The NASA Prandtl wing tip is symmetrical, so it seems twisting a symmetrical foil is sufficiently 'asymmetrical'. Explaining why that is the case, is well out of my league tho...
Select to expand quotefangy said..
This is meth for me and now causing the list of jobs given to me by the Minister of Household Affairs to blow out to several pages.
maybe you should do some aerodynamic tests to establish a better understanding of the tip vortices, and see if you're getting somewhere. Be sure to keep the Ministers list close, you know, somewhere close to the air flow so the Minister can rest assured that you're getting to it soon.
I've heard those industrial sized tree shredders like the councils use create a decent amount of air flow.
Select to expand quotefangman said..
The NASA Prandtl wing tip is symmetrical, so it seems twisting a symmetrical foil is sufficiently 'asymmetrical'. Explaining why that is the case, is well out of my league tho...
Im not sure why people confuse aerodynamics in free flow to hydrodynamics in surface tension, they are not very similar in behaver.
Air and water do not react in the same way to forced movement, water propellers and impellers look nothing like gaseous propellors.
Plane wings, rear stabilisers, and trim tabs etc etc would never be effective under water and worse still on and around the the surface. Equally boat hulls don't make great areoplane fuselages!
I guess the point I am trying to make is that the bell shaped lift distribution comes partly by having a symmetrical section at the tip but also by having washout, If you add washout to a fin it is no longer symmetrical, it works better on one tack than the other. Therefore you cant actually make a symmetrical fin with a true bell shaped lift distribution as you can never have zero lift at the tip on a symmetrical fin (unless the fin is running at (zero AOA, which it doesn't)..... I have played with it on foil wings and it works...but they are not symmetrical foils.
Enough of my hijacking......
Select to expand quotepeterowensbabs said..
Im not sure why people confuse aerodynamics in free flow to hydrodynamics in surface tension, they are not very similar in behaver.
Air and water do not react in the same way to forced movement, water propellers and impellers look nothing like gaseous propellors.
Plane wings, rear stabilisers, and trim tabs etc etc would never be effective under water and worse still on and around the the surface. Equally boat hulls don't make great areoplane fuselages!
Agreed dealing with water is a bit more taxing on the grey matter. Luckily Navier, Stokes and Reynolds plus a few more did most of it just shy of two hundred years ago. They were the seriously good at maths kids who probably sat down the front of the bus. (If only they had invented buses by then) The result is that we can take a foil and look at its behaviour at all sorts of speeds in fluids of varying density and viscosity. The tricky bit for fins is that water will only tolerate a certain pressure drop before it decides to change phase into a gas of completely different physical properties and behaviours. And all this interaction has to happen within a few hundredths of a second as the fin passes by each water molecule. Chuck in some turbulence, seeding events, surface roughness and it's just mind boggling... well it is for my crusty ol' brain.
Select to expand quoteFlex2 said..Mr Milk said..
Are the fins stiff?
You can control the stiffness of the fin by adjusting the infill density, the type of epoxy, the amount of carbon rods and additives like cotton flock or milled glass fibre. (Probably can use more exotic filaments to make stiffer but hasn't seemed necessary so I haven't tried yet even though I've bought the filaments) To answer your question:- Yes, you can make them as stiff (or as flexy) as you want. Photo below shows a 3D printed fin and a quality G10 fin (Tribal WS 31) both loaded at 18cm from the board with a 20Kg bag of salt. I couldn't measure any difference in deflection angle using Fusion 360. I have been playing around with all this and working on anther video. Trouble is both lack of wind for testing and me being slow at editing videos means I can make fins much faster than videos. 
Have you considered using a donor fin as a base and printing a new exterior "sock" to your specific tolerances? Seems to me a super stiff 3d printed skin with those awsome looking flanged bases potted over a donor core using something as hard as spa bond may be a great solution. That way the head that engages is already to spec, fitted with inserts and presumably set at 90 degrees to the box, ground down to fit inside and then you add on your super duper new shape and voila! Just an idea! Maybe a new use for those thousands of dud fins being traded on sea breeze every day!! Your thoughts?
