The surf session was no good, either. But good at the same time.
I struggled to catch waves, mainly due to not enough recent surfing, and chickening out. After 90min, also due to noodle arms.
I caught a few, but never got on an open face much. Burleigh Heads, mildly choppy conditions with light onshore winds, 2m faces, inconsistent sets of 5-6 waves, trade-wind easterly swell, high tide, crumbling but too fast to keep up easily.
The GullWhale7-F-25mm did not break or come out of the box, but it did take in visible amounts of water. Weight went up from 163g to 168g. It was still floating well when I switched it for the HARFTUB95-O-12.5mm at about 90min into the 2hr session. Considering that the transparent PLA seems to be the most leaky one for some reason, that's still promising (for eventually making fins that float and do not leak).
Carrying an extra fin in the back of my wetsuit went well.
Snap-in feature without BSPs or screws or anything more complicated than a fore roll-pin went well, too. I had applied a tiny amount of hot melt glue to the aft end of the GullWhale fin before leaving home, and with the epoxy+silk base, the roll pin can take the load while pushing the fin into the box. Very solid, very easy, minimalistic, universally adaptable to all fin boxes that are reasonably within usual specifications. I'll stick with that for now. And I'll go to the trouble of using the drill press next time. I have a baffling inability to drill a straight hole through a fin tab. I even spent money on an imperial (Bah!) drill bit today. But there is a silverlining to that cloud, too: It is labelled with metric measurements as well. Soon the 3/16 will be in smaller print than the 4.76mm, but it's nice to see any progress.
So, 4.76mm drill bit and drill press will be used next time, and the roll-pin holes should be sweet.
I think I felt a difference in paddling speed when I took out the GullWhale7-F-25mm and put in the HARFTUB95-O-12.5mm. But due to noodle arms, and variable amounts of chop and sweep, I cannot say confidently that I was not imagining it. But it makes sense. I assume water resistance is similar to wind resistance. Whittled down to what I can recall and understand (probably wrongly!), the formula goes like this:
Frontal area * (f***ng x amazingly x complicated * CONSTANT-FACTOR) * speed ^3
So because the GullWhale7-F-25mm is deeper (250mm vs 239mm) and thicker (25mm vs 12.5mm at the base), it should have around 2.1 times the drag of the HARFTUB95-O-12.5mm.
I thought about a way to measure this resistance to paddling, but it's not easy and it's still not entirely free of potential for bias:
Go to the lake, when it's very calm, with a board that glides well and several fins, and a fishing rod.
Wade in to the optimal depth to forcefully launch the board toward deeper water, while it is attached to the fishing line and rod via the leash plug. Set reel to free release to minimise error from line drag. Once the board has stopped moving, reel it back in and while counting the revolutions of the reel as a relative measure of distance travelled.
It could even be done single or double blinded, depending on how many family members I can get to help. Probably none! HAHA! But I think I can be objective enough to push the board as hard as I can every time. I think I could detect a 25% difference in drag that way, easily.
Easily except for all the mozzies, so I'm not likely going to actually do this.
It turns out the weak and crappy nature of the support x-beams was due to a paradoxical effect: I though I might try to make them a little slimmer, but Slic3r then decided to use a layer of infill between the perimeters. It actually used 1.5 times as much filament.
Believe it or not, some, maybe a lot of fine tuning for 3D print designs can be done by ear. If the machine makes noises that sound like it's happy, then it's a good file being printed. If it sounds like it's straining, then it might be possible to change things slightly to make the machine happy.
If you cannot hear the difference between a happy machine and a stressed one, then I cannot explain it to you.
So.....Printing the earlier version of the GullWhale fins involved using very similar x-beam supports, but the printer made pleasant noises while hopping from x-beam to x-beam and making a layer at a time. Since I tried to make the x-beams a little slimmer, it sounds like a cacophony. It still worked for the much less demanding GullWhale7 fin, but the extreme demands for the Dea_Weeder7 forced me to look into what is happening.
The crappy one is 4mm wide with 0.9mm thickness and the blue line in the middle.
The good one is 5mm with 0.8mm thickness.
The theoretical thickness of an extrusion as per the settings I used is 0.45mm. Having learned enough to be dangerous, I figured that 2 x 0.45mm make 0.9mm and that would work, but the older 0.8mm setting works much better.
