The fin base and screw tabs see incredible loads. Any non fiber reinforcement is doomed for failure sooner or later.
The Load must be spread over a wide area, both lateral stresses, and stress on the fin tab(s)/roll pin from any frontal or rearward hit.
Also the roll pin, is best eliminated in favor of two tabs in my opinion. The roll pin needs to be located precisely so that the bottom of the fin is jammed flat against the bottom of the box at the same time the pin is touching the top of the groove within the box. Much easier styped, than accomplished.
Also it seems there are some differences in the location of the groove in boxes. The ball spring plungers from the early days of MrMiks fins would be pushing on the edge of thge bottom of the groove in my box, trying to push the fin upwards out of the box. as such the BSP's were a hindrance to a tight fit and would seek to help eject the fin from box in my board, and the BSP holes also induced weakness in lateral strentgh and fin tab strength as well.
Without the ability to print/drill holes or square up from base of fin into fin body to add carbon bars/rods, then fiberglass needs to be run from the sides of the fin into the base, and a good amount of it which would require thinning of the wood below in order to not sand it all off to get it to fiit
Super hard and strong woods will of course last longer with teh grain going from tab up into fin base, but will fatigue and break This wood grain orientation also makes the screw tabs on single fins weaker. Plywood is generally so soft that even encased in epoxy it will compress and allow rocking of the fin on teh box which then accellerates the wobble and stresses involved on the weakening tab while allowing it to suck water and get even softer.
Having fins break at the base or the tabs just shearing off barely touching sand, is a session ruiner. Even with another fin on hand one needs the tools and light and perhaps reading glasses to remove the broken bits and reinstall another.
..............
I've had two more sessions on my 6'11 with a new toe side rail fin and either the half size GW fin or the half size AW tubercle fin in the center box. My new toe side rail fin has more surface area than previous, with more surface area in the tip, and every frontside wave I got, when going slow, it seemed the board pivoted around that larger rail fin and simply pushed the tubercle center fin sideways for a lack of drive and tail drifting shoreward. If I was going faster then laid into the bottom turn it felt great, but going slow was frustrating, and generating speed on mushier gutless waves was a lesson in futility as it seemed there was little fin to push off of.
The first session I got so frustrated with the lack of ability to generate speed, as the swell had not really filled in as hoped at that time, I went back and got my longboard with one of the most recent GWhale fins, and laid into a few backside bottom turn top turn combos that felt simply awesome and reminded me just how awesome this fin feels.
I'm currently thinking the Gwhale fins cut to half size used in shortboards as center fins is not a valid strategy for gutless conditions where one needs to generate speed. I've moved them back overhanging the back of the box, but I've found on my 6'8" that I liked the smaller center fin moved unnaturally forward. So moving the tubercle fins further back to account for the high aspect ratio is widening the fin cluster and at slow speeds it is not feeling good. Once I am going at a good clip it feels good, but on my toeside rail on the drop, avoiding a bottom turn there is a lack of drive, and that slippery feeling prevents accumulating more speed which could then eliminate the slippery feel.
I generally only ride this board in chest high +, but eagerness to try the board and new fins has me trying it in lesser wave size, and I am struggling. Also quite rusty due to Covid closures, which are happening again as cases surge.
I am not quite ready to write off the Drela AG10 'batfin' without more sessions, but I think its flat sided sharkier predecessor, was faster and crisper. I glued a zippered fin size pocket to the thigh of my Shortjohn, and will carry at least one spare center fin and perhaps the sharkier toe side rail fin in future sessions.
Iam also thinking I should modify a Gwhale fin to sit further forward and be deeper. Something about the fin being too far back on a round pin is bothering me.
Also while the super high aspect ratio Gwhale fin works insanely well in my traditional longboard, perhaps a more maneuverable multifin shortboard needs fin rake and tip area to force larger turning radius and more drag, and modifying the Gwhale from full sized fin to half sized shortboard center fin, is a formula which cant really translate.
