It is my understanding that the foil and toe of our side fins help turn our boards better. I can’t figure out(in terms of physics) what works about the way we set our fins to the bottom of our boards. (If I say anything that is wrong please correct me, I think I know what is going on, but I could be wrong.) First the foil of the fins are shaped to create a force similar to that of an airplane wing. Having a flat inside and a curved outside creates a force toward the curved side of the fin. ( check out http://www.howstuffworks.com/airplane3.htm for a pretty good explanation of lift as it applies to airplane wings and some explanations of the Bernoulli principal not being 100% correct.) Anyway, if we set our fins parallel to the stringer this should do nothing because the force created by one fin works in the exact opposite direction of the other fin, and because the forces are equal they cancel each other out. Of course we do not place our side fins in the board this way, we add toe. The force created by the fin is still perpendicular to the flat side of the fin but the flat side is no longer parallel to the stringer. This force can, however, be broken into components that are either parallel or perpendicular to the stringer. The perpendicular components should still cancel each other leaving a parallel force driving the board forward. These forward forces(one created by each fin) sounds good, but they probably are counteracted by drag and still doesnt explain how it helps turning. In order to over simplify I have ignored cant, drag coefficients and many other things I think have nothing to do with the situation. Sorry for writing so much, but my hope is that it helps anyone explain where my reasoning is wrong. Of course none of this appiles to single fins. All questions and comments welcome, that is the point of all this after all.
bSmith, One critcal factor that you have left out of your equation is that the board is on rail when you turn. The affect is that the disengaged fin does not counter act the the forces that are derived fronm the flow of water across the engaged fin. If the engaged fin were parallel to the stringer there would be less drag and more down force which would engage more of your rail and tighten up the way your board rides. The tow of the fin changes the angle of attack that the foil presents to the water flow. It increases drag and makes the board easier to turn.
bSmith,>>> One critcal factor that you have left out of your equation is that the > board is on rail when you turn. The affect is that the disengaged fin does > not counter act the the forces that are derived fronm the flow of water > across the engaged fin. If the engaged fin were parallel to the stringer > there would be less drag and more down force which would engage more of > your rail and tighten up the way your board rides. The tow of the fin > changes the angle of attack that the foil presents to the water flow. It > increases drag and makes the board easier to turn. Thank you. And the cant added in presents more of a perpendicular angle of resistant surface are to the force of the water when the board is up on that rail in it’s turn.
For a fin to be disengaged wouldn’t it have to be out of the water or somehow have less water flowing over it. I know this happens in some turns, but not all. I agree that it creates more drag, but i still can’t visualize how the drag helps with the turn. For the record I am not an experienced shaper, only 5 boards completed. I really want to know why these things work. Also, just a thought, if the fin was parallel and more force was created to pull more of the rail in the water wouldn’t more of the rocker be utilized, since there would be a greater amount of surface area in the water? this probably doesnt work, the way i think things work probably isnt how they really work. and is it toe or tow?
For a fin to be disengaged wouldn’t it have to be out of the water or > somehow have less water flowing over it. I know this happens in some > turns, but not all. I agree that it creates more drag, but i still can’t > visualize how the drag helps with the turn. For the record I am not an > experienced shaper, only 5 boards completed. I really want to know why > these things work. Also, just a thought, if the fin was parallel and more > force was created to pull more of the rail in the water wouldn’t more of > the rocker be utilized, since there would be a greater amount of surface > area in the water? this probably doesnt work, the way i think things work > probably isnt how they really work. and is it toe or tow? Neither one of these guys steered you wrong. No, the outboard fin usually doesn’t pop all the way out of the water in a turn. The inboard fin goes deep and the outboard fin goes shallow. Water on the surface moves more easily than water under the surface. When a fin pushes surface water the water rises into the air. An air pocket forms on the back side of the fin. When a fin pushes deep water, the water has no place to go, and water has to displace the water on the pushing side of the fin. The deeper the water the more it resists movement. In a straight line, thrusters pull deep water from the center under the board throwing it up and out. The bottom of the board prevents air from displacing the moving water. As a result, the board displaces water, sucking into the water and slowing down.
