First, why use a toothbrush? I have found that toothbrushes are great waterproof motors. I have tested them so far to depths greater than 5 feet with no leaks. Most other store bought submarine motors leak at these depths and eventually rust up.
Step 1: Obtain Materials
1.Duct tape(shiny silver duct tape is better, but it is more expensive)
2.Anything buoyant enough to float your toothbrush, I used a Bawls bottle, but you could use anything else, I have even used a plastic bag.
3.Colgate motion toothbrush
4.Half an easter egg, mine was 3.4 cm(1 1/4 inches) in diameter and fit the toothbrush perfectly
6.Lego axle 3 studs long.
7.Lego axle connector
8.Alignment tool 10 stud long axle and what I like to call an i-piece
9. Cowling, Axle 3 studs long, i piece, and cowling piece.(Optional, cowling piece helps prevent the propeller from undergoing "spontaneous mechanical failure" if it hits a wall)
10.Disposable lithium batteries(optional, but they will make your torpedo go much faster)
11.Hot glue gun with hot glue
Step 2: Attach cross axle to toothbrush
Heat up the hot glue gun and read the following, twice, this is a hard step.
Turn on the toothbrush and keep it on. Then insert the 3 stud axle into the axle connector Inject glue into axle connector. Don't put so much glue in that it overflows, try to fill it about Â¾ full. Then push the axle connector stud assembly onto the part that turns the toothbrush head. Now quickly push the alignment tool onto the stud assembly until the stud is aligned. Let it cool and check quality by holding the spinning axles in front of a CRT display. If it wobbles it will work fine if it doesn't it will work great. To fix a mistake wobble the axle off and extract the glue with a paper clip.
Step 3: Attach easter egg and propeller
You can skip the next step if you have lithium batteries and you can get the toothbrush out of the water if it crashes. Lithium batteries are lighter and more powerful than normal batteries so they provide enough thrust to keep the toothbrush afloat, however it will mostly go in circles, and will sink if the propeller stops moving.
Step 4: Attach to buoyant object and begin POOL DOMINATION!!!!
Now repeat these steps until you have an army of toothbrush torpedoes and TAKE OVER THE LOCAL SWIMMING POOL!!!!!!!!! MWAHAHAHAHA!!!!! THEN GIVE THEM SOME LOW LEVEL INTELLIGENCE ADD AN EXPLOSIVE AND HOLD THE WORLD'S SHIPPING LANES CAPTIVE!!!!!!! MWAHAHAHAHA!!!!
Or you could just race them against each other. The rules of toothbrush racing are simple get from point A to point B before everyone else. Oh yeah, you also can't use a rudder to keep it going straight to make things a little interesting.