The tubercles and one's leash are also an issue. My leash much too easily sinks and gets caught on a tubercle, and even when I notice it when sitting on my board, and try to extract it while sitting, I cant unless I get off the board and undo it by hand. I've tried neoprene sleeves to make the leash float with little success and with more drag . I've an old 'back up' leash from the early 90's( XM) which is much better in this regard.
My fear with tubercled rail fins is the leash getting caught even easier, and trying to ride a wave with leash wrapped around a fin is frustrating to say the least. I've not used a leash on my Longboard so it has not been a factor on that.
The fin base and screw tabs see incredible loads. Any non fiber reinforcement is doomed for failure sooner or later.
The Load must be spread over a wide area, both lateral stresses, and stress on the fin tab(s)/roll pin from any frontal or rearward hit.
...
This fin snapped yesterday.
I used an experimental method to insert the carbon bars into the fin with bees wax and gum turpentine. The fin was printed in Oct 2018 and the carbon bars were inserted in April 2019.
The fin snapped under similar conditions that usually make them snap under wrcsixeight's feet: Pulling off a wave with a forehand turn, the fin snaps at the top of the wave.
The bars still smell of turpentine, I made the wrong assumption that the turps would evaporate through the PLA faily quickly and make the wax more stiff and sticky.
None of my fins which have the carbon bars glued in (either epoxy or polyurethane) have snapped, yet.
There are also longitudinal carbon bars in the base, they did not break because they experience no significant load under the surf move described above.
My resin cheater coat turned out a little thick. So I ran my finishing pass again and I am very please with the results.
I am also very pleased with how the base has turned out so far. Next ones I'll probably use some chopped/milled fiber for even more strength, but sawdust was all I had on hand to thicken the resin for the base.
Here are a couple pictures from after the second finishing pass. Definitely a big difference when using plywood.
I'm going to document the full process for the backside. In case anyone wants to try and duplicate and/or suggest improvements. At this point there are a lot of steps, but a huge improvement of end product.
Here is a fin I cut out recently. It has a non-tubercled brother that hasn't been cutout yet. The outline is based off a supermarine spitfire wing. Still using plywood, but the plys look pretty cool on this one.
Cool, Swaylocks is the hub of tubercled fins research :)
Since this thread is about design, I'll say something about the geometry of the tubercles. The tubercle effect varies in function of the amplitude and frequency of the sinusoidal LE in relation to the chord length. The effect is small when the bumps are tiny and more pronounced with bigger ones.
But there is an upper size limit at which the effect becomes detrimental as it generates too much drag. The maximum is about 20% of the chord length. On the lower limit, under an amplitude of about 5% they barely influence the fin's behaviour. 10 to 20% of the chord length seems to be the sweet window.
Too big bumps generate vortices that will need a bigger portion of the chord length to develop and will "run out of fin", so some of the drag is not transformed in better after-stall performances. Too small bumps and the vortices are too small and carry less energy, so they can't keep the boundary layer attached much beyond the normal stall angle of a given fin.
Basically you trade a little bit of performance at low AoA for much better performance beyond the stall angle. Bigger bumps smoothen the stall more while reducing lift and adding drag to do so. Smaller ones will have a more subtle effect on the stall.
So you can tune in the amount of tubercle effect that you want. Small bumps if you still want to be able to have a marked but not too sudden fin release, while having virtually unchanged performance before stall. Big bumps for a super smooth stall that you'll be able to recover from, without having really completely stalled.
There's also an effect of the frequency of the sinusoidal, more pointy peaks or flatter ones behave differently. Also the type of foils, but we'll keep that for later, I'm not sure I made sense in the above already.
So this last fin seems to be at the upper amplitude to chord limit. Beyond that the drag trade-deal is getting less interesting. This fin should have a smooth stall, very much like the GW7.
