3D printing large surfboard fins

General Discussion
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Yes, I started with the ‘Dummy fin base’ https://www.swaylocks.com/comment/526163#comment-526163 and then moved on to the Tough Fin Base TFB https://www.swaylocks.com/comment/528318#comment-528318 and https://www.swaylocks.com/comment/528469#comment-528469

And then I went on to try 3D printing to make the Universal Tough Fin Base (UTFB) https://www.swaylocks.com/comment/529299#comment-529299
I made a couple of printed split moulds for making a UTFB, and then got side-tracked.

I went down the rabbit hole of printing fins as a single part, with or without reinforcing steel or carbon rods.

I might have to re-visit the split mould 3D-printing idea for making a UTFB, although printing the UTFB and then making a silicon split mould would result in a much sturdier mould, which would be good for dozens or hundreds of casting of UTFB’s.

But, I’m sooo close to pulling it off without messy hand-layering of cloth and resin into a split mould, or grinding down perfectly fabulous materials…just like the last 2 years…HAHA!

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More objective strength testing with less material wastage is in preparation:
These red strips are a section taken out of roughly the middle of a AlbatrossWhale-7-F-20mm fin, with the hole at the depth of the fore tubercle, which is about the middle of the fin.
Clamping the strips in a vise and then snapping them off (with scales attached to the hole) will allow me to measure relative strength of test strips that were printed from different materials, or with different printing parameters and/or post-processing methods.
I print 4 at a time, so I can apply different post-processing methods to otherwise fairly identical items and then compare strength.

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It’s cool to see you take a more objective approach to the destructive testing. That 1 hand/2 hand test doesn’t really lend itself to being easily/accurately repeated.

Something else worth considering is your target strength. In seeing most of those fins snap points being cleanly at the base, I almost see that as more of a feature than an issue. I’d rather snap a fin cleanly at the base, then have it rip out a box and do a lot of damage to the board.

It’s finding that happy balance where the fin is strong enough to endure most typical forces that will act on it during a session, but not to overbuild it so much that the weak point is the finbox/foam connection.

From that standpoint, you could do a test panel of a finbox and foam and test it to destruction. Figure out what that load is, then aim for just below that, as your target strength for your fins. That would give you a clear goal to aim for and you can know if you’ve gone too far and are just overbuilding, which is useful to know. Maybe you want that, but it’s good to be able to know at what point you pass that line from an R&D perspective.

It’s been super interesting to watch you go down this path and share your experiences so openly! Thanks for sharing your journey.

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A couple interesting YouTube videos:

Numbers from the destruction test video:
44kg / 430N / 97lb test

https://www.youtube.com/watch?v=bbofgXfvZWA

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True, but it only destroyed fins that I knew were likely too weak and were clogging up space. A quick and dirty test to get a rough result. What I would like to find is a device to measure maximum weight or force and automatically record it, similar to the one used in one of the videos you posted later. But I would prefer it to have a ‘pull’ mechanism rather than pushing, because that is so much easier. I have an engine hoist that could break any fin. For now I’ll try to ‘calibrate’ test strips so that they can be broken with a little hand held set of scales, either fishing scales or a luggage scale.

Also a very good idea, but not easy in execution. Boards will differ a lot regarding how strongly the box is glassed in, just to start.
For now, I know that a fin that I cannot break with one hand can still snap in the surf, and that fins similar to the ones that did snap during real use can be broken with both hands.
It is not likely that I suddenly achieve a huge leap in fin strength beyond what is required, unless I use the stainless steel rod insertion method.
I also know that the flex reduces markedly when exceeding the ‘two hand breaking strength’, and at least some rotational flex is desirable.
It’s quite difficult to make 3D printed fins any stronger than the ones I have made so far, and I think I just need a little bit more to get into the ‘zone’ of sufficient strength.
Material fatigue will be a whole other question, but if the fins had a lifespan of a year, that would be great for prototyping.

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The more objective strength testing of standardized sections of a fin is more complex than I thought.
Lots of interesting results, and more questions popping up than getting answered.
I tried to ‘anneal’ half of the test sections by boiling them in water for 2hrs and then letting it cool down over night.
Measurements of thickness in 4 places and weight before and after boiling were recorded.
Result: Both PLA and ePA-CF take on water and actually swell rather than shrink. Their weight increases which must be the water.