Select to expand quotepeterowensbabs said..
Have you considered using a donor fin as a base and printing a new exterior "sock" to your specific tolerances? Seems to me a super stiff 3d printed skin with those awsome looking flanged bases potted over a donor core ........
Can you pint directly on to G10 (or carbon)and get good adhesion/compatibility?
Select to expand quotepeterowensbabs said..Flex2 said..Mr Milk said..
Are the fins stiff?
You can control the stiffness of the fin by adjusting the infill density, the type of epoxy, the amount of carbon rods and additives like cotton flock or milled glass fibre. (Probably can use more exotic filaments to make stiffer but hasn't seemed necessary so I haven't tried yet even though I've bought the filaments) To answer your question:- Yes, you can make them as stiff (or as flexy) as you want. Photo below shows a 3D printed fin and a quality G10 fin (Tribal WS 31) both loaded at 18cm from the board with a 20Kg bag of salt. I couldn't measure any difference in deflection angle using Fusion 360. I have been playing around with all this and working on anther video. Trouble is both lack of wind for testing and me being slow at editing videos means I can make fins much faster than videos.
Have you considered using a donor fin as a base and printing a new exterior "sock" to your specific tolerances? Seems to me a super stiff 3d printed skin with those awsome looking flanged bases potted over a donor core using something as hard as spa bond may be a great solution. That way the head that engages is already to spec, fitted with inserts and presumably set at 90 degrees to the box, ground down to fit inside and then you add on your super duper new shape and voila! Just an idea! Maybe a new use for those thousands of dud fins being traded on sea breeze every day!! Your thoughts?
Thats a great idea.
With a template, you could cut a simple standard triangle shape to insert. Also, different thickness of donor fin, different flex.
Also , if you smash the plastic bit off, you still have a tiny bit of fin to get you home.
I think printing onto a core may be a bit tricky.
The printers work by adding layers which are flat, the first layer is generally bonds itself to a heated platen the next layer bonds to the first and so on a so forth.
You could possibly post fit a core, but that is essentially what Fangy and Flex are already doing with carbon rods.
Currently Flex prints the foil vertically, this method gives the best finish but mechanically it is the weakest method of printing.
You can print the fin horizontally but are limited by the size of the build plate (mines 290x290mm), you would also have to build the fin in two halves, these would ten have to be bonded together. but it's going to be a small fin unless you have a wizzbang machine with a really large or continuous build plate.
The biggest issue with printing fin horizontally is the layer lines, on a steeper incline (front of the fin) these lines are less visible at the rear they would be very noticeable.
Printing fin vertically hides a lot of these sins.
For larger custom fins you need another of our mad creators, Pepe with his larger CNC router can bang them out of G10.
But then you need to go down the rabbit hole of fin design, and that's one deep, dark and convoluted hole. Being able to design in 3D cad helps.
Select to expand quotepeterowensbabs said.
Have you considered using a donor fin as a base and printing a new exterior "sock" to your specific tolerances? Seems to me a super stiff 3d printed skin with those awsome looking flanged bases potted over a donor core using something as hard as spa bond may be a great solution. That way the head that engages is already to spec, fitted with inserts and presumably set at 90 degrees to the box, ground down to fit inside and then you add on your super duper new shape and voila! Just an idea! Maybe a new use for those thousands of dud fins being traded on sea breeze every day!! Your thoughts?
Yep. Way, way back the initial plan was to CNC an aluminium or titanium universal base and tenon arrangement and simply slip different skins over the top held in place by a grub screw. The quotes I got to do it this way were eye watering to say the least, not to mention the minimum order requirements. I like the idea of reskinning old fins, and certainly the recycling side of it. I think the process might be do able on thicker foils: for example on a foil that is 16mm wide (width of the box) and say *4mm for each skin thickness, that leaves only 8mm thick 'tenon' to slide the shell over. IDK, but wonder if the remaining tenon base would be thick enough to be strong enough?? Bonding the skin and the underlying tenon will obviously be helpful, plus shedding load onto the fillet. Of course this will require a different base for every foil rather than being interchangeable, but as you say the fin bases are already out there doing nothing, so no big deal on that score. Certainly printing the epoxy filled skins with a predetermined recess for the tenon is do able.
