Update 02-Aug-2015:
• Uploaded the smaller 3-blade propeller model seen in the video.
• Tweaked the coinweight models. The coinweight_threads part would sometimes split apart when inserting the coinweight_clamshell part. I beefed up the part that was breaking and left more room for the clamshell to be inserted.

This toy is a rubber band powered, propeller driven, underwater torpedo. Assemble the components, wind up the propeller, and see how far it goes underwater. Designed in re arrangeable sections that change the Toypedo's dynamics, the Toypedo is made for thought-provoking pool-time fun for kids. How far can you make it go?

Things kids can learn:
• How moving weight and buoyancy affects the travel arc. More buoyancy in the front will make it curve upward - and more weight in the back will do the same!
• Newton's third law - When the rubber band twists the propeller-end one direction, it also twists the other end the opposite direction. Easily illustrated by removing the fins; the back section will spin much faster. This is why fins are important!
• Inertia - Stack some coin-weight sections on the Toypedo and see how it affects the acceleration.

Reconfiguring ideas
• Swap out the fins for another backwards propeller (using the double-male to connect)
• Get a longer or shorted rubber band, add or remove hollow shaft sections as needed, and see what effect it has.
• Minimize weight, maximize buoyancy and aim the Toypedo upwards. Will it come rocketing out of the water like a killer whale, or will it not have enough velocity for a dramatic exit?
• Aiming the Toypedo upwards, see how much weight it takes to keep it from moving upwards when the propeller spins.
• Add buoyancy to the back, and see how much it curves down.
• Add weight to the back, and see how much it curves up.

Redesign Ideas
• Design your own optimized propeller
• Redesign the Body assembly to reduce drag.
• Design a shaft section with a big flat disk to demonstrate drag.
• Design a spring-driven system and get rid of the rubber band.

Small propeller test: https://www.youtube.com/watch?v=FBwe6xVVXEU
Large propeller test: https://www.youtube.com/watch?v=ITS9-C_qkNI

Other Thoughts
• My favorite thing about this toy is how modular it is. Anyone from the community can come up with ideas to improve the design, and because it's designed in sections, they can print and test their ideas without needing to reprint or redesign the whole thing. I'm eager to see what folks come up with!
• Note that there is a .zip file labeled "Gearassm". This file contains parts for a coil-spring powered version that uses bevel gears to reduce the gearing. Even with the current gearing, though, I found that the springs still acts too fast. When released, the propeller would go "Blrrrrp" for a fraction of a second before the springs finished their action. I do believe that this idea can still work, it just needs a lot more gear reduction to let the propeller spin longer. Due to time constraints, I've focused my efforts on the rubber band powered version.

Non-printed parts required:
• Rubber band. The one I'm using is 120mm diameter, 3x2mm cross-section. Experiment with other sizes! You can add or remove hollow sections to tune the tension to your rubber band.
• U.S. Pennies. Each coin-weight section holds 7. Recommended to have 14 pennies for 2 weighted sections.

Assembly instructions:

→ Body Assembly:
• Pass the Body_rubberband_loop part through the front-end shaft of the Body part.
• Screw the Body_male part to the Body_rubberband_loop part. These should spin easily in the shaft of the Body.
• Insert the Body_female_hollow part into the tail-end of the Body part, ensuring that the threading is facing the tail-end. Line the threaded holes of the Body_female_hollow part up with the holes in the Body part. Screw in the set screws.

→ Rubber band assembly:
• Unscrew the Body_rubberband_loop part from the body and pass the rubber band through its eyeloop. Loop the rubber band back around and through itself, and tighten it down. Pass the free end of the rubber band through the hole in the Body_female_hollow part and pull all the slack through. Pass the rubber band through hollow sections (screwing each one in after passing the band through) until you run out of room. Then hook the rubber band on the end of the Blank_section_hook part (note that the hook part has a thin section that needs to be cut away), and screw it in. Now reassemble the Body_rubberband_loop part to the body. The rubber band should have little or no slack.

→ Everything else assembly: Combine sections as you see fit. See what you can come up with!

Recommended print settings:

→ Material: I used PLA. Feel free to try other materials, and let me know how they work out.
→ z-res: 0.1mm for any threaded or gear parts. 0.2 can work, but the threading may be tight. For non-threaded parts, go with whatever suits your fancy.
→ Supports: For the propeller, I recommend using a raft with grid supports. For any other parts that need supports, use a raft with line supports.
→ Infill: I was using 20%. If you use more infill, it will mean that you need less pennies to weigh it down.

Part descriptions:

• Blank_Section_Hollow: Standard shaft component. Hollow to let rubber band pass through if desired.

• Blank_Section_Floater: Like Blank_Section_Hollow, but it balloons out to increase buoyancy. Also hollow for rubber band.

• Blank_Section_Hook: Shaft component that has a hook to attach the rubber band to.

• Coinweight_Threads: Shaft component that includes a housing for the coinweight_clamshell part. The coinweight parts hold up to 7 pennies, and they act as ballast to add weight where needed to balance the Toypedo.

• Coinweight_Clamshell: Housing that holds 7 pennies. Snaps inside the coinweight_threads parts. When clipped in, spin it open to insert or remove coins; to close, spin the other way until the clip snaps in securely.

•Body: The main function of this part is to allow the front-end of the Toypedo to spin freely while still keeping the axis aligned with the tail-end.

• Body_Male: Connects the body to the front-end. This part screws onto the body_rubberband_loop part through the shaft of the body.

• Body_Rubberband_Loop: Holds one end of the rubberband. To attach rubber band, feed one end of the rubber band through the eyeloop, and then feed the other end through the new loop formed by the rubber band.

• Body_Female_Hollow: Secures the body to the tail-end. Hollow for rubber band feeding.

• Body_Setscrew (x2): Fixes the body_female_hollow to the body.

• Baitboat_5blade_Propeller: 5 blade propeller. Fits on the front-end (or tail-end if connected with double_male).

• Fins: Help straighten the tractory, and helps keep the tail-end from spinning (which reduces number of turns you get at the propeller).

• DoubleMale: Used for fixing propeller to the tail-end.