I've tried some of MrMik's harftubs with more tubercles, I believe it was 15 or 20 vs the 9.5, once. I kept thinking I was dragging kelp the whole ride, only to flip the board over after kicking out, and finding nothing on the fin or nearby in the water. The next harftubs that arrived had 6.5 larger turbucles and this felt faster and slightly less forgiving, but overall better.
These impressions underfoot on boards I am very familiar with, seems to be opposite of the science. with 6.5/7 larger tubercles feeling less draggy and less forgiving than 9.5 tubercles.
Then I tried the slightly deeper Gwhale fin with 7 tubercles and rarely if ever rode another harftub again, I think the only time was dragging the video camera behind it.
I've a whole bunch of early harftub pure PLA fins never tested, as the GW7 was just so good, but it is even higher aspect ratio, so not directly comparable to harftubs regarding tubercle size and number.
Though I'm willing to try a GW9 or a GW5, or a spit fire, Though I am not really racking much water time these days.
These impressions underfoot on boards I am very familiar with, seems to be opposite of the science. with 6.5/7 larger tubercles feeling less draggy and less forgiving than 9.5 tubercles.
Interesting, it could be that the fins with more tubercles were falling under the 10% chord ratio, or some other interactions, like the foil profile used (smaller tubercles don't work very well with thick foils). Or the angle of the slope between peaks could be the culprit. It seems that you prefer large amplitude, low frequency tubercles, which makes sense. I am surprised that you find them less forgiving though and also the more draggy feeling of the smaller tubercles. I would be very interested if you could find the time to post pics of the very best and worst fins that you tried. Just a few, like the 2 worst and 2 best with a short description of your impressions.
I am trying to extract rules of thumb out of more than 60 scientific papers, my own simulations and actual surf testing. While there seems to be a consistency in the variation of the tubercle effect in relation to its geometry, there are also configurations that jump out of the pack, in good or bad. There's so many variables, some tubercles will show an advantage at very specific AoAs or Reynolds numbers. The thickness of the foil also changes which tubercles configuration is more favorable. Anyway, it is obvious that the idealized conditions of the studies are not the same as real surf. But I think some sort of general rules can be found while the final verdict will obviously be found in actual surfing conditions.
phillipjohnw wrote:
When you say there is an amplitude sweet spot between 10-20% chord length, are you referring to the chord length of the airfoil or the chord length of the fin outline itself?
The chord length of the airfoil at a given position along the span. In other words, let's say that the chord length of the fin at a peak is 100%, so if the the chord length at the next through is 80% that, we have a 20% chord lentgh amplitude. This is just an easy way to characterise the bumps for the sake of finding some sort of general rules. Then there's the frequency (distance between peaks) but for now it seems that most designs, including yours, intuitively choose a frequency of around twice the amplitude, which is a good zone to be in.
Again, I know that real life surfing is the only judge but I think general rules can be helpful to devellop new fins. Just like there are general rules about rocker, toe-in etc. Of course these rules interact between each other in a real board and can give all kinds of weird results in some cases. I just get stoked at the mental exercise of visualizing flow over different configurations and how the performance envelop is morphing with the geometry.
The impressions formed with more vs less tubercles were all on the older harftub fin design.
It was a while ago, I did write down my impressions soon after surfing them, usually within hours, and will have to find dig through a lot of old Emails between MrMik and myself, as my memory of waves rode and impressions of various fins from three years ago is quite faded. Once I rode the GW I was done with the harftub, nearly having a panic attack when it started to break and I had no backup GWhale. I gave the best harftub 6.5 to Zack Flores, who said it went unreal in some Salina Cruz pointbreak on his midlength. Have not seen him in a long time, but have pics of it on my external hard drive somewhere, and MrMik seems to keep quality records of specific fins, when I can provide the dates printed into the fin bases.