If you want to do the experiment I can send you 3D file of a fin for you to print, fill, and bond. If you are really patient (lots of stuff on my drawing board waiting to be printed), I could do a finished skin and mail it to you to bond to your own base to try out.
*I would defer to whatever Prof Flexafario says is the minimum thickness we can go as he has much greater experience than I do in this area.
Select to expand quotefangman said..peterowensbabs said.
Have you considered using a donor fin as a base and printing a new exterior "sock" to your specific tolerances? Seems to me a super stiff 3d printed skin with those awsome looking flanged bases potted over a donor core using something as hard as spa bond may be a great solution. That way the head that engages is already to spec, fitted with inserts and presumably set at 90 degrees to the box, ground down to fit inside and then you add on your super duper new shape and voila! Just an idea! Maybe a new use for those thousands of dud fins being traded on sea breeze every day!! Your thoughts?
Yep. Way, way back the initial plan was to CNC an aluminium or titanium universal base and tenon arrangement and simply slip different skins over the top held in place by a grub screw. The quotes I got to do it this way were eye watering to say the least, not to mention the minimum order requirements. I like the idea of reskinning old fins, and certainly the recycling side of it. I think the process might be do able on thicker foils: for example on a foil that is 16mm wide (width of the box) and say *4mm for each skin thickness, that leaves only 8mm thick 'tenon' to slide the shell over. IDK, but wonder if the remaining tenon base would be thick enough to be strong enough?? Bonding the skin and the underlying tenon will obviously be helpful, plus shedding load onto the fillet. Of course this will require a different base for every foil rather than being interchangeable, but as you say the fin bases are already out there doing nothing, so no big deal on that score. Certainly printing the epoxy filled skins with a predetermined recess for the tenon is do able.
If you want to do the experiment I can send you 3D file of a fin for you to print, fill, and bond. If you are really patient (lots of stuff on my drawing board waiting to be printed), I could do a finished skin and mail it to you to bond to your own base to try out.
*I would defer to whatever Prof Flexafario says is the minimum thickness we can go as he has much greater experience than I do in this area.
Pm me and we can chat. My issue is not enough computer time now, my printer and CNC sit idle, but potting a skin over a g10 donor base is something i think i could do easily. No body in this side of the industry seems to use spa bond, our go to for non visible but super strong joins in boat building/repair. Only thing it does not like is gel coat.
Select to expand quoteHave you considered using a donor fin as a base and printing a new exterior "sock" to your specific tolerances? Seems to me a super stiff 3d printed skin with those awsome looking flanged bases potted over a donor core using something as hard as spa bond may be a great solution. That way the head that engages is already to spec, fitted with inserts and presumably set at 90 degrees to the box, ground down to fit inside and then you add on your super duper new shape and voila! Just an idea! Maybe a new use for those thousands of dud fins being traded on sea breeze every day!! Your thoughts?
Hadn't considered this before but is actually a really good idea to refresh old fins or for more conventional pointers that have had their day. I literally just purchased my first 3D scanner today (a Creality CR-Scan Ferret Pro which is currently going here in Oz at good price with 0.1mm accuracy at $549AUD..this is roughly same accuracy as better 3D printers). I got it to scan my boards in and clone them as a certain someone who promised to send me a good board design hasn't delivered after 2 weeks of promises...he will deliver but kinda like Musk, might take some time. I'm impatient so thus the 3D scanner.
In theory the process would go like this: You scan your old fin to 0.1mm accuracy, it should scan all the imperfections. Import to Fusion 360 (or your CAD program of choice). Convert the Mesh to a body. Design your new fin. Subtract the scanned old fin from the designed fin. Use the push pull command to give a 0.4mm or desired tolerance. Print the fin sock(vertically) in as many parts as required with whatever exotic or plain filament you want. Wall thickness can be as low as 0.4mm (or 0.2mm if you go the smaller nozzle) Mix up a little bit of epoxy, coat the old fin with this and stuff it in the 3D printed "sock". Sand the new fin to your spec and job done. Probably cost $1. My scanner should arrive within a week and I have an old pointer lying around and will do exactly that and update progress here.
