Keelfin with 3d printed internals

Hi guys,

I’ve been messing around with the 3d printer and achieved what I think is a good result on my rudder.
I printed a two part mould (is it “mould” or “Mold”) and used that to create a FG rudder with a balsa core and a profile that is extremely close - if not identical other than surface finishing issues - to the CAD version.

So it’s time to scale up and look at the keelfin.
My immediate issues arethe need for at least 2 pieces per half, an extremely long print time at relatively high quality and will necessitate me aligning the pieces with possible joint issues . These were non-issues on the rudder, and non of them are insurmountable but it got me thinking of alternative methods to use the printer more economically. So I have a hair-brained idea that I’d like some input on.

If you build a version that has the carbon rod placed between two sheets of Carbon/FG then you get a very good keelfin but not much control over the foil shape.
So what happens if you take the carbon tube, and do it like the guys make plane wings?
This has the bonus of the carbon rod for strength but also gives me a nice long tube to align the bits of printed frame.
Then skin this skeleton.

Here is my basic idea. The framework would need to be cut into two sections per half to fit on the 3d printer build plate, so these pics are what one half of the completed keelfin frame would look like.

That structure is 2mm thick for the vertical walls which is pretty good. In the Slicer software it estimates 4g for that entire half (which excludes the weight of the carbon rod)
So total on the 3d print will be in the region of 10g assuming that my choice of framework is correct? I can make it in pretty much any way we want to try.

This is how they would have to be printed,

Ignore the settings on there :slight_smile:

Would you put more vertical struts at the front, back, more foils at the lower region by the bulb - Educated guesses would be most welcome :stuck_out_tongue:
I’m wondering about two carbon rods?
How do you then go about skinning that?
Possibly thin balsa which then gets FG/Carbon (I’ts actually quite a nice idea for boats wanting a very exact shape with a wooden finish on the keel?)
Fill with styrofoam, sand, skin using Claudio’s clamped foam method?
It will need traditional finishing work but at least the foil shape should be quite exact.

I’m going to give it a bash but if you guys can see/think of any pitfalls I’m missing or possibly better methods let me know.


Looking at the pictures… I have some theory questions…

That planform is 60mm at the top and 40mm at the bottom, vertical trailing edge with a slightly swept back front angle.
Apparantly this works on plane wings to prevent stalling of the flaps by pushing the turbulent air to the tips. I could find nothing on this for keelfins. I figured that the rudder is closer to the top so pushing turbulent flow/vortices further down makes sense. Knowing my luck it should be the other way around…is there a correct way?
Would you put the rod running vertically down like I have it in the picture or running from the thickest section of the top foil to the thickest section of the bottom foil?
Do you ever make the top foil shape different to the bottom foil shape - eg NACA 0010 60mm chord top to NACA 0010 40mm chord at the bottom vs. NACA 0010 60mm chord at the top and NACA 0012 40mm chord at the bottom. (I’m just using those profiles as a reference) Is it better to keep them the same unless you are worried about the structural integrity of it getting too small?

Hi Andrew,
Using a tube it is necessary to maintain the same profile from Top to Bottom since the tube have the same diameter. The Fin has thus a rectangular shape.
If you whish a tapered form, then the tube should be also conical in order to keep the same profile % to allow the same flux speed and efficiency.
Actually I started designing and build a tool to make a rectangular Fin since I will use a normal carbon tube. The skins are prepared separately with a simple to make jig/tool that I will illustrate in a separated tread the Esterel 45".
All that aiming to get a low weight Fin.

The principle is illustrated in the figure


I would think you need to split each half again port - starboard. printers dont like shapes on the bottom, it prints on a flat bed. supports might be a pain as everything is so small at the trailing edge.

Hi guys,

Claudio - The tube in that pic’s diameter is decided by the bottom, thinner foil so the fin can still have the taper, but at the loss of strength. That’s why i wondered about adding two?

RGSailor (I don’t know your name -apologies) - Those pictures are only one half, so it’s flat on the bottom and their is no need for supports. I would then print the mirror of that and glue. With the Slic3r Prusa Edition I can keep the bottom portion with extremely thin layers and go to thicker layer heights fromhalfway up. Make sense?

Thanks :slight_smile:

Hi Andrew, probably I did not express myself well, what I was supposed to says is that tapering the Fin from top to bottom by keeping the same tube diameter you are forced to change the thickness “percentage” . I’m not good in aerodynamics, but I suspect that the change of profile ‘thickness’ along the Wing/Fin from top to bottom it may be not good for the performance /efficiency. see pics

Oh, I assumed we wanted a taper … oops. I read up quite a lot on wing design and the taper seemed to give benefits.
I wonder if the guys on the boat building forums ( full scale) will have an answer to that. I’ll see if I can find more info on it. I’ll probably need you guys to decipher it for me though lol.
All the aerodynamic info seems to jump from basic info to extremely advanced and far beyond my grasp of mathematics.

Perhaps a future test is on the cards :slight_smile: