienced from these kinds of configurations since the late sixties, at least not much later(?) (Or whenever tri-fins started showing up on Long Island in large enough numbers to catch my eye. I'm not very sure about the dates here, I'm not much of an historian. I am sure that the first shortboad I actually owned, was a single fin.) That there was something special about them was never in any doubt, but I've never felt that 'thrust' properly captured my experience with them. But I have to admit, given the model described above, I can see how that could be interpreted as a sensation of 'thrust', compared to the way one surfed a single fin configuration, where such a feed back mechanism is not operating. Of course this may not be the only way they are operating, as once again I've failed to address turning etc.
Nevertheless, the nature of the experiment to find out whether or not this kind of mechanism is operating seems simple enough.
Of course there's alway just going out and seeing if it works, as opposed to knowing why. I seem to be interested in doing both... people are what they are.
Thanks, that was fun.
kc
If you have not directly experienced thrust, and you haven’t, you ought to get on a board with a proper three fin setup and experience it. It will simplify this sort of discussion quite a bit.
Also, people seem to be underestimating the “plow” effect. Under conditions most commonly seen, displacement and planing generate roughly equal amounts of vertical lift to support the surfer in the water (you only need to look at a few pictures and remember Galileo).
There is no planing on water (with half the force supported by displacement) without plowing. And remember, the plowing effect will increase or decrease if you lift or lower the nose, respectively. Lifting the nose increases planing forces, lowerering the nose decreases planing forces.
That being said, thrust comes because
the rail fin is oriented relative to the centerline (think of how a sailboat tacks with a mainsail oriented relative to the keel)
When forces are generated on the rail fin, they will want to spin the rider out of the turn (turn the nose too fast). The rear fin gets into a near stall configuration, and it has a longer lever arm, so it stabilizes the rail fin in a position that allows it to generate thrust. This is why a 3 fin board can generate thrust much more easily than a 2 fin board. The outside rail fin is mostly a nuisance. You turn on 2 fins at a time.
hth
I’ve read a few reviews of the rotating fin system, and there aren’t that many–maybe because what people really notice is that the feedback from the outside fin is sorely missed. It’s self-evident that drag off the outside fin’s tip would be valuable feedback when that fin is engaged–that tip seems to be a valuable tool that can be exploited–if it’s in the toolbox–as I mentioned before I think this footage shows it being exploited: http://www.youtube.com/watch?v=aUo23cqgRhU but of course there are probably a billion surf vids you could watch and if you surf multi-fin boards competently, you’re probably not finding the outside fin a major nuisance…
Ok Im in cuz I prefer talking about fin function much over water particle behavior. I really like your summation there Blakestah. I was in agreement upon first reading, I still mostly am in agreement. But just to make this thread crazier than it already is...
This is why a 3 fin board can generate thrust much more easily than a 2 fin board.
One could easily argue that a three finner is more 'thrustier' because there's two fins providing thrust, the inside rail and center, as opposed to just one in a twin fin. And it is the fore-aft space between them that helps to eliminate too much pivot and provide directional control and thus thrust. The center fin plays a dual role: thrust and direction controlling rudder.
Keep in mind that when I think of thrust, I think of BOTTOM TURNS. In a typical modern bottom turn, the center fin generates thrust, IMO. It is during smaller radius turns, top turns, that the center fin becomes more of a brake. It works fine because even tho youve slowed at the top, youve got maximum or near maximum potential energy again which as we know is the most potent energy source in surfing. It could easily be argued that modern thruster surfing is a constant accelerating/decelerating process. Unless the waves are really good and fast....then I couldnt care less what you'd call it, Id just call it a RUSH!
As far as the outside fin being a nuisance, and Janklows response to that...that one is tricky. Altho I havent tried your system, I would have to assume that part of the affect is that with less outside resistance the board will tilt on rail much quicker. Ive had boards that do that (too quickly) and there seems to be comfortable limits to tilting on rail response. My sense of it is that if the response is too quick its harder to time your bottom turns. Maybe it is that controlled tilt that is what is being 'missed'. Dont hold me to those reviews Im just quoting J-low. It is also likely that certain boards and styles would benefit more from your system - maybe a slower railing board/surfer would benefit. Heavier surfers really dont have much problem tilting the board over so maybe their feedback may not be the best - but a lighter surfer may dig it. My guess is that the pros of your system (less resistance) likely outweigh any cons. Im thinkin the 7'4" compsand I built in January would work well with your system.
So Blakestah, do you have a board design preference for using your fin system?
Oh and btw, the plow affect varies with speed.
Keep that in mind when your running your CFD computer sims.
I cant wait until one of you select dudes get to the "final answer".
Im sure it will revolutionize surfboard design as we know it.
Maybe then everyone will be riding the same "ideal surfboard", because the mathematically proven physics says it is so.
I want one.
Had a thought: if I surf only lefts, I can crudely simulate your system's behavior by either removing the outside fin or using a tiny one. Yes?