One thing I’ve noticed on Foil Zone is that there doesn’t seem to be much experimentation with home-built three-bladed folding props, especially 3D printed ones.
So I decided to have a go at making one.
I started with blade geometry based around the Wageningen propeller tools, then designed a folding three-blade hub around it.
The folding requirement changes the blade design quite a bit. With three blades, they need to be narrow enough to nest together cleanly when folded, but I also wanted the roots to be very substantial because that is where most of the mechanical load is going to end up.
The blade roots have generous fillets and quite a lot of material around the pivot area. The deployed load is also intended to transfer through the root/stop area rather than relying entirely on the pivot pin.
The photos here are only PLA development prints. They’re just for checking geometry, clearances and the folding mechanism.
The real version will be printed much more heavily in PET-CF, with plenty of wall thickness and a very dense root and pivot section.
The nice thing about getting the folding hub sorted first is that I can then experiment fairly easily with:
diameter
pitch
blade area
blade profile
root shape
possibly different materials and print orientations
The three blades now fold together quite neatly, which was the part I thought might become awkward.
There’s still some work to do before I’d trust it at several thousand RPM in the water, but so far this has been a really fun little side project.
Thanks Jan! I appreciate the feedback, but I think you might be thinking of standard, flexible PETG, or perhaps standard PLA-CF where the chopped fibers can sometimes hurt Z-layer adhesion.
Pure engineering-grade PET-CF is a completely different class of material. Because this folding prop will face heavy hydrodynamic loading and high centripetal forces right at the pivot pins and deployment stops, PLA is highly susceptible to mechanical creep and fatigue. Under sustained high torque, PLA permanently deforms and stretches over time, which causes blades to lose their pitch or fail at the root.
When printed properly—especially inside an actively heated chamber which maximizes interlayer bonding and crystallization kinetics—PET-CF handles these stresses significantly better:
Stiffness (Flexural Modulus): PET-CF sits around ~5,300 MPa compared to PLA’s ~2,500 MPa, preventing the blades from flexing and losing efficiency under load.
Ultimate Strength: It features an ultimate tensile strength of ~74 MPa and a flexural strength of ~131 MPa, giving it the structural integrity needed for a high-RPM hub.
Environmental Stability: Unlike PLA, which absorbs water and suffers from hydrolytic degradation over time under dynamic marine stress, PET-CF has a saturated water absorption rate of under 0.4%, maintaining its strength underwater.
The PLA prints are purely a quick, cheap way for me to validate the nesting geometry, clearances, and deployment stops before committing the expensive carbon composite filament to the final version!
Thanks for the summary.
I have done my number of printed props and used them on the water. They all fail sooner or later along the layer lines. But it’s your choice.
If we want to share AI stories, this is mine.
If you mean Z-direction / inter-layer strength, PLA is actually a strong baseline.
A useful rule of thumb is:
PETG (plain) > PLA ≈ PETG-CF > PET-CF for pure layer bonding, although the exact order can change a lot with brand and print temperature.
*The key point is that carbon fiber usually hurts inter-layer adhesion somewhat. The fibers greatly increase stiffness in the XY plane, but they don’t bridge layers effectively, and the filled polymer has less ability to diffuse across the layer boundary. Prusa explicitly notes that its PETG-CF has lower toughness than normal PETG. *
So compared with normal PLA:
PLA: generally very good layer fusion when printed hot enough, but the resulting part is relatively brittle.
PETG-CF: can have good layer adhesion and usually tolerates deformation better than PLA, but CF reduces bonding versus unfilled PETG.
PET-CF: much stiffer and stronger in-plane than PLA, but Z-strength is often the weak point. It benefits significantly from higher nozzle temperature, low cooling, slow outer walls, and very dry filament.
This is why something like PET-CF can have dramatically higher XY bending strength and stiffness than PLA—Bambu lists roughly 131 MPa vs 76 MPa bending strength and 5.32 vs 2.75 GPa bending modulus—without necessarily being stronger when you try to split the print along the layer lines.
