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.