3d printing small props

Discussion in 'Boat Design' started by jakeeeef, Jun 6, 2022.

  1. jakeeeef
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    jakeeeef Senior Member

    I'm experimenting at the moment with a small outboard, a mercury single cylinder 2.2 hp, on a 2.4 m inflatable zodiac tender. I've derestricted the nominal 2.2 to nominal 3.3hp by the well documented removal of the restrictor washer behind the carb.
    I'm now experimenting with various lift surfaces under the boat.
    The next issue I will run into as I reduce the hull drag is propeller pitch. As this engine will be designed to push boats at 3 or 4 knots.

    Do 3D design and printing companies exist that can:
    *Scan the hub part of my prop
    *Tweak the pitch and blade shape (my blades are horrible weed resistant shape which was a selling point of the motor back in the day)
    *Print a prop, or a selection of props with different higher pitches and more efficient blade shapes to fit my engine.
    *Out of a reinforced plastic material that will be be strong enough.

    This last one might be surmounted more cheaply by getting a blank for the prop printed undersize so I can vacuum bag the outside of the prop in carbon and epoxy.

    I would imagine that the powerboat racing people would be into 3d printing props to experiment and rapidly tweak and optimise pitches and blade shapes, but maybe the strength of commonly available printing plastics just isn't there yet?
     
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  2. kerosene
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    kerosene Senior Member

    I am keenly waiting for other people to react. I have been thinking of this but for a lower power situation - a trolling motor in 0.8-1.5hp range (input power I guess so shaft up to about 1hp).
    My idea was to print but leave "channels", recessed bands, that would be easy to fill with a laminated composite tape.

    javaprop is very cool and recommended too for seeking the optimal prop - but you still need 3D wizardry to get a print file.
    Rick (not anymore a member but his posts are up) made very efficient props by simply bending, welding and filing. Try to search from this thread:

    Pedal Powered Boats https://www.boatdesign.net/threads/pedal-powered-boats.23345/
     
  3. messabout
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    messabout Senior Member

    I have a 2.2 Mercury, perhaps similar, or the same as you describe. My little engine is probably a rebranded Tohatsu because its details suggest that it is of Japanese origin. It starts and runs very well. The downside is that it is a single cylinder vibrating machine. It is particularly vibratory at low RPMs. It soothes out a bit when running faster. I have not removed the restrictor washer behind the carb.

    The prop is the original one. This little engine will actually plane my 15 ft 6 inch flat bottom skiff.. It can drive the boat at about 12.5 to 13.5 MPH. The full weight of the boat, motor, me, and a few assorted things like paddles, is 330 pounds. The speed has been measured only on moderate to light air days with only a modest chop on the water surface.

    I can think of no overwhelming reason to change the prop. Have you tried your engine with the stock prop?
     
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  4. jehardiman
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    jehardiman Senior Member

    I'm not sure a 3-d printed PVA or "reinforced plastic" prop will have sufficient strength; especially if "higher pitched". As Messabout points out, 2.2 hp is more than enough for a small zodiac tender. That motor should drive a "2.4 m inflatable zodiac tender" much more than "3 to 4 knots". Now using it as a yawl boat for a larger vessel ..... that is a different matter.
     
  5. jakeeeef
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    jakeeeef Senior Member

    Not yet. I've only so far tried the stock prop under the standard inflatable with no planing surface underneath- just to get a baseline number. 4.5 knots, but with a huge white sternwave behind that I will soon be getting rid of. I'd be concerned about the RPM that yours is running at 13.5mph. I've got a little tacho I can rig up on it.
    Once your up and planing on a very low resistance hull, I'd imagine RPM might become the limiting factor.
    I've devised a way of rigging a low water pickup very easily, have rigged for a fair bit of setback and, I'm thinking ahead here, but would ultimately like to run with just the bottom blades in- which would require the appropriate blade shape. So, go from my 4.5 knots baseline to... who knows what!
     
  6. baeckmo
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    baeckmo Hydrodynamics

    We routinely use 3D printed impellers and housings for prototype testing of industrial pumps. Just recently run a printed plastic impeller, od about 450 mm with a power input of roughly 300 kW at 1750 rpm without problems. The present unknown is stability over time, like creep and chemical degradation, but for pleasure boating equipment I don't see any serious limitations.
     
