For fun, I wanted to custom design a propeller so that I could effectively match it to the hydrodynamic characteristics of the AUV and a drive train to try and improve efficiency. Luckily, I found a design tool which makes the propeller design easy — MH Aerotool’s JavaProp. Although the JavaProp is meant for airplane propeller design, it’s easy to change the medium parameters by inputting the density, kinematic viscosity, and speed of sound for water in the options tab to produce designs suitable for usage in water.
Recall previous posts where I used OpenFOAM to analyze the flow around the AUV hull to determine the drag and velocity profile in the propeller disc. I can input these parameters into JavaProp to tailor the design to my AUV! Granted the flow profile is a fairly simplistic linear input with 2 parameters, it’s probably good enough for my purposes. Inputting the drag and dimensions into JavaProp is straightforward. For a preliminary cut, I chose the following parameters:
Clicking “Design It” gives me a quick design. After some modifications, the rough design gives me about 76% efficiency, and that’s without really thinking too much into it. Lots of information is generated as well to better understand the propeller’s performance.
The best part, however, is that it outputs the optimum propeller geometry given a selection of profiles for different points along the propeller:
The awesome part is that you can export the profiles as a surface to manipulate in external programs – Through some CAD elbow grease with your program of choice, it’s possible to convert the surface into a complete propeller. One caveat, the trailing edge needs to be thickened to be manufacturable later, but I accomplished this easily with the custom settings available in JavaProp.
The 3D prop was created in OnShape by intersecting the exported surface along a series of planes, which creates a series of profiles used for a loft feature for a single blade. From there it’s just a matter of rotation pattern feature to make three blades, adding a hub, and adding some fillets at the root.
Shapeways has recently introduced a trial of the HP Jet Fusion 3D printer, so I figured that would be a good material to do a trial print in. The results were surprisingly good! I had originally planned to smoothen the propeller out and use it to make a plug to create a mold, allowing me to cast propellers out of a much stronger urethane material. Although not smooth enough to be a perfect propeller, this new material may be tough enough to use the water itself!
The keen eye will notice that I didn’t print the specific design shown above — I actually 3D printed an earlier revision, which didn’t have an adjusted flow profile. Once adjusting the flow profile for reduced flow near the root of the blade, the chord of the profile at that point increased to compensate.
Next steps will be to experiment with different ways to smoothen and stiffen the propeller. Either filling and sanding or adding a very thin layer of fiberglass tissue and epoxy could work. Either way, 3D printing propellers seems very feasible, and a very reasonable cost. The best part is, I can experiment with different ways to optimize and easily print multiple variations to actually test.
One unknown, however, is how the print will deal with pressure if used directly at the AUV’s test depth — Any porosity may cause problems so some testing will be required before putting it into use.