For a fin to be disengaged wouldn’t it have to be out of the water or > somehow have less water flowing over it. I know this happens in some > turns, but not all. I agree that it creates more drag, but i still can’t > visualize how the drag helps with the turn. For the record I am not an > experienced shaper, only 5 boards completed. I really want to know why > these things work. Also, just a thought, if the fin was parallel and more > force was created to pull more of the rail in the water wouldn’t more of > the rocker be utilized, since there would be a greater amount of surface > area in the water? this probably doesnt work, the way i think things work > probably isnt how they really work. and is it toe or tow? The correct term is toe. Inorder for both fins to be equally engaged throughout a turn you would have to surf down onto the flats and swing the nose around like an old long board turn. Thrusters are design to be ridden in the power zone of the wave. The high side fin, in any turn, may not always be on hundered percent out of the water. But, it is also not buried and compressing the flow of water across it’s foil either. Hence, the terminology that I used engaged and disengaged. When water flows across a surface it will infuence the surface to orient itself such that drag is minimized. Take a flat piece of plastic and hold it in a flowing current. The faster the current the more the flat piece of plastic will be influenced parallel to the flow of water. Drag disrupts the flow of water. So, it makes it easier to to change directions away from the existing flow direction.
The foiled thickness of surfboard fins seems to be a factor of the mechanical means used to attach it to the board today. Does this “form over function” make a difference? Earlier threads noted that the variable of speed is also an important consideration in deciding what works best. I still have an old molded T.Morey? plastic fin that is every bit of 1/2" thick. This thicker profile appears to me to be a more efficient foiled design than a 3/8" (or less) seen in todays modern glass or molded fins. I’m looking at this from the perspective of an average day, takeoff, turn, drive and cutback. Could we do better with a thicker fin profile. (can you tell I’ve been grinding on fin blanks and resetting broken glass-ons the last few nights?) Tom S.>>> It is my understanding that the foil and toe of our side fins help turn > our boards better. I can’t figure out(in terms of physics) what works > about the way we set our fins to the bottom of our boards. (If I say > anything that is wrong please correct me, I think I know what is going on, > but I could be wrong.) First the foil of the fins are shaped to create a > force similar to that of an airplane wing. Having a flat inside and a > curved outside creates a force toward the curved side of the fin. ( check > out http://www.howstuffworks.com/airplane3.htm for a pretty good > explanation of lift as it applies to airplane wings and some explanations > of the Bernoulli principal not being 100% correct.) Anyway, if we set our > fins parallel to the stringer this should do nothing because the force > created by one fin works in the exact opposite direction of the other fin, > and because the forces are equal they cancel each other out. Of course we > do not place our side fins in the board this way, we add toe. The force > created by the fin is still perpendicular to the flat side of the fin but > the flat side is no longer parallel to the stringer. This force can, > however, be broken into components that are either parallel or > perpendicular to the stringer. The perpendicular components should still > cancel each other leaving a parallel force driving the board forward. > These forward forces(one created by each fin) sounds good, but they > probably are counteracted by drag and still doesnt explain how it helps > turning. In order to over simplify I have ignored cant, drag coefficients > and many other things I think have nothing to do with the situation. Sorry > for writing so much, but my hope is that it helps anyone explain where my > reasoning is wrong. Of course none of this appiles to single fins. All > questions and comments welcome, that is the point of all this after all.
