After sanding for about an hour with various files and 240grit sandpaper:
Total weight of PLA plastic now 202g.
This means about 6.5% of the material needed to make this fin is being wasted on support structures for printing. Not too bad considering that PLA is made from renewable resources and is potentially bio-degradeable (but only in a nice hot compost heap, it's not going to rot while surfing!).
I am now using different ball spring plungers (BSPs).
These ones have a plastic (?or nylon?) ball instead of a stainless steel ball.
They are a little bit more expensive than the full SS variant, but almost 1g lighter for each BSP.
That weight difference might be what it takes to make these fins float, eventually.
These photos show how the UTFB is secured to the fin box, so it can release in a frontal as well as in a rear impact scenario.
One of the difficulties of 3D printing a large surfboard fin, even when using PLA, is warping.
3D printing these fins using multiple colours increases the warping risk, because the enclosure needs to be opened, or left open for the whole print. The cool drafts or sudden temperature changes tip it over the edge.
The first attempt to 3D print this HARFTUB101 fin ended with the fin tearing itself apart in the orange part.
I'll send off some more fins for testing by wrxsixeight later today.
They are only roughly cleaned up after printing and need to be sanded before use. They have surface rough spots and need a bit of wet sanding all around, which takes about an hour or so. It works very nicely under water, zero dust.
I'll include 6 BSPs with plastic balls, this brings it right up to the 500g airmail limit which seems to be the most economical option to send them.
Wrcsixeight had a good idea for how to try smaller fin versions: Cut off a tubercle from the tip of the fin and sand it round again. So he'll probably try that with this HARF9TUB fin.
The other fin is a Wanderfalke 2 fin, or a Gull-whale fin, based on the McCoy Gullwing fin. I'm not certain this one is quite optimal, because I made up an unusual foil for it.
Surffoils has one of them (but with hollow tip area, and this one is solid). The foil is an exaggerated Eppler 168 like foil, with concave areas in front of the trailing edge. I have surfed an identically shaped fin just once (but printed hollow and then filled with carbon rods and epoxy resin) and the waves were poor and crowded, and the board brand new, so I'm not certain how it compares to other fins. I thought it felt extremely loose, and now I think this might have been very high lift generation at low surfing speeds. Or the loose feeling was just due to the new 8'4'' McCoy Nugget. The fin could have issues like cavitation at higher speeds, it only had a little bit of testing so far, but Surffoils likes it.
Thanks to Hans I now have high resolution finFoil files of the Wanderfalke 2 fins to 3D print them (with and without tubercles) with unmodified Eppler 168 foil. I have not printed them yet, just one printer is not enough for the intensity with which I am working on this at the moment. Long print queue....
However, my general plan is to send one Wanderfalke2 fin together with a HARFTUB fin to wrcsixeight at a time, because it adds up to the most economical postage weight that way. Next time I might have a smooth fin with the same foil in the pack, or a tubercled version with the Eppler 168 foil.
Here it is, after 12h44min printing time:
This fins purpose is to establish a baseline to evaluate how they work with low amplitude, high frequency tubercles.
This can then later be compared to fractally arranged tubercles of same frequency and number on the fin, like in the purple + blue HARFFTUB143 shown above.
I figure if fractally arranging the tubercles has a beneficial effect, then the effect would show in many of the possible versions. That would then be sufficient cause to try to find the optimal fractal arrangement. There may of course be several, for different purposes.
I was cheering too soon...A closer feel and look at the green HARFFTUB101 reveals that it has warped.
What happened is this: The warping forces got too strong and the distal (tip end) support pad came off the print bed. See photo, it shows a concavity that is not just limited to the centre as usual, but it has breached the outer perimeter where the pad lifted off the print bed.
The fin did not split like the last one, but the problem occurred in the same general area in the upper half of the fin. It can be felt, the fin is foiled properly at the base, but towards the tip there is a twist / step in it in the forward part.
I will do three things to hopefully prevent this failure:
1) Scrub the PEI print bed sheet with 0000grade steel wool and acetone, then wipe clean with acetone and paper towel.
2) Increase the print bed temperature from 65/55degC to 80 / 70degC (first layer temp / all other layers temp).
3) And close the printer with the top half of the box at some time early in the print, and leave it closed until at least the fin is finished. This allows to have different colours for 3D printing the trailing edge of the fin, and the fore part of the fin tab (so as to 3D print every fin in a unique colour combination).
Looking forward to trying the Fins you sent and will be eagerly checking the mail.
This one has pictures included, full PDF download for free.
Mimicking the humpback whale: An aerodynamic perspective
This comprehensive review aims to provide a critical overview of the work on tubercles in the past decade. The humpback whale is of interest to aerodynamic/hydrodynamic researchers, as it performs manoeuvres that baffle the imagination. Researchers have attributed these capabilities to the presence of lumps, known as tubercles, on the leading edge of the flipper. Tubercles generate a unique flow control mechanism, offering the humpback exceptional manoeuverability. Experimental and numerical studies have shown that the flow pattern over the tubercle wing is quite different from conventional wings. Research on the Tubercle Leading Edge (TLE) concept has helped to clarify aerodynamic issues such as flow separation, tonal noise and dynamic stall. TLE shows increased lift by delaying and restricting spanwise separation. A summary of studies on different airfoils and reported improvement in performance is outlined. The major contributions and limitations of previous work are also reported.
Mimicking the humpback whale: An aerodynamic perspective (PDF Download Available). Available from: https://www.researchgate.net/publication/304512350_Mimicking_the_humpbac... [accessed Aug 13, 2017].
I think I have improved the support system for the HARFFTUB fin series by a great lot today.
Not printed yet, though. Surprises are possible.
However, the idea is that I have replaced the cylindrical pads under the fin with elliptical pads. They are much thicker in the middle, and gradually loose thickness towards their edges. I hope this will prevent bubbles of PEI sheet forming in the centre of the pads, where the most intense warping forces try to pull the fin off while it is being 3D printed. I hope that this will result in a much better spread of the forces on the PEI surface of the print bed. At the same time, it reduces the footprint on the print bed, so I have more options for where to put the fins for printing. While a fin is being 3D-printed, the area of the print bed that was used for the previous 3D fin print gets time to flatten out out again.
The ugly cubic stabilisers have also been replaced with elliptical stabilisers. I think it takes longer to render, though.