Select to expand quotemr love said..
I would have to question how relevant bell shaped lift distribution is to a symmetrical shallow water weed fin.
Who said my fin is symmetrical?
Select to expand quoteFlex2 said..mr love said..
I would have to question how relevant bell shaped lift distribution is to a symmetrical shallow water weed fin.
Who said my fin is symmetrical?
Well thats a good point!!!
Select to expand quotesailquik said..
Can you pint directly on to G10 (or carbon)and get good adhesion/compatibility?
no
This printed fin sock concept sounds awesome 
I don't have CAD design software or a printer,but I do have a neighbour with one who is happy to print stuff if I can find it on Thingiverse. I recently had an Axis foil shim printed which works a treat.
Jim, I think this wasn't intended to be a fin only discussion, so I have one request: can someone please design a clew pulley molding like the Chinook ones, that can then have the pulleys and bolts attached 
Select to expand quotepeterowensbabs said..
Pm me and we can chat. My issue is not enough computer time now, my printer and CNC sit idle, but potting a skin over a g10 donor base is something i think i could do easily. No body in this side of the industry seems to use spa bond, our go to for non visible but super strong joins in boat building/repair. Only thing it does not like is gel coat.
Done
Select to expand quoteStretchy said..
This printed fin sock concept sounds awesome
I don't have CAD design software or a printer,but I do have a neighbour with one who is happy to print stuff if I can find it on Thingiverse. I recently had an Axis foil shim printed which works a treat.
Jim, I think this wasn't intended to be a fin only discussion, so I have one request: can someone please design a clew pulley molding like the Chinook ones, that can then have the pulleys and bolts attached
ya Stetchy, the intention of this thread was not 3D printed fins but how 3D printing can solve/evolve/reduce cost of many things associated with any vaguely wind related sports that is accessible to anyone. I for one cannot hope to keep up with the many endless possibilities but hope others with share here their ideas/wishes here. My near term bucket list is the high $ stuff in descending order: Board, sail, Fin (sorted), harness line, other (like outhaul pulley etc)As far as the outhaul pulley goes the Severne one you can get for $25 at 2nd Wind (call ahead) is way better than Chinook as you can leave them installed on your many sails. (I got 6 to instal on upper and lower clew hole for my 3 fav sails) However, thats yet another example where 3D printing can save big bucks as really should only cost $2 for the stainless bits.
Select to expand quotemr love said..
The Prandlt bell shaped lift distribution wing uses twist....but once you use twist on a fin it becomes asymmetrical.....so how can you do it on a symmetrical fin?
Pretty simple. Just build the fin similar to how sails are made. Morph/Warp them for each tack. Trivial with a 3D printer using TPU. Initial inspiration/credit to Simon Faag who (as far as I can tell 27 years ago) the only one to try morphing fins with his warping asymmetrical foils. For those unfamiliar he used a stiff mast at leading edge and training edge. However, he did not consider Prandtl and its way easier to just twist the foil and leave the shape of foil intact. i.e. just loose Simons's trailing edge mast and beef the leading edge mast as strong as possible. The result is Flex's Flex Fin. Or F3^3 Warp fins use passive warp/twist, V4 will use active/controllable twist. The yellow part of print shown earlier and below is TPU 95A HF. V3 just came off printer, V4 parts just arrived (photo with both) No clue about the modelling/theory, V1-V2 fins tested definitely improves on water experience in terms of feeling locked in and heading upwind. So far about 120 Km on V1 and V2. Not just me, tested with way more experienced pilot twice in light and nut case wind and have them on record stating is way better than their (high quality$) delta fin. Plan is to get other pilots to give their feedback over time to eliminate any bias I may have. Trouble is here we now have only intermittent winter squalls to test so will be slow to give updates.