The correct term is toe. Inorder for both fins to be equally engaged > throughout a turn you would have to surf down onto the flats and swing the > nose around like an old long board turn. Thrusters are design to be ridden > in the power zone of the wave. The high side fin, in any turn, may not > always be on hundered percent out of the water. But, it is also not buried > and compressing the flow of water across it’s foil either. Hence, the > terminology that I used engaged and disengaged. When water flows across a > surface it will infuence the surface to orient itself such that drag is > minimized. Take a flat piece of plastic and hold it in a flowing current. > The faster the current the more the flat piece of plastic will be > influenced parallel to the flow of water. Drag disrupts the flow of water. > So, it makes it easier to to change directions away from the existing flow > direction. Hi Tom, There is one thing in what you say above that speaks to someone like myself who is trying to enter the frontiers, if there are any, of fin design. I think there are so I’ll keep experimenting. What I’m referring to is how a flat object wants to hold its orientation in fluid flow that surrounds it. This is an obvious truth and some may pass it off as incidental but this one fact alone is the reason different foils respond differently. I’m building fins of several templates with different foils. The next step will be to listen to what surfers say about how they act differently. What I’m getting at is simply this: When we look at extremes a wide based shallow foiled fin will be much stiffer at a given speed then one that is foiled rounder and has less a narrow base, and it seems to follow that the wide base fin with a shallow foil will create more turbulence as well. Nature has allot to teach about what shape works the best for a given application. I wonder if there’s room for serious improvement in fin design? What do the fin gurus out there think about this? This is a valuable thread. It speaks directly to why things work the way they do and I would like to compliment all those contributing there views here. Swaylocks Rocks! Good Surfin’, Rich
The foiled thickness of surfboard fins seems to be a factor of the > mechanical means used to attach it to the board today. Does this > “form over function” make a difference? Earlier threads noted > that the variable of speed is also an important consideration in deciding > what works best. I still have an old molded T.Morey? plastic fin that is > every bit of 1/2" thick. This thicker profile appears to me to be a > more efficient foiled design than a 3/8" (or less) seen in todays > modern glass or molded fins. I’m looking at this from the perspective of > an average day, takeoff, turn, drive and cutback. Could we do better with > a thicker fin profile. (can you tell I’ve been grinding on fin blanks and > resetting broken glass-ons the last few nights?)>>> Tom S. Hey Sterne It’s my contention that a thicker fin than the ones we’re used to seeing, all other things being equal, is a better fin as long as it’s foiled properly. That’s one of the design concepts I’m working on but like any new idea, if this is really one, it does not catch on easily. A retorical question seems to say it all: For ever hundred in the crow how many are willing to think independently or are willing to scan for a new frontier? In surfboard design as well as fin design thankfully there still are a few of us. What we accomplish is just to be part of dynamic process. It’s why we surf, why surfboard design continues to evolve and why surfers are able to more and more things on a wave. What’s next? Who knows but there are sure to be some surprizes. Good Surfin’, Rich
Halcyon- Thanks for your thoughts. Paul Jensen has some nicely made single fins that are locked up in the Board Archive. He let it be known long ago that through his own testing he could tell a difference with the thicker templates. Surely there have been some compromises made based on labor, materials and more recently the cost of a more complex mold. I’m just going to have to layup some thicker fin sheets and see what comes of it. Anyone else have some thoughts on this? TS>>> Hey Sterne>>> It’s my contention that a thicker fin than the ones we’re used to seeing, > all other things being equal, is a better fin as long as it’s foiled > properly. That’s one of the design concepts I’m working on but like any > new idea, if this is really one, it does not catch on easily. A retorical > question seems to say it all: For ever hundred in the crow how many are > willing to think independently or are willing to scan for a new frontier? > In surfboard design as well as fin design thankfully there still are a few > of us. What we accomplish is just to be part of dynamic process. It’s why > we surf, why surfboard design continues to evolve and why surfers are able > to more and more things on a wave. What’s next? Who knows but there are > sure to be some surprizes.>>> Good Surfin’, Rich
Halcyon->>> Thanks for your thoughts. Paul Jensen has some nicely made single fins > that are locked up in the Board Archive. He let it be known long ago that > through his own testing he could tell a difference with the thicker > templates. Surely there have been some compromises made based on labor, > materials and more recently the cost of a more complex mold. I’m just > going to have to layup some thicker fin sheets and see what comes of it. > Anyone else have some thoughts on this?>>> TS Hey Sterne, What are the board specks you going to make these thicker fins for? Best, Rich
thanks. it makes more sense to me now, but it raises the question of why are fins are so flexable. stiffness would optimize the turning capiblity of the fin. it seems to me that shorter, thicker fins with wider bases might be better. and of course since everything in surfboard design is a compromise, i imagine the board would be less “forgiving” with stiffer fins, but surley another aspect of the board can be changed to regain the forgiving effect or we can suck it up and not have a forgiving board. reguardless of my forgiving fins assumtion being correct, i hate that word in surfboards. “forgiving” to me means unwilling to learn how to surf that board correctly (by correctly i mean optimizing its performance capablities). before i go on maybe this should be another post. i am going away for the weekend and dont want to start something until i get back, but someone else feel free to make a post about “forgiving” surfboards.