The GW's variables have been the thickness, and the flex, both lateral and rotational, flanged base or unflanged, same planshape. I don't think it ever occurred to me to inquire for a different size and shape of the tubercles of the GW7, as it never felt like it was lacking anything just subtle differences of great speed and response. The fatter fins turned better and were more forgiving but did not have the turbo button, the thinnest fins straight line trim quicker with turbo, and less forgiving, requiring more precision initiating and through turns. I recall saying if I were rusty or at a slower peeling wave wanting to do lots of harder turns, I wold want the thickest Gwhale until I scraped off the rust, then the thinner fins for that thrilling 5th gear turbo spark on more down the line waves.
The most flexy fins felt the worst to me, but the stiffest was not quite as good as the second stiffest. The one fin with a very rotationally flexy filament was way too weird. Weird through the initial loading of it, and uncontrollable as to when the flex would unload where it would unset the rail and change the line chosen.
The experimentation tapered off in this area, as it became too difficult to really notice the extremely subtle differences when a fin would deflect 14mm or 18mm when i'd apply 10Lbs of load to the second highest tubercle in my flex testing jig, and when I'd not really be able to perfectly replicate how one fit in my worn stretched fin box compared to the next in terms of how much the fin base flexed in the box itself.
Right now the GW fin in my longboard is one of the stiffest and certainly the strongest with the highest amount of carbon bars cross section epoxied to the interior reaching as close to the tip of the fin as possible, to the best of my abilities, ensuring the best possible adhesion of pultruded carbonbar to PLA.
I've made spacers before and behind the fin to fill in the fin box holes as there is no need to move the fin up or back anymore. Interestingly when swinging the boards tail underwater, filling in the voids in the box before and after the fin added resistance to swinging the board's tail underwater, making it a more effective paddle. Can't really tell any difference up and riding, though it should feel even quicker.
I'll see what I can compile from my Emails with MrMik and find, or take new pics of the relevant 2 best and worst harftubs.
I'll be honest, I didn't put much thought into the size, shape, or number of tubercles. I just used what looked right for the outline I was using.
When you say there is an amplitude sweet spot between 10-20% chord length, are you referring to the chord length of the airfoil or the chord length of the fin outline itself?
Cranked out another one. I like the shape, but the last one seemed a little too small for the longboard I was using it in. I also did a resin halo. Halo didn't come out the best. I think I used too much catalyst and it cracked a bit.
The fin base and screw tabs see incredible loads. Any non fiber reinforcement is doomed for failure sooner or later.
The Load must be spread over a wide area, both lateral stresses, and stress on the fin tab(s)/roll pin from any frontal or rearward hit.
Also the roll pin, is best eliminated in favor of two tabs in my opinion. The roll pin needs to be located precisely so that the bottom of the fin is jammed flat against the bottom of the box at the same time the pin is touching the top of the groove within the box. Much easier styped, than accomplished.
Also it seems there are some differences in the location of the groove in boxes. The ball spring plungers from the early days of MrMiks fins would be pushing on the edge of thge bottom of the groove in my box, trying to push the fin upwards out of the box. as such the BSP's were a hindrance to a tight fit and would seek to help eject the fin from box in my board, and the BSP holes also induced weakness in lateral strentgh and fin tab strength as well.
Without the ability to print/drill holes or square up from base of fin into fin body to add carbon bars/rods, then fiberglass needs to be run from the sides of the fin into the base, and a good amount of it which would require thinning of the wood below in order to not sand it all off to get it to fiit
Super hard and strong woods will of course last longer with teh grain going from tab up into fin base, but will fatigue and break This wood grain orientation also makes the screw tabs on single fins weaker. Plywood is generally so soft that even encased in epoxy it will compress and allow rocking of the fin on teh box which then accellerates the wobble and stresses involved on the weakening tab while allowing it to suck water and get even softer.
Having fins break at the base or the tabs just shearing off barely touching sand, is a session ruiner. Even with another fin on hand one needs the tools and light and perhaps reading glasses to remove the broken bits and reinstall another.
..............
