Human Powered Boat (HPB) Propeller design
Human Powered Boat Propellers

By Warren Beauchamp  1/05/07

Model airplane propellers are the most common and easiest to acquire human powered boat propellers. The APC fiberglass props such at the 16x16 model and 14x14 models are well sized for the power ranges and speeds encountered in HPB applications. Purpose built HPB props like the Wavebike props and the Jake Free props are more efficient, but are harder to come by. 

Rick Willoughby has designed several iterations of propellers using stainless steel bar stock, which have worked quite well for him, and he has kindly shared what he has learned with me. His latest props were designed with the aid of a web-based application called Java Prop These props have the potential to be much more efficient (faster!) than the generic airplane props, and are inexpensive to build.

You could build a basic 16" HPB prop from 1.5" x 1/8" steel bar stock, by simply welding the blades onto the hub, twisting them to approximate a prop shape, and then rounding the edges. It would work a lot better than a powerboat prop, but still be far from optimal. 

By using 1.5" x 1/4" bar stock, and spending some quality time with a bench grinder, you could create prop blades with an E193 foil shape as in the drawing below.

This simple change will result in a 10% increase in efficiency, which will result in about another 2 lb/ft in thrust.
This MA409 foil is even more efficient, but a bit harder to create from a flat bar.
For even greater efficiency (speed!), you will need to do twist the blade with specific angles at specific distances from the hub (radius). Also you will need to cut the width of the blade (chord), to specific widths. One way you can do this is by loading a settings file which is already set up for HPBs into JavaProp. JavaProp will tell you the radii, chords,  and blade angle.
This picture, which was generated by JavaProp shows a 16" prop. 

To replicate this design, first download this settings file by right-clicking on the link and saving it to your PC. Remove the .txt from the end to make it easier to find.

There are a whole bunch of water prop specific prop design parameters that are in the settings file, but I'm going to try to keep this simple. Once you have loaded the settings into JavaProp and played with it a while it will become easy.

Next start the JavaProp application. Once you get it running, use the "Load..." button on the "Options" tab to pre-load the proper settings. Click the "Design it!" button on the "Design" tab to run the simulation. You will see a plot like the one above, along with a table of the design parameters on the "Geometry" tab.
You can then play with the options on the "Design" page to see how they change the characteristics of the prop.

Note that the "Velocity of Rotation" is RPM. A velocity of 2.7m/s is 10kph. To optimize your design, you can use the calculators on this page to figure out what your hull or sprint speeds are.

Increasing the length of the prop from 16" to 20" improves the efficiency by 2%, but the blades start getting very thin which may allow them to be too flexy. May not be worthwhile.

Alternatively, you can just use the 16" prop that Rick designed for me. He has provided this table showing pitch angles for the various radii. Note that the rate of pitch change changes about half way down the prop blade. Rick has run simulations that say this is more efficient. You can click on the plot to see it bigger.

Here's a text file with all the radii, chord and pitch data for this prop. The prop detailed in the text file is intended to be an easy one to make. Here's another for a 18" prop.


Rick has provided very nice directions to make the prop, and I have edited them.
  • You will need to cut two pieces of  steel 180mm (7") long.
  • Mark off a line at 85mm from the inside.
  • Place the inside end in the vise and twist 33 degrees using a wrench placed at the line.
  • Place the outside end in the vise and  twist 15 degrees using a wrench placed at the line.
  • The resulting blade will have a total twist of 48 degrees.
     
  • Cut the blade chord widths as noted. From looking at the plots above it appears that 1/3 of the chord width is toward the leading edge of the blade, and 2/3 of the width is toward the trailing edge.
  • Shape the blades to approximate the E193 or MA409 foil shape with an angle or bench grinder.
     
  • Weld the blade to the hub. The base of the blade should be held at 67 degrees to the line of the shaft.
  •  A single pitch block or suitable fixturing will ensure that the blade stays in place while welding. Cory has provided very good pictures on twisting the blades and using the single pitch block.


The most important aspect on DESIGNING/making a prop is matching the prop to your application.  A perfectly made prop can be an absolute dog if it is not matched to the application. 

 

Here are some views of the prop for Ricks V11 HPB.
1/15/07
I'm building the 18" prop that Rick sent me the data file for. I picked up a 3 foot length of 1.5" x 1/8" weldable steel at the hardware store to make the blades out of. I then drew a line at 1/2" from the edge down the length of the steel to mark the 1/3 line. After marking the radius tick marks, I figured out the lengths for 1/3 and 2/3 of the chord width for each radius, and marked them on the blade. 

Here's the blade, ready to cut out.

After a little quality time with the jigsaw, and some touchup with a file, one of the blades is born. I used the new blade as a template for the second blade, and cut it out.

You can see that the trailing edge of the blade has more of a curve than the leading edge, due to the 1/3 - 2/3 measurements.

I twisted the blades according to Rick's instructions above. I kept the Seawind prop handy to make sure I made the twist in the proper direction. I will need to reinforce the blades down by the hub as they are pretty long and skinny.

This picture shows the blades next to the Seawind prop. They will be a couple inches longer, but much narrower. It will be interesting to see how they perform. I now need to locate some suitable material to make the hub with.

3/14/07
It's finally nice enough weather to braze outside. I jigged up the blades using the usual assortment of magnets and clamps, making sure to mount the blades at 18 degrees, and in the proper orientation.

 
Once they were brazed, I cut out small rectangles of steel to reinforce the root of the blades. This will stiffen them immensely. Note that the stiffeners are attached to the "top" of the wing section.  I'll braze these on and then start shaping the leading and trailing edges of the blades.

I acquired some 1/4" steel bar stock for propeller version 2. This will allow me to better shape the airfoil.

3/18/07
I brazed the stiffeners onto the prop root, as well as a nut to clamp the prop to the driveshaft. Next it was time to get out the angle grinder. I used it to make the leading edge rounded, and to shave down the top side of the trailing edge to simulate a wind shape. This prop will be little more than a flat blade prop with shaped edges.
Here's the prop on a Seawind drive unit. I still need to fashion some type of fairing for the prop.
I painted the prop just to see what it would look like. Not too bad. As this is the version 1.0 prop, I'm not going to worry about the prop fairing until I know if it's going to be useable.

I'm going to be building a 20 inch prop next, which will have a better airfoil contour.

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