Here, you can see how it performs; It shot down the 12' course in about 2 seconds. That's roughly 4 miles/hour, or 3.5 knots!
Step 1: Materials and Tools
This is a 3D printing project, so I needed access to a 3D printer. I ordered my prints from local makers on 3D Hubs.
- T-pin, 2 inches
- Model boat propeller
- High stretch rubber bands or strings
- Rare earth magnets
- A file and sand paper, about 150 grit
- Model boat bearings or beads
- (optional) Pour-on epoxy
- 3D modeling software. This instruction uses Autodesk Fusion 360
- Needle Nose Pliers
- Dial caliper
- Work gloves and eye protection
For epoxy pour-on coating: brush, disposable measuring cups, a piece of plywood larger than the boat, screws.
Warning: Note that as the boat's performance improves, the high speed propeller becomes more dangerous. The propeller can slice your fingers at the rate of dozens per second at the same spot! Wear safety gloves and eye protections!
Warning: Do not let children play with the propeller. This is not a toy. But, if your neighborhood kids beg nonstop and create chaos in your garage shop, one loop of measly 1/16" thin rubber might not hurt them too badly, as long as they wear thick gloves at all time.
Step 2: Boat Design
The goal is to use less than 8 inches of rubber to travel 12' of water. I decided to use a simple, proven designs and made iterations to improve its performance.
Hull Type: I chose the catamaran design for maximized stability. This design also allow room for rubber band axle to go full length between the hulls to maximize performance.
Buoyancy and weight: I use Autodesk Fusion 360 to estimate the weight and volume and calculate whether the model has enough buoyancy.
Strength: The boat has to withstand frequent impacts with the wooden wall of the moat.
Step 3: Select a Propeller
I use propeller instead of paddle because propeller designs are inherently far superior. Propeller design is an advanced art and science on its own, which I decided not to tackle. I didn't print the propeller myself because they need to be perfectly polished to be effective. So I used some off the shelve model plane propellers for this project.
However, a rubber band propeller needs to be different from electric or gas motor propeller because rubber bands have limited rotations. Here are a few important features when choosing a propeller:
Size and direction: larger diameter propellers move more water, but it also causes instability(propeller walk). Left hand propellers, for example, causes the boat to turn right. The stronger the torque, the sharper the boat turns.
Note: One way to mitigate this problem is to use a two propeller, counter rotating design. But it increases the complexity dramatically. Another way is to use a duct.
Blade pitch: Since there are limited rotations for each rubber band, I need a propeller to travel further with less rotations. This means choose a propeller with a large blade pitch.
Zippercraft used a model boat propeller with pitch 1.6" - that's how far one rotation travels in perfect condition. So, in theory, it should only require only 90 rotations. (length / pitch = 144" / 1.6") However, during the race, I needed about 120 rotations to make it reliably make 12ft. This overhead is slippage and delayed release, more on that later.
Number of Blades: Propellers with more blades have more blade surface area, so they can accelerate faster, but they also have more drag resulting in lower top speed. My tests show that very fast accelerations could be detrimental to the boat's stability. Zippercraft's propeller had three blades.
Leading Edges: An efficient propeller needs sharp leading edges. Before each session, I would sand the propeller blades to ensure the edges are sharp.
Step 4: Model the Deck
To make the deck, draw a rectangle on the XY plane the size of the boat, and extrude downward 1.2mm.
Step 5: Model the Center Beam
To draw the center beam that holds the rubber band:
- Turn to the YZ plane and draw a rectangle. The length of the boat body and the depth of the center beam.
- Draw a spline to carve out the rectangle you created. This will be the space for the rubber band. As pictured.
- Extrude the top portion of the drawing to create the body of the center beam. The extrusion should be symmetrical to the origin plane.
Now you have the deck and the center beam that holds the rubber band.
Step 6: Model Propeller Axle and Bearing
To minimize friction, use a small metal axle. I used a 2" T-pin as the propeller axle. I cut a slot in the tail of the propeller to secure it on the T-pin's head.
Tip: If you can't find a T-pin long enough, you can use a piano wire.
The bearing's job is to permit the axel to rotate freely. This means having minimum contact with the hub of the propeller. Here are a few options:
- The Zippercraft uses an off the shelf model boat bearing,
- Beads work fine, too. Like that big pearl earing nobody is using.
- Print your own bearing
- Draw the bearing profile as shown.
- Use create - revolve to model the bearing body
If you print your own bearing, make sure the inside is smooth. I use a drill.
Step 7: Model the Propeller Holder
Now the propeller holder at the end of the center beam,
- Create a drawing at the end of the beam, perpendicular to both the deck and the center beam.
- Draw two concentral circles. The inner circle diameter is the size of the bearing plus a bit of allowance.
- (Optional) Draw a rectangle from the top of the beam to the top of the outer circle you just created. This is a pole for extra strength to hold the propeller.
- Now you have the drawing, extrude the hoop between the circles, thickness should make it strong enough to withstand the strong force of rubber bands.
You now have the rear of the rubber band assemble.
Step 8: Model the Front Rubber Band Hook
Now create the hook to secure the rubber band in the other end. To maximize the length of the rubber band, put the hook right at the front of the deck.
- Go back to the center beam drawing, and add a vertical rectangle about 5mm from the front of the deck. This is the stem of the rubber band hook and its reinforcement.
- Add another rectangle at the bottom of the previously created beam and extend it to the very front of the deck. This is the lower half of the rubber band hook.