I've had two more sessions on my 6'11 with a new toe side rail fin and either the half size GW fin or the half size AW tubercle fin in the center box. My new toe side rail fin has more surface area than previous, with more surface area in the tip, and every frontside wave I got, when going slow, it seemed the board pivoted around that larger rail fin and simply pushed the tubercle center fin sideways for a lack of drive and tail drifting shoreward. If I was going faster then laid into the bottom turn it felt great, but going slow was frustrating, and generating speed on mushier gutless waves was a lesson in futility as it seemed there was little fin to push off of.
The first session I got so frustrated with the lack of ability to generate speed, as the swell had not really filled in as hoped at that time, I went back and got my longboard with one of the most recent GWhale fins, and laid into a few backside bottom turn top turn combos that felt simply awesome and reminded me just how awesome this fin feels.
I'm currently thinking the Gwhale fins cut to half size used in shortboards as center fins is not a valid strategy for gutless conditions where one needs to generate speed. I've moved them back overhanging the back of the box, but I've found on my 6'8" that I liked the smaller center fin moved unnaturally forward. So moving the tubercle fins further back to account for the high aspect ratio is widening the fin cluster and at slow speeds it is not feeling good. Once I am going at a good clip it feels good, but on my toeside rail on the drop, avoiding a bottom turn there is a lack of drive, and that slippery feeling prevents accumulating more speed which could then eliminate the slippery feel.
I generally only ride this board in chest high +, but eagerness to try the board and new fins has me trying it in lesser wave size, and I am struggling. Also quite rusty due to Covid closures, which are happening again as cases surge.
I am not quite ready to write off the Drela AG10 'batfin' without more sessions, but I think its flat sided sharkier predecessor, was faster and crisper. I glued a zippered fin size pocket to the thigh of my Shortjohn, and will carry at least one spare center fin and perhaps the sharkier toe side rail fin in future sessions.
Iam also thinking I should modify a Gwhale fin to sit further forward and be deeper. Something about the fin being too far back on a round pin is bothering me.
Also while the super high aspect ratio Gwhale fin works insanely well in my traditional longboard, perhaps a more maneuverable multifin shortboard needs fin rake and tip area to force larger turning radius and more drag, and modifying the Gwhale from full sized fin to half sized shortboard center fin, is a formula which cant really translate.
The tubercles and one's leash are also an issue. My leash much too easily sinks and gets caught on a tubercle, and even when I notice it when sitting on my board, and try to extract it while sitting, I cant unless I get off the board and undo it by hand. I've tried neoprene sleeves to make the leash float with little success and with more drag . I've an old 'back up' leash from the early 90's( XM) which is much better in this regard.
My fear with tubercled rail fins is the leash getting caught even easier, and trying to ride a wave with leash wrapped around a fin is frustrating to say the least. I've not used a leash on my Longboard so it has not been a factor on that.
This fin snapped yesterday.
I used an experimental method to insert the carbon bars into the fin with bees wax and gum turpentine. The fin was printed in Oct 2018 and the carbon bars were inserted in April 2019.
The fin snapped under similar conditions that usually make them snap under wrcsixeight's feet: Pulling off a wave with a forehand turn, the fin snaps at the top of the wave.
The bars still smell of turpentine, I made the wrong assumption that the turps would evaporate through the PLA faily quickly and make the wax more stiff and sticky.
None of my fins which have the carbon bars glued in (either epoxy or polyurethane) have snapped, yet.
There are also longitudinal carbon bars in the base, they did not break because they experience no significant load under the surf move described above.
Wax-failure_Screenshot from 2020-07-03 08-21-29.png
My resin cheater coat turned out a little thick. So I ran my finishing pass again and I am very please with the results.
I am also very pleased with how the base has turned out so far. Next ones I'll probably use some chopped/milled fiber for even more strength, but sawdust was all I had on hand to thicken the resin for the base.
Here are a couple pictures from after the second finishing pass. Definitely a big difference when using plywood.
I'm going to document the full process for the backside. In case anyone wants to try and duplicate and/or suggest improvements. At this point there are a lot of steps, but a huge improvement of end product.
