For your education and entertainment, here's a rubber band powered submarine that's easy to assemble and might even teach kids a couple things about buoyancy and marine engineering.
You'll need to print two other (really awesome) models for this:
Propeller, a terrific marine-style propeller which works great and doesn't need supports
The amazing 608 Bearing, which is all-plastic (and thus won't rust or contaminate the water with grease)
- OR, if you don't have Airsoft BBs for the above, you can use this 100% printed 608 bearing: https://www.thingiverse.com/thing:3002837 (I haven't tried this personally, let me know how this works for you if you do).
- Print one of each part (hull, nose, aft and driveshaft).
- Glue the propeller halves onto the driveshaft - make sure the propeller is facing the correct way, with the curved edges of the props toward the hull. The propeller halves should be flush against the cone at the end of the driveshaft.
- Assemble the 608 bearing and glue it into the aft section - make sure you press it all the way down into the cavity for it.
- Insert the driveshaft into the bearing until it stops.
- Loop a paperclip through the hole in the end of the driveshaft (this might be tricky, needle nose pliers may help).
- Hook one end of a rubber band (or rubber band chain) onto the paperclip, threading it through the main hull, and screw the aft section onto the hull, Loop the other end of the band onto the hook inside the nose section, and screw the nose section onto the hull.
Flood the submarine with water, wind it up, and watch it go!
Get all the air out of the submarine's compartment before using it.
You can add ballast to the sub to make it neutrally buoyant by putting M8 washers on the threaded rods inside the hull, and securing them with M8 bolts (which you could create and print yourself). Distributing the washers correctly between the fore and aft rods will balance the sub.
Winding up the sub is really easy: just run it backwards in the water, and let the water wind it. This is surprisingly efficient, so be careful not to over-wind and break the rubber band.
- You can store extra bands in the sub's compartment.
Please note that I'm still refining this thing; it's been pool-tested but could be better, especially with regards to neutral buoyancy.
Print the walls at 1.4mm to prevent water from infiltrating, and to get closer to neutral buoyancy.
You should use some infill, but don't go above 20% or so.