Halcyon- I’ve been looking for a ready made fin template along the lines of 5"H x 7" Base to put on a traditional fish outline using FCS plugs. Couldn’t find even glass ons in this old keel outline, so I made up a fin panel and cutout a few. Have been riding them and like the added drive on my boards, offer a very noticeable improvement over the Merrick Twin outline. Just exploring this whole thing from that point of view but I’ve been making single 9.5" fins for my longboards from time to time as well. Using SunCure for making these panels is soooo easy, a sheet goes a long way. I can cutout a fin and foil it in a no time. Shaping a fin is not a whole lot different than shaping a mini surfboard. T.S.>>> Hey Sterne,>>> What are the board specks you going to make these thicker fins for?>>> Best, Rich
Sorry I have not been able to get back until now. I have been on the CAD system working on another fin related project. Velocity of waterflow has alot to do with efficiencies of foils. Think about it in terms of air planes. Crop dusters and survey planes fly low and slow and you will notice that their wing foils are very full. They keep generating lift even at very low air speeds. While the foils of the wings on a Blackbird are very fine. It has to haul ass just to stay flying. Thicker fin foil sections do work better in slower weaker surf. In faster more powerful surf you want finer foils and smaller templates. The tow boards that Parsons and most of the tow pyscho’s are riding, use tiny super fine foil super stiff fins made of G-10 material. They even run a 30/70 filleted leading edge to maintain control without tracking to much. You may even go with shallower cant and toe angles. It just depends upon whether you are looking for less drag and more tracking. So, yes the fontiers of fin design are very wide open for exploration. We have been focusing upon foil thickness and many of our foils are very full compared to the other fins out there. But, we also have some fine fine for higher velocity applications.
T.S., Check with Clyde Beatty. He has a fin template that is close to the parameters you are describing. Just remember that a fin that large is going to generate an incredible amount of torsional load on the attachment point. That’s why Clyde Beatty only uses Red X.
Thanks Tom, this sounds like good info to work from. As far as the weaknesses of the FCS plugs with fuller outlines, I’m fully aware that they could rip out at anytime. I checked them more than a couple times when I christened that first set! TS>>> Sorry I have not been able to get back until now. I have been on the CAD > system working on another fin related project. Velocity of waterflow has > alot to do with efficiencies of foils. Think about it in terms of air > planes. Crop dusters and survey planes fly low and slow and you will > notice that their wing foils are very full. They keep generating lift even > at very low air speeds. While the foils of the wings on a Blackbird are > very fine. It has to haul ass just to stay flying.>>> Thicker fin foil sections do work better in slower weaker surf. In faster > more powerful surf you want finer foils and smaller templates. The tow > boards that Parsons and most of the tow pyscho’s are riding, use tiny > super fine foil super stiff fins made of G-10 material. They even run a > 30/70 filleted leading edge to maintain control without tracking to much. > You may even go with shallower cant and toe angles. It just depends upon > whether you are looking for less drag and more tracking.>>> So, yes the fontiers of fin design are very wide open for exploration. We > have been focusing upon foil thickness and many of our foils are very full > compared to the other fins out there. But, we also have some fine fine for > higher velocity applications.
