MobZ said..
Ok, advice taken, mast track back. Getting comfortable. Feeling some magic. Thanks.

Smiles on dials is what we are all about. Fab to see MobZ, and also, what a beautiful looking spot. I agree with the above peeps, your set up and trim looks sweet. All you need to do now is burn some old kit in your backyard as a sacrifice to Huey and you will be pushing Kato in no time. ( and yes Kato's effort still messes with my head)

FF WTF#4 With each iteration Flex prints for me I am getting better at the resin side of things, and the overall number of stuff ups is getting less. There is a reduction in the diameter of the carbon rods to taper the stiffness toward the tip a little. The stainless fin bolts are now embedded deep within the fin foil itself, hopefully taking advantage of the good compressive strength of this technique and minimising its weaker tensile properties. Finally, the relatively lightly loaded rods at the leading edge are all eucalyptus dowel. Dowel is is cheaper, and lighter than same resin volume. Whilst, not having the strength of the carbon rod, it is a small fraction of the cost.
The epoxy resin I use is expensive, but has the advantage of a hard surface and allows me to pour thick sections (5cm) without problems with a runaway exothermic reaction. The downside is that it takes a week to cure. My patience is being sorely tested.

peterowensbabs said..



Can you do a post cure in an oven or hot box to speed up the cure to max hardness?

I am a tad "Once bitten twice shy" after I melted the first one. I do get that it's post cure so it's a different scenario, but should I be concerned about melting the original 3D print framework? I am not sure how of how much structural significance, if any, it plays in the final scheme of things. I have been leaving it out in the sun under a dark rag, so it's getting a degree of post cure at least.

ka43 said..
Thats a sweet looking fin Ross!!!!

It's got some funky stuff going on with the foil as it transitions from root to tip, so we will see if it goes as well as it looks...

Would have to disagree with previous statement that 3D prints have stuff all structural integrity. In two dimensions they are as strong as whatever material you print with. You can of course 3D print with metal if you want to take it to the extreme. Check out 'Relativity Space' who are making fully 3D printed rockets. The main weakness in hobbyist 3d prints is the layer to layer bonding and I've been pretty successfully getting around that and making useable fins and other stuff using a system of transfer plates and bolts to crush the layers together or gluing the print around thin stainless steel sheet to take the tensile loads. Another way is just adding a single layer of fibreglass over the print.

Then I discovered gyroid infill which allows filling the void space with epoxy or expanding foam etc. I would suspect that the result is similar to fibreglass where the combination of epoxy makes the result stronger than either of the components. The beauty of 3D prints is you can adjust the infill amount so instead of trying to melt out the print you could just reduce the infill to 0% which would just be the shell. However, I've been using 15% infill as this seems around the point of most strength, cheap cost and fast print. These fins are printing in 5hrs or less.

The last few fins Fangy has been posting pics of all have carbon stiffeners or wooden dowel to save epoxy cost. However, in addition to costing more and making it more complex to design/fabricate I think this is decreasing the overall strength, especially in thin sections as it hinders getting the epoxy into all the spaces. I've just finished a couple of fins that have zero carbon stiffeners or steel bolts and the result appears extremely strong and stiff.

So strong and stiff it appears plenty of space could be used for other stuff. Other stuff could be lots of things. I put a whole bunch of ideas into one of Fangy's funky fins but there were so many new variables in play it was a bit of a dogs breakfast. See video below. In this fin I stuck a battery, a charge port, a LED and some simple latch electronics triggered by a reed switch to test the power supply to run a servo. I made it an identical fin to Fangy's but also put a flexible trailing edge and flexible bulb. The idea was to do an apples to apples comparison to test the flex idea. Got this idea from Rick who sails regularly at my home spot who made his own fin with a mono film trailing edge. He reckons the fin 'feels smoother' and points upwind better. I used 0.5mm PETG sheet but this may not be flexible enough based on footage from fin cam except for use as a bulb. The apples to apples test didn't go as planned as Fangy had cut a small notch in his fin and this seemed to make his fin much more immune to spinout.

So I've made a couple of standard Fangy 22's, which I have many hrs on so comparing if this flexy trailing edge stuff does anything should be easier. One has a 0.15mm mono film edge and one with 0.5mm PETG.

There is a big safety advantage of a flexible trailing edge that it stops fin cuts. A flexible bulb also eliminates the real possibility of being impaled. It seems like a good concept that it copies nature more. Fish fins and bird wings are strong at the leading edge and completely flexible to extremely delicate at the trailing edge.

Enough waffle for now. Video below for those interested.

You two guys never fail to dazzle me with your inventiveness and perseverance. !!!

Nice work, Flex, keep going! I don't think flex is bad per se (no pun intended ). I think the problem with fins that are too flexible is that you also get twist, which basically reduces your angle of attack, and makes your lift-to-drag ratios go down. I think Kashy's "secret" for making the best formula fins was to give fins a lot of flex with minimal twist. Slowy might have mentioned something about orienting the layups for a similar reason, if I understood and remember him correctly.
Twist is obviously more of an issue with pointers or thinner weedies, but might still matter for deltas. I wonder what the best orientation of the carbon or steel rods be to minimize twist while allowing flex?

You'd have to draw a diagram to convince me that a raked fin can produce a downward force due to flex. If it just flex (without twist), then it's still and upward force that is produced.

I can see how a weedie that is too flexible might feel like it is pulling the board down. But a fin that's too flexible due to bad design or construction will also have a lot of twist (unlike a good formula fin, with has a proper layup to have flex without twist). The twist reduces or eliminates the angle of attack on the lower part of the fin. So instead of producing most of the lift, the lower part only produces drag, making the fin basically equivalent to a shorter fin with more drag. The higher drag / lower lift combination will have the effect of pushing the nose of the board down.