  7. jehardiman
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    jehardiman Senior Member

    300 kW? 400 hp? I've done sintered metal that high, but not even carbon reinforced epoxy has held up otherwise. Just a touch and it turns into a pom-pom.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Yes, we were starting with reduced rpms since we were not convinced about the strength, but after complete performance mapping at low rpms, we decided to test to the limit. No drama at all, to everybody's surprise. Now this doesn't mean that It would stand continuous load at that stress level, but as a "plastic sceptic" I have to rethink......
     
  9. jehardiman
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    jehardiman Senior Member

    Ok, clean water prototype testing for pumps I can see going that powerful. And for sub ~5hp I could also see a 3-d printed ABS or PVA if properly UV protected. ( Of course most mid range outboards can now be found with injection molded FRP props, but that's way beyond most homebuilders.) But I'd still worry about debris and shallow water. It only takes a look at the USS NIPSIC wheel to know why NiAlBronze is the preferred open wheel material.
     
  10. jakeeeef
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    jakeeeef Senior Member

    I'm not bothered about debris or shallow water. Happy for it to fail if I'm daft enough to hit the bottom with it. This is for testing and prototyping. What material, process and fill were you using for the pump impellers Baeckmo?
    I don't know anything about 3D printing yet, but I've just joined a local hacker space that has a bunch of 3D printers and the software and people who can teach me how to use it, but don't know what they are, what their capabilities are, how to use them yet. Steep learning curve ahead. They also have casting equipment, a 3 axis CNC milling machine, that can mill things like a cylinder head, so shouldn't struggle too much with a prop. But I still like the idea of 3D printing props so I can run out 4 or 5 of them at different pitches for comparison. Maybe play with blade shapes too. Then once I've got the one I want, maybe cast it or CNC mill it?
     
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  11. mc_rash
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    mc_rash Senior Member

    I don't want to stop you from 3D-printing a prop. Actually I really would like to see the results how it worked and what the behavior of the prop is in water.

    I own a Ender 3 which prints pretty well with PLA filament but there is not much force required to break printed parts in direction of the layers. Sure there are other printers (resin,...)/ print techniques/ print settings which could lead to a better, stronger print but I guess it would be easier to 3D print a prop and use it to make a mold for casting a metal prop. Except you have access to knowlegde/skills/industrial printers, maybe even a metal 3D printer.

    But all in all I would like to hear your experiences 3D printing and testing propellers!
     
  12. mc_rash
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    mc_rash Senior Member

    The prop will experience different kind of stresses (bending, twisting, vibrations, accelerations,...) and a "normal" PLA-printed prop will not be able to withstand this forces. Strengthening with Epoxy might delay the destructive process.
     
  13. baeckmo
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    baeckmo Hydrodynamics

    The pump impellers we use for testing are printed with an off-the-shelf polyamide plus glass. But the hub design is quite careful in order to avoid "hard points" though.
     
  14. Skyak
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    Skyak Senior Member

    This is not an extrapolation. It has been done with some success and failure. The first thing is to make sure you actually need something outside of what is conventionally manufactured -mass production outboards tend to have a comprehensive selection of relevant pitches.
    When you have done the prop selection work and determined what you need, and that it doesn't exist, you need a model. I doubt you need to do any scanning, the hub can just be directly modeled. You might find someone already posted one on thing verse, or you will meet capable modelers at your hacker space, or there are loads of contract modelers. Note that the blades are a free form but mathematically precise. There are people who don't know they are not capable of that. If you don't find a local I would go to the software user blogs -fusion 360 has loads of capable users.
    When you have the model (preferably parametric) you can put out an stl file and shop for printing. The common consumer 3D print is "fused deposition" which has a grain to it and needs a support structure. Filled plastics are stiffer and stronger but not between layers. Polycarbonate is my favored premium material for good layer strength, stiffness, high temp, and ductile failure.
    It might be easier to just get an SLS (selective Laser Sintering) part from one of the big service bureaus. SLS needs no support structure, gets about 90% of plastic strength in all directions -but no long fillers). If unfilled plastic has sufficient properties, SLS can make your part. If plastic doesn't have the properties for your part, there are expensive direct metal printing, and there are "lost wax" style casting options of high quality.
     

  15. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    I would first investigate if Mercury has any other props available.
     
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