0702201321_copy_1512x2688.jpg
0702201321a_copy_1512x2688.jpg
Here is a fin I cut out recently. It has a non-tubercled brother that hasn't been cutout yet. The outline is based off a supermarine spitfire wing. Still using plywood, but the plys look pretty cool on this one.
0722201623a_copy_1512x2688.jpg
0722201623_copy_1512x2688.jpg
Cool, Swaylocks is the hub of tubercled fins research :)
Since this thread is about design, I'll say something about the geometry of the tubercles. The tubercle effect varies in function of the amplitude and frequency of the sinusoidal LE in relation to the chord length. The effect is small when the bumps are tiny and more pronounced with bigger ones.
But there is an upper size limit at which the effect becomes detrimental as it generates too much drag. The maximum is about 20% of the chord length. On the lower limit, under an amplitude of about 5% they barely influence the fin's behaviour. 10 to 20% of the chord length seems to be the sweet window.
Too big bumps generate vortices that will need a bigger portion of the chord length to develop and will "run out of fin", so some of the drag is not transformed in better after-stall performances. Too small bumps and the vortices are too small and carry less energy, so they can't keep the boundary layer attached much beyond the normal stall angle of a given fin.
Basically you trade a little bit of performance at low AoA for much better performance beyond the stall angle. Bigger bumps smoothen the stall more while reducing lift and adding drag to do so. Smaller ones will have a more subtle effect on the stall.
So you can tune in the amount of tubercle effect that you want. Small bumps if you still want to be able to have a marked but not too sudden fin release, while having virtually unchanged performance before stall. Big bumps for a super smooth stall that you'll be able to recover from, without having really completely stalled.
There's also an effect of the frequency of the sinusoidal, more pointy peaks or flatter ones behave differently. Also the type of foils, but we'll keep that for later, I'm not sure I made sense in the above already.
So this last fin seems to be at the upper amplitude to chord limit. Beyond that the drag trade-deal is getting less interesting. This fin should have a smooth stall, very much like the GW7.
_____________
We Are One
I've tried some of MrMik's harftubs with more tubercles, I believe it was 15 or 20 vs the 9.5, once. I kept thinking I was dragging kelp the whole ride, only to flip the board over after kicking out, and finding nothing on the fin or nearby in the water. The next harftubs that arrived had 6.5 larger turbucles and this felt faster and slightly less forgiving, but overall better.
These impressions underfoot on boards I am very familiar with, seems to be opposite of the science. with 6.5/7 larger tubercles feeling less draggy and less forgiving than 9.5 tubercles.
Then I tried the slightly deeper Gwhale fin with 7 tubercles and rarely if ever rode another harftub again, I think the only time was dragging the video camera behind it.
I've a whole bunch of early harftub pure PLA fins never tested, as the GW7 was just so good, but it is even higher aspect ratio, so not directly comparable to harftubs regarding tubercle size and number.
Though I'm willing to try a GW9 or a GW5, or a spit fire, Though I am not really racking much water time these days.
Interesting, it could be that the fins with more tubercles were falling under the 10% chord ratio, or some other interactions, like the foil profile used (smaller tubercles don't work very well with thick foils). Or the angle of the slope between peaks could be the culprit. It seems that you prefer large amplitude, low frequency tubercles, which makes sense. I am surprised that you find them less forgiving though and also the more draggy feeling of the smaller tubercles. I would be very interested if you could find the time to post pics of the very best and worst fins that you tried. Just a few, like the 2 worst and 2 best with a short description of your impressions.
I am trying to extract rules of thumb out of more than 60 scientific papers, my own simulations and actual surf testing. While there seems to be a consistency in the variation of the tubercle effect in relation to its geometry, there are also configurations that jump out of the pack, in good or bad. There's so many variables, some tubercles will show an advantage at very specific AoAs or Reynolds numbers. The thickness of the foil also changes which tubercles configuration is more favorable. Anyway, it is obvious that the idealized conditions of the studies are not the same as real surf. But I think some sort of general rules can be found while the final verdict will obviously be found in actual surfing conditions.
