This is an oldie, and I guess probably originated as a variation on the wind up elastic planes.

It's fairly simple, and a great one to do with kids.

Date made: June 2013
Approx Cost: £0 (recycled materials)
Approx Time: 0.5 to 1hr
Difficulty: Easy

Step 1: What You Need

Materials you will need are:

*2no bottles (I used one 500ml for the propeller, and one 600ml for the body, but you could experiment with different sizes and shapes)
*3no elastic bands
*2no paper clips
*1no piece of wood about 150mm long (maybe an old broken ruler? I ended up using 2 plant markers)
*1no bead

Tools you will need:

*knife or scissors (I used a combination of both)
*drill or some sort of punch (I used a 2mm drill bit)
*long piece of hooked wire (coat hanger?)
*random tools for bending the body paper clip (you'll see!)

Step 2: Propeller

Take one bottle (ideally with big ridges on the bottom) and cut the bottom off.

Cut holes in one side of each ridge (the same side on each). This will give you a propeller shape as per the photos.

Then drill a hole in the middle of the propeller and the other bottles cap, just big enough for a paper clip. Drill an offset hole in the propeller to lock off the paper clip too.

Next bend the paper clip and feed it through the cap and the propellor as shown in the photos. I ended up using a bead between the propeller and the cap to reduce the friction.

Step 3: Sub Body

Next prepare the body of the sub by drilling two holes in the end of the bottle. One in the center and one off center.

Bend and feed a paper clip through the center hole, locking it off with the other off center hole.

Bend a hook in to the paperclip that is inside the bottle! This requires a bit of patience, a bit of jiggerypokery, and use of some random tools you have to try and bend in to shape inside the bottle!

Or if you had some long nosed pliers you could bend outside the bottle and then feed the paper clip through the inside of the hole!?! You'll find a way!

Step 4: Fit Propellor to Body

Attach the elastic band between both paper clips, you will need to use the long hooked wire to do this - using it to 'fish' inside the bottle.

Once connected between the two, screw on the cap.

Step 5: Add Stabilisers

Use two elastic bands around the bottle to secure the piece of wood as a stabilizer.

This wood will act as 'fins' to ensure that the body remains still, whilst the propeller spins. Without it, the body would spin as well as the propeller.

Step 6: Find Buoyancy and Play

To use just wind up, as you would an elastic band plane, and launch in water!
Really you want the propeller to turn so that it pushes the water backwards. If you are unsure which way that is, just try both!

If the bottle is full of air, the propeller will sit above the water where it won't do much.
If the bottle is full of water, it will sink and will be too heavy to move.
Experiment with different amounts of water in the bottle to try and get the propeller to sit just right in the water.

Experiment with the position of the stabilizers.

Here are some links to a couple of videos:

My elastic band wasn't very strong, it was quite thin. Use the thickest, strongest elastic band you can, so that you can wind it up loads, that way you will get some reasonable power.

If you get enough power, and get the sub neutrally buoyant by getting the amount of water right, you should be able to get it to move underwater.