Here's a little thought experiment. Let's take a pointer fin with a layup that prevents twist, but allows flex. At speed, the fin deforms into a J-shape, as we have seen in underwater movies of formula fins. Basically, the fin acts as a J-type foil.

Now we rebox this fin so that it is now tilted back 45 degrees, and sail it at speed. The side ways forces bend it into the same J-shape. Looking at the flexing tip, the front edge is now lower than the rear edge, basically creating a negative angle of attack in this section. When this happens, that should indeed create a downward force, sticking the board to the water.

I think you and others have re-boxed pointer fins this way, so you probably verified this in practice. But what you saw is not an inherent limitation of weed fins - it's a result of a layup that was never intended to be used at a weedie angle, and is oriented wrong after re-boxing. For a weedie, you have to change the layup (or other reinforcements) so that flex is similar to a pointer - that is, perpendicular to the direction of travel, not the length of the fin. I know that the best weedies out there have a carefully designed layup, but the weedies that don't work well probably do not. For re-boxed pointer fins, I'd expect them to work well only if they are really stiff or short.

Yes, I think Fangy's alu fins are a very good example of how well stiff weedies can work. According to the designer of the Tectonic Weed and Speed Demons, one reason these fins are very good is that they use a much higher grade GFC than most other fins, which also makes them stiffer.

I'm starting to understand the effects of flex, twist, and bend curves a bit better, thanks to this discussion. Makes me wonder how many weed fins were designed by someone with a good understanding of the relevant factors. But I know of one brand designed in your area where that's certainly the case .

Early days but got our first wind in weeks today so did first test of the pure 3d print FF22 without any carbon stiffeners with two types of flexible trailing edge. In my haste to get out the door I left the original Aluminium FF22 in the shed so the test wasn't quite as planned. Didn't bother with fins cams etc.

However the results were a little surprising so sharing here. Please take all results with a pinch of salt as was sailing Melville which offers wind all over the place so further test required.

First both fins all survived with no issue despite pushing as hard as possible. This is a major milestone as reduces cost, complexity and speed to build.

Little to no spinouts and easy to recover if they did. Despite the fins being able to flex like in above photos it didn't seem noticeable. Fastest run was on the stiffer trailing edge fin but only because wind was stronger/lucky gust.

Most startling was the the upwind ability of the fin with mono film 'super flex' trailing edge (blue one in photo above) even though had less wind. Like Rick (the guy I got the idea off) said it gave a dramatic increase in upwind ability with no downside in speed. Attached polar plots (sorry boardsurfr, from GPSAR as can measure angles) show 8+ deg improvement upwind on both tacks. Hard to believe a windshift could have caused this but also hard to believe 5mm of flex plastic can do this. Logs on KA72 if anyone wants to check.

I can highly recommend PVC as a fin material. ( short fat fin ). If you guys had access to a CNC mill. A bit flexy and tough as nails. I would think it perfect for the fins you guys are making.

I was about to delete all the video from the FF21.400.264.11 comparison test done earlier at Coods (Flex trailing edge vs Fangy stiff) as concluded it wasn't valid test as had two variables not the one hoped but realised I hadn't even looked at the footage. It was immediately apparent that the fin wake is considerably smaller for the flex trailing edge fin. Of course this reduced turbulence is what is expected as boardsurfr points out. This seems much more apparent in the smooth than the choppy stuff. Since Fangy had also introduced a small notch (which appeared to help with spinout) the effect may be caused by the notch and/or the stiffer trailing edge. I felt like I could push less on the flexy fin (which was opposite expected) which may explain the reduction in wake. Despite this, both fins headed upwind at exactly the same angle and speed. The flexy trailing edge in this case was 0.5mm PETG so I am expecting even less wake for the 0.15mm mono film fin if I stick a camera on. I uploaded a video (on my channel if you really interested) but snapshot below so you don't have to watch it to get the idea.

Bit of an update. Took the pure 3D printed FF22 fins with the flexy edges out for another test but also remembered to bring the regular Al FF22 and camera. Was a lot choppier and windier and thought it would just be a spinout session. The two printed fins were completely solid in the chop, especially in the bear aways and had zero spin outs. The regular Al fin spun out a lot heading upwind but this was potentially due to it worn down to 18cm.
Interestingly, all 3 fins performed exactly the same upwind, though to be fair the 0.5mm trailing edge fin was tested last and wind/chop had died off and really only had one run. The fact the fins survived (just) the chop fest and performed better in some regards was a win. Looking at the video of the fin wake, it is difficult to call if any one fin had less or more wake.

After the session I noticed stress cracks forming around the rear embedded nut so thought better not risk heading out on these fins again. Made another mono film trailing edge fin up with nuts deeper into fin plus a few carbon stiffeners to see if stiffening the fin improves upwind performance and eliminates the cracking.

Whilst that was curing I went out on the light fin with the flexy trailing edge again but this time with a cutout filed into the rear like Fangy had done but bigger. I didn't bother with a camera but probably should have as the fin wake appeared much larger. Fin handled the chop much better with the cutout.

The conclusion is the cutout has a bigger effect on the visible wake than the flexy edge.

Attached pics of the stress cracks and the new fin. Seems there is a shortage of 8mm carbon rod which is the ideal size for fin stiffeners in the center of foil so had to go with 6mm and 4mm. Interestingly the addition of the carbon only decreased the flex by 1.5deg with the same 20Kg load. Hoping to test this new fin out tomorrow.

Flex2 said..

Beautiful