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Home made prop shafts and props for the model boat builder

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I was having an email conversation with a friend last week about model boat building as he is in the process of building from scratch.  We got to talking about running gear and I suggested he build his own, not realising he had.  It got me thinking, I haven't made a prop shaft in over 30yrs.  At that time I was building model boats and wanted to make my own shafts, rudders etc, using machine tools.  I thought, lets have a go with what is in the workshop now.  I found some brass bar, 1/8" stainless tig rods and carbon fibre tube (kite/arrow shaft), no brass tube.

One of the problems with off the shelf carbon tube other than it is mostly pulverised fibre, the bore of the tube is never concentric.  You can buy wound tube, and that is also something I would like to try making at some point.  After cutting the tube, it needed boring to open out for the bearings, this was done with a carbide burr, note the paper towel to collect the dust, nasty stuff.  I dab the ends in Superglue to lock together any fibre that might have started to come loose after machining.  Ideally a close fitting collet is the best way to hold these, but as I am just playing the ER collet was good enough.

For the prop shaft I cut up some 1/8" stainless tig filler rod, I was very surprised how concentric this was with very good surface finish.  First job get the end to be machined red hot with a blow torch, make sure the stainless is annealed.  I could now turn the shaft end carefully down to 3mm and die thread the end.  By the time you finish this the stainless is starting to work harden, actually useful at this point, grind a lead on the shaft at both ends and it's done.

Next up was the bearings, I had intended to use phosphor bronze, but I would have needed to cut up some new stock and for playing brass would do as my stock matched the carbon tube at 6mm diameter.  The bearings are machined, drilled and reamed +0.002" oversize.  I just happened to have a suitable reamer.  The bearings have some small grooves on them to take the glue, or in this case Loctite used to bond them into the prop tube.  Use the shaft to keep the bearings lined up.  The result after 30 mins, a finished prop shaft with smooth action.  I did the bearings different lengths so you can distinguish which end is which as the prop has more support than the coupling end.

I made some Acetal thrust washers and brass collet to complete the shaft.  I enjoyed making the shaft so much I made a second one as they don't take too long to make.

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.

Nice work!  :beer:

The propeller looks like it would make a fine mold for casting  :dremel:

Between work and other jobs, I have been having a bit of a prop printing frenzy.  Printing out propellers and testing different methods of finish on them.  They also get water tested to see how they perform and hold up.  The larger 55mm 2 blade props seem OK, less than 1mm blade thickness and they start to flex quite a bit.  PlastiKote metallic paint does a convincing job of making them look the part.  Not bad for FDM prints, got to wait a while longer for the resin printed versions for testing.


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