Select to expand quotemr love said..
I would have to question how relevant bell shaped lift distribution is to a symmetrical shallow water weed fin.
Is the Prandtl wing by its nature not asymmetrical anyway? And 2nd the wing needs to be more than 20% longer than with an elliptical wing... scholarworks.calstate.edu/downloads/q811km846
To reduce cost further for prototyping and also test build concept to 3D print a board I tried filling a fin with expanding foam. This brought the fin cost to $13 AUD. It worked much better than expected even though I broke the fin during excessive load test. Seems can eliminate the carbon stiffeners or replace them with wooden dowels to bring cost down further for a sub $10 fin.
The tensile strength of you print is very surprising.
Is it relying on the bond strength between filaments, or is it printed the other way?
Lots of good stuff in there as always Flexafario. Thanks for taking the time to film the infill testing. Q's: what was the infill % that you have settled upon, and how many perimeters?
Select to expand quoteSchobiHH said..
Is the Prandtl wing by its nature not asymmetrical anyway? And 2nd the wing needs to be more than 20% longer than with an elliptical wing... scholarworks.calstate.edu/downloads/q811km846
Yep, Agree with Q1 one, and agree with part two IF I was aiming for the full Prandtl bag of tricks. For me (not speaking for Flex) I simply wanted to play around with tip vortex mitigation strategies.
Select to expand quotedecrepit said..
The tensile strength of you print is very surprising.
Is it relying on the bond strength between filaments, or is it printed the other way?
For the test samples they are printed the best way for the applied load so the tensile strength is of whatever the filament you are using. I thought the fin was printed the worst way but since the failure was roughly 45 deg across fin cutting through many layers seems printing in this orientation is almost optimal??. For sure the print or the foam alone are very weak and easy to break. The combo however is orders of magnitude higher. Make the print with PA6-CF (best spec hobby affordable filament??) and should be roughly 2 1/2 times stronger for same mass.
My surface layup has 2 layers of glass at about 30deg in opposite directions.
I'm wondering, if you can do something similar, can you overprint a second layer with the fin the other way?
Select to expand quotefangman said..
Lots of good stuff in there as always Flexafario. Thanks for taking the time to film the infill testing. Q's: what was the infill % that you have settled upon, and how many perimeters?
All prints so far have had 2 perimeters (or about 0.9mm wall thickness). For strength/stiffness it appears you want higher infill. Haven't gone higher than 15% infil but my guess would be higher infil ratios will yield a stiffer result. There is of course a trade off, higher infil is more expensive (heavier) and slower print but needs less epoxy which is more expensive than filament. Upping the perimeters to 3 would make the foam fin pass the 20Kg load test. In the case of expanding foam it's hard to get expansion through all the pore space for higher infils. For that case it appears optimal is between 5-10% infil. For large volumes like a board should be able to push infil lower to 2-3% but no clue if practical till try that.
Select to expand quotedecrepit said..
My surface layup has 2 layers of glass at about 30deg in opposite directions.
I'm wondering, if you can do something similar, can you overprint a second layer with the fin the other way?
that one is tricky...at least with current level of easily available current tech. There is new class of 3d printers that allow printing on different planes that the nerds are working on but this is currently out of reach of most...however only a matter of time before this becomes consumer level (search 4 and 5 axis 3D printing to see more):- example here:
Select to expand quoteStretchy said..
This printed fin sock concept sounds awesome
I don't have CAD design software or a printer,but I do have a neighbour with one who is happy to print stuff if I can find it on Thingiverse. I recently had an Axis foil shim printed which works a treat.
Jim, I think this wasn't intended to be a fin only discussion, so I have one request: can someone please design a clew pulley molding like the Chinook ones, that can then have the pulleys and bolts attached
Stretchy, ask and though shall receive. Bunnings special outhaul pulley. Works better than the commercial ones for $0.50 each and last forever. Leave them on your sails in both clew positions for quick rig up and quick change on water. I use 12 for my 6 sails. Files and more details on link below.
www.printables.com/model/913756-outhaul-pulley-for-windsurfing
