Sitting on the beach, finishing off a beer one day, [Rulof] realized that if he put a motor in the beer bottle with a propeller at the bottle’s mouth, he could attach the result to his leg and use it to propel himself through the water. Even without the added bonus of the beautiful Mediterranean waters through which he propels himself, this is one hack we all wish we’d thought of.
These particular beer bottles were aluminum, making cutting them open to put the motor inside easy to do using his angle grinder. And [Rulof] made good use of that grinder because not only did he use it to round out parts of the motor mounting bracket and to cut a piston housing, he also used the grinder to cut up some old sneakers on which he mounted the bottles.
You might wonder where the pistons come into play. He didn’t actually use the whole pistons but just a part of their housing and the shaft that extends out of them. That’s because where the shaft emerges from the housing has a water tight seal. And as you can see from the video below, the seal works well in the shallow waters in which he swims.
The batteries for powering the motor go in a separate watertight PVC cylinder mounted on his upper body, with watertight seals for the wiring going from the battery cylinder to the bottled motors. But how to make a watertight on-off switch? For that [Rulof] put a reed switch inside the battery housing. The reed switch energizes a relay, and the relay electrically connects the motor to the batteries. He made sure to mount the reed switch near the PVC cylinder’s wall. To close the reed switch he brings a magnet outside the cylinder to near where the reed switch is inside the cylinder. To open the reed switch again he moves the magnet away. So the reed switch remains inside the watertight cylinder while the switch is opened and closed using a magnet field from the magnet outside the cylinder.
[Rulof] isn’t the only electric fish in the sea. There’s also this fin propelled robofish that can communicate with others of its kind using Sonar. And then there’s jet propulsion, sucking water in one intake and expelling it out the other as does this ROV.