Test strips printed with more PLA flow are heavier (as expected), and they warp more during boiling than parts printed with less filament flow.
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When boiled, all test strips warp in the same direction in which they would have warped if they had not been attached well to the print bed surface during printing.
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More flow = more warping when boiling.
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Different methods of achieving a solidly printed test section result in different severity of warping when boiling.
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The different methods also result in very different printing times, 1.5 x longer for the method that has the least warping during boiling.
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The ePA-CF parts warp the least during boiling, but their flex properties are profoundly affected by water content.
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Next steps:

Dry half of the annealed parts and check if warping and weight reduce again. Measure if any shrinkage has occurred due to boiling. Calculate how much scaling is required for a 3D printed fin so it fits neatly after annealing. 
       
Put half of the not annealed parts in cold water and check if they also warp in cold water and how much water they take on.
  
And then try to decide at which moisture content level the destructive testing should occur.
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Regarding a device to measure load on the fins and peak load before they break, I am not sure what the device is called, but in the following video the guy is using one to test different crimpers of electrical connections and it does measure the peak load, and goes upto several hundred pounds, as in another videoed test he drills and taps fiberglass for machine threads, and also measures load on his anchor line when backing down on it.

https://www.youtube.com/watch?v=Os_rEObglnM&list=UUoPqTkOluQsuu3RpGnxVwFw&index=70

Sorry do not know how to embed on this forum.

https://www.youtube.com/watch?v=ZclHNSxr1xs

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Another attempt to make a fin requiring less post-production manual work went seemingly very well:

The Black Beauty AlbatrossWhale-7-F-20

The sides have not been sanded, just a little bit on the leading edge and the trailing edge.

The double screw-plate makes it easier to fit to fin boxes of varying depth, just sand back the bottom until it fits perfectly, then screw in gently.

Strength of screw plate protrusions is so far unknown, I hope they are sufficiently strong.

It does not snap with one hand pulling in the vise and feels very strong while doing it. A double-hand destructive test is not a good idea (I just can’t do it!') because it will not be representative of real world conditions. That is because this fin has so much flex that it reduces the load placed upon it by the water, while the destructive load test continues to apply the same force although the fin starts to bend and rotate.

It remains to be seen how floppy it gets when it’s done absorbing moisture from the air. Multiple carbon rods in the base and fin should keep the lateral flex within acceptable limits.
The rotational flex is already good and it will continue to soften over time, maybe too much.

But, this will allow to test for optimal rotational flex, because the stiffness / softness is reversible by drying. This fin can easily take 80C without deforming, likely even higher. Leaving it in a hot car or an oven will drive out the water and make it stiffer again.

I don’t think the flex pattern will change much during a 2-3 hr surf session. The water absorption is a bit faster in water than in air, but not rapid. i guess the water can only penetrate into the deeper layers at a slow rate, so there is a ceiling effect for how quickly the fin softens.

I think there is also a ‘preferred moisture content’ at which the polyamide in the fin will be happy, and change up or down only very slowly.
It gets more flexible and more tough against snapping with increasing moisture content.

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In YouTube, click on share, click on embed, copy the text and paste into the post.

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The euphoria did not last long…
Upon inserting it in another board the aft screw extension snapped off.
Maybe it would be more resilient if I wait until the polyamide has gone tough and flexible, but ‘customers’ would certainly break this more often than acceptable.
I have glued it back together and will go and surf it.

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I surfed the ‘AlbatrossWhale’ fin (after repairing the screw plate extension by melting it) and so found out that I was very wrong about the time it takes for the flex pattern to change.
After an hour in the salty water, it is very rotationally flexible indeed, much more than before. I cannot help but believe that it is too much rotational flex.
But the lateral stiffness remains acceptable, due to the carbon rods.
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I did not get any good rides, which had nothing to do with the fin. The competition was too fierce.
The day before I surfed with a PLA AlbatrossWhale fin and found it extremely loose where I had placed it in the box. It pivots almost as if there is no fin in the board. These fins need to be installed a bit further back than other fins.
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The dual screw plate approach works well, thanks to wrcsixeight for the suggestion!
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It seems to me that I have at least succeeded to make a better screw plate: 3D printed Carbon-PA with a stainless steel nut in it. The conical entry for the screw makes it very easy to hit the right spot with the screw. I’ll probably make them from white PC eventually to make them easier to see in low light conditions. I hate when fiddly stuff like trying to get a fin fitted in the cold morning air before dawn delay getting into the water. I can also print them a touch thicker so they ‘stick’ in the fin-box channel and can be left in the box in-between sessions.

