For my entry in the iRobot Create contest, i've made a pool skimmer that uses the create's wheels to power it. The process can be done entirely mechanically - no programming necessary. Before I start, I'd like to acknowledge the fact that parts of it do not look a apristine as they could. This is due to the fact that instead of going and getting all new parts, I recycled what components I could.
Step 1: Materials
This is probably the hardest step for me to type, because things very so much depending on what you have, but I will tell you what I used - you should use whatever is available to you.
2 ball bearings
Bike inner tube
4 Chinese take out containers
Spare blocks of wood
Various PVC pipe fittings
1/2 in. conduit pipe
Multi purpose caulk
Heat Shrink Tubing
A lot of my materials were improvised based on what was happening. For instance, I already had pipe, and when I got the bearings, they were different sizes, hence the PVC fittings.
Step 2: PVC Box
You will need 2 15x15in squares of pvc, and 4 15x6in rectangles.
Drill a hole the size of your conduit in the middle of 2 rectangles as close to the bottom as you can, allowing space for the bearings. i.e. put the bearing at the bottom of the plexi, and trace the inside of it to know where to drill.
If you don't have the right sized drill bit, go one size up, and make a bushing out of the inner tube.
Be warned that when using any power tools on the plexi, it will start to melt as you go, so you will have to stop frequently.
Use the caulk to glue all but one of the squares together. (so it's a rectangular prism without a top) Book ends are helpful for keeping the pieces in position while they're drying.
Step 3: Water Test 1
Now that you have a box, put some duct tape over the holes you drilled, and stick it in the water. Push it down into the water to see if there are any leaks. If there are, use caulk to plug them. Repeat this process until there are no leaks at all. (allow it to cure each time)
Step 4: Paddle Wheels
These can really be made out of anything, I used plastic from Chinese take out containers...the size doesn't matter too much, I made them as big as I could with those containers. Make a hole in the middle the size of the conduit, and use JB weld to put them on.
Step 5: Other Parts
All of this should be improvised as needed. You'll need something to raise the robot, you'll also put the bearings onto this. You need a small block to push up the front wheel, and you'll also need to put tape over four the cliff sensors. Velcro the robot to the raised stand.
Bushings - You'll want some kind of rubber bushings for it..I used inner tube.
A lid is optional, but recommended.
Heat shrink tubing over the conduit helps with grip.
Styrofoam around the edges help with floatation, and if you glue some in on the insides, it will keep the robot in place. Be sure not to glue all the sides, but make one removable so it will be easier to remove/replace.
Net - I used a pool net for mine. Take a larger piece of pvc, drill a hole in it, put the net in, and stick a screw through the aligned holes. Glue the pvc so that it's about halfway in the water.
Step 6: In the Water
Note for next time: the bushings did have a small leak, the holes for the axles were cut a bit off, and the seal wasn't perfect. I would also try and make the axles treaded, for better traction against the bumpy wheels.