Once the shafts got made, well..errrr you need some propellers. I could turn a hub and cut up some brass sheet to solder in the blades. I hated those style props in my youth, they looked horrible on a scale boat. I figured now was a good time to learn how to design proper marine propellers.
First job look at a commercial prop 3D model, all I am after is the blade outline. No point reinventing the wheel, I projected the blade outline on the disk in the background. Next I had to decide the pitch I would like for the prop, in CAD I can specify a helix to match my pitch. I sweep the helix with a line to create a shell, this shell now has the blade outline projected on the shell. Cut the shell away leaving the blade and array it 3 times to generate the 3 blades. I am also able to unroll a single blade, this could then be cut out of brass sheet, rolled and soldered onto a hub.
A hub needed adding to my blades and this is a simple tube. The blades at the moment are just shells, they need to be built up into real world thickness, so they can be made. I could offset the surface to produce a specified blade thickness and add some shape to the outer edges. Not much better than sheet blades. I wanted to have nice fillets in the roots of the blades, so these needed modelling. Another aim was to also design blades with proper form, thick roots tapering away to the tip and edges. That is the modelled blade on the right of the three.
Well they had to be 3D printed didn't they. The first blade printed was 30mm and came out awful, in part due to the nozzle size and also the design. You can see the step layers and poor edge finish. I thought oh, this is going to be bad for the tapered blade, so some more work was done refining the printing process. Success, with the right support and an increase in blade size to 35mm I got improved surface finish.
The printed props got tapped so they locked better onto the shaft, forgetting its a right hand tap and right hand prop. It's very simple in CAD to mirror my design for a left had prop that won't try to unscrew in forward motion. A shorter hub has also been designed so a stainless 3mm nyloc nut can hold them on.
The edges of the prop just need a light sanding to neaten up the edge. These aren't strong propellers, suitable for displacement models, you would be pushing your luck on fast electrics, but if they were printed in PETG they might work, but surface finish would be an issue. Some DLP resin printed versions are in the works, so see how they turn out.
I hope you find this interesting, seeing my journey in learning how to make something I have used for as long as I remember without ever really understanding what went into the design of a marine propeller. Hydrodynamics, like aerodynamics is a fascinating subject.