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The dual screw fin plate method makes me happy, as it allows a might better fit in the box with less wobble, and makes fitting fins into my worn/stretched box much simpler. also the roll pin, being much thinner than the thickness of the channel does not really keep that side of the fin firmly planted.
Bertleman might have liked the wobbly fin, but I equate this to loosening up the nuts to the very ends of the threads on skateboard truks, and allowing the wheels to slide in and out 1/2 inch.

The AW fin is even looser than the GW? The looseness and quickness of the GW fin without loss of any hold is rather impressive. Usually one trades off one attribute for another and finds or learns the acceptable balance. Seems that is no longer true. Quicker and looser with more hold and extremely forgiving, the GW fin is awesome. I’ll gladly take some seaweed or kelp for a ride on occassion, rather than go back to my traditional fin.

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I do not know yet if the AlbatrossWhale’s drive and hold is as good, or better or worse than the GullWhale fin’s. Just not enough (ANY!) quality waves yet.

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I only just now got around to watching the second video. That was interesting because I had so far assumed that Roy’s fins were designed using FinFoil, but the video looks like that was not the case.

However, while I have limited understanding of how open source software development works, I think it is entirely possible that both FinFoil and whatever program PDM are using are based on a common ancestor, or on each other, or a bit of both.

 

 

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The second video shows some very interesting stuff, mainly that the fin was lofted which can be done with most 3D CAD programs. How they brought in the hand drawn sketch for dimensions and added the bumps after lofting is clever. Still more work IMO than finFoil…

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Another lesson learned: 

PU (in this case ‘Gorilla Glue’) is quite tough, but it flexes too much.

When using carbon tow soaked in PU, the fin base bends too much under load and then separates from the PLA. 

It stays attached like a living hinge rather than snapping off, but I’ll go back to using epoxy for now. It had to be tried out…

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Finally my shins robbed me of the HARFTUB-7-S-13mm today. Some of the weave next to the finbox is now visible but a fingernal dragged over it doesn’t feel it so I guess I will leave it be. 

I won’t be putting the GulWhale 7s 11mm back in so I will go back to the WG2. 

The HARFTUB-7-S-13mm was a clear favourite: as fast as the WG2 but with more control. Regards the Gulwhale, I need more testing in better waves. 

Many thanks again to allow this testing :slight_smile:

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My pleasure!

I’m not sure if I understand you correctly.  What happened?

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Yes more details please.  What type of board are you using it in, how much use has it (the fin) gotten?

How did your shins take it out?  Do you think if a regular fiberglass fin in there it would have damaged your shins and board to a much greater degree, assuming the same impact?  Can you post pics of the portion which remained in the finbox?

 

The harftub7s13 is my Second favorite fin.  It is a predicatable  and brutally efficient and reliable machine, but I did not feel it have the accelleration to max hull speed of the WG2.  The Gullwhale7s11mm however does have that  torque/accelleration, and perhaps more so, on 90% of the wave face.  There is something about the WG2 when that winglet and hull of the board can tap into the wave energy in that 10% that is quite surprising, but the WG2 also, in my experience with my boards, would track, requiring I just hold on for a bit before lowering the rail and then pushing progressively harder.

 

The GW fin just responds with no delay required and I feel it is just as quick, if not more so, but for that 10% of  tail positions in the wave, where the WG2 is in its  rocket thrust mode.  I also have issues with the Wg2 perhaps being too flexible for my 100Kilo weight.  I think when it unloads after a hard front side bottom  it unsets the rail.  many times I have fallen onto the board in this position when loaded hard frontside. Backside no issue though.

 

With lots of experinece with both fins, I just do not feel i would choose the Wg2 over the GW fin in my board at the wave I ride, ever, unless the GW was visibly damaged, and I did not have the harftub7s13 with me.

 

Super gutless  tiny waves with longer wallspeeling quickly, I  would choose the Wg2 over the harftub7s13, but if acquiring speed is not an issue the harftub7s13 just turns and flows better, in my opinion, but the GW fin is a step above that, also IMO.

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I had a look at my records to check how these fins were made. See https://www.swaylocks.com/comment/545506#comment-545506 for photos

Both fins have no carbon rods, they are a ‘Hybrid’ material blending epoxy and PLA evenly throughout the entire fin and base, no hollow spaces.

Unfortunately it appears that the strenth against snapping off is not sufficient with this build method.

One of wrcsixeight’s fins made the same way snapped off, too.

So be careful with the remaining fin, it might be better to not surf it.