This is a fun project to do and entertain your 4-8 year old kid, give him small tasks to do and he'll love you for it. Only motorized this time, next time it'll have remote control and stuff. The project costs nothing but time, 1 old 3V car motor, couple of old batteries, foam board (blue in my case), an iron and some wires with an on/off switch. Enjoy it, and try it.
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Step 1: The Powertrain
This is probably the most important part of all, you'll have to attach some sort of a drive shaft to the litle motor, which by the way is overhauled motor from an old toy, rewound with a thicker wire than the original and less winds and the brushes were replaced with a thicker copper brushes (made by myself as well). This overhauling enables the litle buzzer to go from lets say initial 500 rpm to well over 2500 rpm, and the sound is like it's going to fly. You'll hear it in the video. At the end of the shaft, attach some nicely cut and bent pieces of metal that will act as prophellers, and your done. Glue used for this matter = instant glue.
Step 2: The Source Aka Battery Pack
You can use whatever batteries you want, ni-cd, ni-mh or even li-ion from a cell phone or similar. I used two ni-mh 1.2 V which were supposed to be dead anyway, but with a shock from powerful source, they are revived and ready for action. The two antennas are actually the two charge connectors, cause I had a feeling that my boys would want to play with it more than once. Just hook them to a cell charger for couple of hours and thats it. After some duct tape to protect them from the water, they end up looking like the second pic.
Step 3: The Body
Cut some pieces of foamboard to make it look like a catamaran, I would think that 1:10 ratio in height and lenght is a proper size to begin with, make it a little big bigger, mine is 25cm but it looks a bit short. Just curved it nicely all around the edges (that's the part where your kid works) and you are done.
Step 4: Join the Body and the Powertrain
Just glue them together with a heat melting silicone, any other glue will be too agresive to the foam, and try to glue the motor in line with the body, otherwise your cat will have a mind of its own.
Step 5: The Power to the Powertrain
Glue the batteries to the body and solder the wires to the motor and a switch. It's now officialy ready for testing. Just look at the video to hear the sound it makes.