| Human Powered Boat Propellers |
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By Warren Beauchamp
1/05/07 |
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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. |
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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. |
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This simple change will
result in a 10% increase in efficiency, which will result in about
another 2 lb/ft in thrust. |
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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. |
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| 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. |
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| 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. |
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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. |
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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.
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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. |
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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. |
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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.
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| 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. |
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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. |
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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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