thanks. it makes more sense to me now, but it raises the question of why > are fins are so flexable. stiffness would optimize the turning capiblity > of the fin. it seems to me that shorter, thicker fins with wider bases > might be better. and of course since everything in surfboard design is a > compromise, i imagine the board would be less “forgiving” with > stiffer fins, but surley another aspect of the board can be changed to > regain the forgiving effect or we can suck it up and not have a forgiving > board. reguardless of my forgiving fins assumtion being correct, i hate > that word in surfboards. “forgiving” to me means unwilling to > learn how to surf that board correctly (by correctly i mean optimizing its > performance capablities). before i go on maybe this should be another > post. i am going away for the weekend and dont want to start something > until i get back, but someone else feel free to make a post about > “forgiving” surfboards. bsmith, You are partially correct about fin stiffness. Base rigidity is fundamental in maintaining a clean flow between the tail rocker and the fin base. However, tip flex is also important. If we revisit my previous example of placing a flat piece of plastic in a current, you will note that as you change the angle of attackto the flow of water across the plane more and more turbulence will develop at the trailing edge, as that flat plane is turned. If we allow the tip to flex to a small degree you will relieve alot of that turbulence. Water has a skin tension that will allow it to remain attached to the foil surface through shallow radial angle changes. But, if you induce to much tip flex the water will release and set up a cavitation problem. Basically, your fins loose their ability to produce lift and they become inefficient. The trick is to have as secure and rigid a base attachment as is possible and a progressively tapered foil from base to tip. Different surfing scenarios require different degrees of all the characteristics that affect fins. But, our basic fundamental premise at Red X is solid base and progressive foil taper. If you want to see some other factors we consider check out http://www.wetsand.com/article-email.asp?ID=21&CatID=102
Hi Tom, You discussion of the affects of tip flex are very interesting. I struggle with the truth of it but I’ll bet some nautical engineer, which is what we surfboard innovaters are to some digree, can lay it all out for us. The explaination of what works best and why is always a challenge to the writer. Some of us are allot more professional that others to be sure. You may be that person. I’m just holding out a little before I make my final decision. How different templates factor into this is maybe the ultimate variable. Good Surfin’, Rich
T.S.,>>> Check with Clyde Beatty. He has a fin template that is close to the > parameters you are describing. Just remember that a fin that large is > going to generate an incredible amount of torsional load on the attachment > point. That’s why Clyde Beatty only uses Red X. I have to agree with Tom about the FCS choose. Frankly I think you’re asking for trouble with a wide based fin.
Sorry I have not been able to get back until now. I have been on the CAD > system working on another fin related project. Velocity of waterflow has > alot to do with efficiencies of foils. Think about it in terms of air > planes. Crop dusters and survey planes fly low and slow and you will > notice that their wing foils are very full. They keep generating lift even > at very low air speeds. While the foils of the wings on a Blackbird are > very fine. It has to haul ass just to stay flying.>>> Thicker fin foil sections do work better in slower weaker surf. In faster > more powerful surf you want finer foils and smaller templates. The tow > boards that Parsons and most of the tow pyscho’s are riding, use tiny > super fine foil super stiff fins made of G-10 material. They even run a > 30/70 filleted leading edge to maintain control without tracking to much. > You may even go with shallower cant and toe angles. It just depends upon > whether you are looking for less drag and more tracking.>>> So, yes the fontiers of fin design are very wide open for exploration. We > have been focusing upon foil thickness and many of our foils are very full > compared to the other fins out there. But, we also have some fine fine for > higher velocity applications. Hi Tom, When I consider how fast a surfboard travels it seems to me that 35 knots on a 70 foot fast is close to top end speed. So when we talk about shallow foils I wonder about their value at the relatively slow speeds we travel. Of of course an Airplane would struggle to get of the ground a this speed so I wonder if shallow foils only create more drag and less drive. Certainly a narrow base fin with a shallow foil will create less turbulence then one with a wide base. Hmmmm… I look at our brothers that live in the sea and how their drive components are foiled and my questions seems to be answered there. Of course the circumspective view is always the best one so I’ll keep my mind open to new ideas and I’ll bet I’ll learn something. It good to hear your perspective on things. It’s very informative indeed. At this point for me its round foils for more drive and lift and minimal bases for less drag. The templates and foil crossections I’m using are bit of a secret at this point but if you interested email me. I’ve done all little studing about it over the years and I may be right, but then there always it another surprise around the corner. By the way what’s CAD? Good Surfin’, Rich