The chord length of the airfoil at a given position along the span. In other words, let's say that the chord length of the fin at a peak is 100%, so if the the chord length at the next through is 80% that, we have a 20% chord lentgh amplitude. This is just an easy way to characterise the bumps for the sake of finding some sort of general rules. Then there's the frequency (distance between peaks) but for now it seems that most designs, including yours, intuitively choose a frequency of around twice the amplitude, which is a good zone to be in.
Again, I know that real life surfing is the only judge but I think general rules can be helpful to devellop new fins. Just like there are general rules about rocker, toe-in etc. Of course these rules interact between each other in a real board and can give all kinds of weird results in some cases. I just get stoked at the mental exercise of visualizing flow over different configurations and how the performance envelop is morphing with the geometry.
_____________
We Are One
The impressions formed with more vs less tubercles were all on the older harftub fin design.
It was a while ago, I did write down my impressions soon after surfing them, usually within hours, and will have to find dig through a lot of old Emails between MrMik and myself, as my memory of waves rode and impressions of various fins from three years ago is quite faded. Once I rode the GW I was done with the harftub, nearly having a panic attack when it started to break and I had no backup GWhale. I gave the best harftub 6.5 to Zack Flores, who said it went unreal in some Salina Cruz pointbreak on his midlength. Have not seen him in a long time, but have pics of it on my external hard drive somewhere, and MrMik seems to keep quality records of specific fins, when I can provide the dates printed into the fin bases.
The GW's variables have been the thickness, and the flex, both lateral and rotational, flanged base or unflanged, same planshape. I don't think it ever occurred to me to inquire for a different size and shape of the tubercles of the GW7, as it never felt like it was lacking anything just subtle differences of great speed and response. The fatter fins turned better and were more forgiving but did not have the turbo button, the thinnest fins straight line trim quicker with turbo, and less forgiving, requiring more precision initiating and through turns. I recall saying if I were rusty or at a slower peeling wave wanting to do lots of harder turns, I wold want the thickest Gwhale until I scraped off the rust, then the thinner fins for that thrilling 5th gear turbo spark on more down the line waves.
The most flexy fins felt the worst to me, but the stiffest was not quite as good as the second stiffest. The one fin with a very rotationally flexy filament was way too weird. Weird through the initial loading of it, and uncontrollable as to when the flex would unload where it would unset the rail and change the line chosen.
The experimentation tapered off in this area, as it became too difficult to really notice the extremely subtle differences when a fin would deflect 14mm or 18mm when i'd apply 10Lbs of load to the second highest tubercle in my flex testing jig, and when I'd not really be able to perfectly replicate how one fit in my worn stretched fin box compared to the next in terms of how much the fin base flexed in the box itself.
Right now the GW fin in my longboard is one of the stiffest and certainly the strongest with the highest amount of carbon bars cross section epoxied to the interior reaching as close to the tip of the fin as possible, to the best of my abilities, ensuring the best possible adhesion of pultruded carbonbar to PLA.
I've made spacers before and behind the fin to fill in the fin box holes as there is no need to move the fin up or back anymore. Interestingly when swinging the boards tail underwater, filling in the voids in the box before and after the fin added resistance to swinging the board's tail underwater, making it a more effective paddle. Can't really tell any difference up and riding, though it should feel even quicker.
I'll see what I can compile from my Emails with MrMik and find, or take new pics of the relevant 2 best and worst harftubs.
I'll be honest, I didn't put much thought into the size, shape, or number of tubercles. I just used what looked right for the outline I was using.
When you say there is an amplitude sweet spot between 10-20% chord length, are you referring to the chord length of the airfoil or the chord length of the fin outline itself?
Cranked out another one. I like the shape, but the last one seemed a little too small for the longboard I was using it in. I also did a resin halo. Halo didn't come out the best. I think I used too much catalyst and it cracked a bit.
0726201758_copy_1512x2688.jpg
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