Print is now re-running, after hot-end was cleaned by a couple of cold pulls, filament spool mounted with less friction on top of print bed, support system x-beams adjusted and re-sliced, and fingers crossed.
The second print attempt worked well.
The fin looks a bit like the tubercle fins (re: the side undulations) so maybe it will have similar properties to tubercle fins in the water.
I think I understand much better now the debate about "fat fins vs thin fins".
Result: Fins should not bee too fat or too thin. Who would have thunkt!
They should be just right, and that means a good foil and a good thickness distribution along the wing span, so that the foil actually is close to optimal everywhere.
With typical dolphin-fins, the base length is so long that the width of a standard single fin box is insufficient to make a fin with a good foil. They end up too thin if limited to the width of the fin box for 'ease of production reasons'.
But high aspect ratio fins end up too thick if they are made wider than the standard single fin box. They are "too fat'' even when they are thinner than a dolphin fin that is too thin for it's base length.
That leads to the problem that a high aspect ratio fin has a much smaller area to spread the load of the forces trying to snap the fin off where it leaves the fin box.
And that's why it took hours to fine tune the hollow UTFB, to be as strong as possible in the small available space. My computer gets some major hickups when I try to use 'preview' in OpenScad and I have to turn off many parts of the file when I want to fine-tune a particular part using preview.
I successfully printed the GullWhale7-S-10mm fin, after calculating the thickness distribution.
Anyone's guess if the cord to thickness ratio should be measured at the peaks or the troughs....so I went for half-way in-between for the first print. Making it thinner will require re-adjusting the dimensions of the UTFB-hollow (see above post), but making the fin thicker is just a matter of changing the right value in the OpenScad code, rendering it, exporting it, slicing it, exporting it as Gcode file, and 3D printing it.
I was aiming at Eppler 168 12.45% foil.
Results for Tubercles:
That's pretty even.
Results for valleys:
Number one is a bit of an outlier, otherwise OK.
Thickness of Tubs + valleys:
= ( 0.1141 + 0.1298 + 0.1129 + 0.142 + 0.1155 + 0.1376 + 0.1153 + 0.1391 + 0.1154 + 0.1381 + 0.1131 + 0.1413 + 0.1153)
Divide by number of Tubs+valleys:
≈ 0.1253 = 1.6295 ∕ 13
≈ 1.006 = 0.1253 ∕ 0.1245 (which means I got the result that I wanted to within 0.6% tolerance, proving my math's were appropriate).
Render and print and fill and sand GullWhale-7-S-11mm.
If the 1mm increase in thickness worsens performance compared to the GullWhale-7-S-10mm fin, then try making a 9.2mm version. But only if the 10mm version does not snap off during surfing.
Filling time for GullWhale7-S-10mm: 25minutes from finish of planning stage, to all resin and fibres in the fin. No further need to monitor temperatures, done that before, the thin build of the UTFB will not likely cause heat rise above 40C. If it did, the PLA would eventually melt.
Chopping 5 skeins of silk (yellow x 4, burgundy x 1), mixing West epoxy 105 x 18ml + 207 x 6ml, mixing in the chopped silk, stuffing the UTFB. I think the pressure at the tip of my paddle-pop stick is probably not much less than 1ATM. The fibre will of course expand again, unlike vacuum bagging at 1ATM, but it gets fairly close to high fibre : resin ratio.
Strong enough, I hope. If not, I have some ideas for that (untested and very half-baked so far).
Hi Mik, my sons and I have finally decided today to pass on the 3 Fins you most graciously supplied to us so that other people can enjoy the ride of new design and new construction.
Thank you very much for your generosity and it has been a series of arguements to convince the boys that a gift is best shared.
The fins are still in really good condition, we’ve all had a blast on them but the tubercled Gull Wing was the overall favourite.
Well done Mr Mik, you’ve got a winner.
Thanks, great idea, that would not have been easy to convince your sons, but an important lesson to try to teach.
I'm trying to fix up my blunders with wrong thickness, hopefully I can achieve a reasonably well performing fin without a complete re-design and re-rendering in finFoil.
Dea-Weeder7-F-20.5mm (resized from 25mm) with completely revised internal UTFB structure, no carbon rods.