- Tip: make sure the hook is the same Z axis depth as the bearing, so that your propeller would point parallel to the water surface.
- Extrude this symmetrically in both directions, about 5mm to create the front rubber band hook.
Step 9: Model the Pontoons
I used the loft tool to create the hulls. The pontoons were made with free hand drawings roughly resemble the shape of working catamarans. See the drawings first:
- Create a plane on the front of the deck, perpendicular to both the deck and the center beam.
- One the plane you just created, draw the profile shape of the pontoon's nose. Here I simply drew a thin, horizontal line.
- On the same plane of the bearing holder, draw the rear profile shape. Here I free hand drew a spline roughly a smoothed out upside down triangle.
- Now you have the profiles of the front and rear of the pontoons. you can add another profile between these two planes.
- Create a new plane between the two previous planes (between front and back of the pontoon)
- On this new plane, draw and intermediate profile shape for the pontoon. Here I drew it just bigger than the rear pontoon and roughly the same shape.
- Now you have three parallel profile shapes of the pontoon, use the loft tool to interpolate them to form the body of the pontoon.
- Select all three planes in order
- Use the loft tool to create a new body.
- To create the body of the other pontoon, use the mirror tool to mirror via the center plane.
Step 10: Resize the Deck
The newly created pontoons left part of the deck hanging outside of the boat. Use pull/push tool to remove the excess, but ensure the deck is still securely attached to the deck.
Now the boat's main body is done!
Step 11: Add Fillets
To strengthen the model and make it easier to print and reduce drag, Add fillets everywhere to remove sharp edges. Except of the rear face of the pontoon.
Tip: take special care to fillet near the bearings. You want to maximize the fillet diameter to strengthen it.
Step 12: Estimate Weight and Buoyancy
To determine whether the boat will float. I used Autocad Fusion's feature:
- Select the body to estimate in the browser
- Right click and choose "Properties" and a new dialogue box shows the volume, weight, etc. You might want to change the material to get the right estimate.
- Calculate Buoyancy: 1 liter can support 1 Kg. I used an online calculator.
- Make the weigh of the boat about half of the buoyancy.
If you don't have enough buoyancy, go back to change the shape of the pontoons. In this picture, you can see the shape is enlarged for testing.
Step 13: 3D Print the Boat
Zippercraft was made of PLA, and it seemed to be strong enough to withstand smashing against the walls of the wooden moat during the competition. It was printed with the boat nose sticking upward on Z axis.
If you don't own a 3D printer, find a good 3D hub for the job like I did; I have also attached the .stl file at the end of this instructable.
Step 14: Cleaning and Coating the 3D Print
To reduce drag, remove 3D printing artifacts and coat the boat.
- Remove support structures cleanly
- File off any rafts, skirts and brims on the first layer of the print
- Use sand paper to smooth out sharp edges
- I used pour-on epoxy coating to smooth out the bottom of the boat, as pictured,
- Use a plank as the floor to catch drips
- Drill screws pointing to the top, this is the support for the boat during drying
- Place the boat body bottom up, resting on the screws from the previous step
- Follow the product instructions, mix and pour epoxy coating onto the boat body and let dry
- If you find the top coat isn't enough, brush on extra layers
It should dry up very smooth in high gloss. Run your finger on the bottom of the boat, and you shouldn't feel the layers of 3D printing anymore.
Step 15: Choose the Rubber
Not all rubber bands are created equal. A propeller design requires far more rotations than a paddle design. In this contest, the boat needed to travel 12' to the goal line and fit into 6.5" of space. This means the rubber band needs to be as stretchy as possible, so you should choose one with a higher natural rubber content. This type of rubber band tend to be very soft and pale. I used FAI Tan Super Sports Rubber used in rubber powered plane competitions.
The width of rubber is a critical factor, too. More on that later.
Step 16: Install the Propeller
To make the propeller axle, I used a 2" T-pin, bent to the shape of a hook. Also, the head of the T-pin must hold on to the propeller, so I cut a small slot to fit it.
Step 17: Wind and Release
For competition, winding and releasing a high performing rubber band propeller requires a lot of practice. It flips over; it turns back; it hits things; it runs away into the middle of the lake, etc.
Here are some tips:
- Winding the propeller with high powered rubber can be very dangerous; you want to wear gloves. In one of my boats, I modeled a propeller holder to help holding wound up rubber.
- The distance to travel is mostly related to the number of rotations on the propeller, so you should practice to find out how many rotation is takes to reach the goal.
- When you have an estimate of the number of rotation needed, add loops of rubber to maximize strength. The more rubber, the less rotation you can get., but the boat runs faster. Your goal is to optimize the performance while making the goal distance.
Reference: Zippercraft used 5 loops of 1/8" rubber, 120 propeller rotations.
- You will notice that the torque right after you release the propeller is very strong, causing the boat to flip over or turn the boat back. To avoid this problem, after the propeller is released, I hold the boat for a split second to stabilize it before releasing the boat.
- You might need to add some weight to the left and back of the boat. I used a stack of rare early magnets. This allows me to adjust weight incrementally.
Here's a video showing the delayed release:
Here's a video of practice run:
Step 18: Improve!
Congratulations! Now you have something that, hopefully, moves on water. Time to make it better.
The Zippercraft project went through 4 iterations in total. Using local 3d Hubs allowed me half day turn around, very useful for fixing problems and improving performance.
(I have attached the model STL file in mm.)