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Walk on water challenge

Updated Monday, 12th June 2006

The Basilisk or Jesus Christ Lizard ‘runs’ on water by moving very fast, slapping its foot down hard on the water such that it creates an upward reaction. The challenge is to build a machine that can do the same … [WE WANT YOU TO HAVE FUN - BUT TAKE CARE WITH THIS CHALLENGE. DON'T ATTEMPT IT ALONE, AND ONLY TRY YOUR DEVICE SOMEWHERE SAFE, LIKE A SWIMMING POOL]

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crash test dummy

Is it possible for a human being to walk on water? If we look to the animal kingdom we find that it is a feat that some insects and one particular reptile have already mastered.

Insects like pond skaters and water-measurers don’t just float, they glide around on the surface of the water without ever breaking through it, using a property of water called surface tension. Surface tension produces a sort of ‘skin’ on the water; this skin is formed by the way molecules at the surface of the liquid are attracted to each other. A water molecule is made of an atom of oxygen attached to 2 atoms of hydrogen. The oxygen is a little bit negative and the hydrogen a little bit positive, and because opposites attract, each water molecule is pulled towards its neighbour and they join to form a skin.

The Basilisk or Jesus Christ Lizard however can also walk, or rather ‘run,’ on water. It is too heavy to either float or be supported by surface tension. It has powerful legs which give it a terrific running speed, a very long tail for balance and slightly webbed feet to increase the surface area of the foot. It ‘runs’ on water by moving very fast, slapping its foot down hard on the water such that it creates an upward reaction. The lizard creates an air pocket as it slices down with its foot, and then it pulls its foot out again quickly before the water has time to fill the hole and the lizard starts to sink. In fact if Jesus Christ Lizards do slow down, they sink … but they’re good swimmers.

Now that you know how the Jesus Christ Lizard performs its miracle have a go a making your own ‘water running Jesus Christ Lizard.’

It’s not easy – but this is how to do it. This is a model for real experts to build. The design is based around a right-angled gear box or ‘T’ box. You could either make this yourself (using an aluminium angle, push-fit bearings and bevel gears) or buy one from a shop. Using a gear box with a 2:1 reduction on it gives more force to the feet – but making the feet spin at the same rate as the drive shaft worked too.

Stuff you need

Materials
• Right-angled gear box (or ‘T’ box).
• 40 mm long section of aluminium angle, 40 x 40 x 3 mm
• 150 mm long section of aluminium angle, 40 x 15 x 3 mm
• 2” x 1” planed timber 400 mm long
• Three solid ½” diameter aluminium rods (20 mm lengths)
• Two 150 mm lengths of 3 mm diameter solid steel rod
• Four M4 grub screws (can be made from M4 bolts)
• Two M6 grub screws (can be made from M6 bolts)
• 70 mm length of M8 studding (threaded bar); (or use a long M8 bolt)
• Four 1” No. 8 wood screws
• One 1” M6 bolt
• One M6 nut
• Two M8 nuts
• Four 12 mm M4 screws
• Plastic for feet (approx 70 mm diameter x 2 mm thick for each)
• Large elastic bands 15 mm wide x 200 mm long

Tools
• Drill
• 3.3 or 3.5 mm; 4.5 mm; 5 mm; 6.5 mm drill bits
• M6 tap
• M4 tap
• Pozidrive screwdriver
• Jigsaw
• Hacksaw
• File or grinder
• Ruler
• Hot glue gun or Araldite
• Pliers

 
study

What to do

1. Take the 400 mm length of timber and give it a vague lizard shape (a thinner tail and thicker body). This makes it more lizard-like and reduces any buoyancy the tail may have had to give a better running angle. Angle the front underside of the wood to give it a more streamlined shape (and possibly to give it a small amount of planing lift once moving).

2. Next step is to fix the gearbox onto the 40 mm length of aluminium angle. Drill four holes in one leg of the aluminium angle, to match the mounting holes at the drive shaft end of the gear box. Then make a hole in the centre of that angle leg, to pass the drive shaft through.

 

3. Drill and countersink two holes in the horizontal leg of the angle. Use two 1” wood screws to fix the horizontal angle leg to the top surface of the wooden cut out, with the vertical leg standing 60 mm from the front edge of the timber. Fix the gearbox to the vertical angle leg so that it is right at the front of the lizard, with the drive shaft pointing towards the tail.

4. File a 3-4mm wide flat on each of the three shafts coming out of the gear-box. Next step is to attach the ‘legs’. First, cut three 20 mm lengths of solid ½” aluminium rod. Then drill a 5 mm diameter hole down the centre of each one. Next, drill a 3.5 mm diameter radial hole at one end of each of the three aluminium rods. Use an M4 tap to give this a thread, so that you can put an M4 grub screw in to fix it to the shaft. At the other end of the two rods, use an M6 tap to make an M6 thread. Then drill a 3 mm hole through the diameter of each of these two rods, about 6-8 mm in from the M6 tapped end. (The 3 mm hole is to accept the 3 mm diameter steel rod used for the lizard’s legs).

 
Adam in water wheel

5. This all seems a bit complicated – and to some extent it is! But the idea is that two of these aluminium rods will be used for fixing the lizard’s legs to the shafts coming out of the gearbox, and the other one will be used for fixing the elastic band to the drive shaft of the gear box. The third aluminium rod has two M4 grub screws in it and a 6 mm hole through its diameter – about 4 mm from the end. This fixes onto the drive shaft with an M6 bolt through the 6 mm hole, such that there is a 4 mm gap between its head and the aluminium rod, and a 4 mm gap the other side (between the nut on the other end and the aluminium rod). The M6 bolt is for attaching the rubber bands that will drive the legs.

6. At the other end of the lizard’s body drill a 6.5 mm hole in the top of the tail, and screw a 70 mm length M8 studding into it to give a firm anchor for attaching the other end of the rubber bands to. On the underside of the tail make a keel out of the 150 mm length of aluminium angle. Cut one leg of it down to the bare minimum needed to fix it to the underside of the tail about 12 mm wide. Then screw it on with some short No. 8 wood screws.

7. To make the legs cut two 150 mm lengths of 3 mm diameter steel rods. Make a hoop at the end of each (by wrapping around a piece of bar) to give a hoop that can be glued strongly to the plastic feet. At the other end, grind or file a 2 mm wide flat on the side of the rod so that it can be held more securely by the M6 grub screw.

 
Adam and lift test

8. To make the feet, cut out two 75 mm diameter circles from some 2 mm thick plastic sheet - the flat tops of aerosol can lids work pretty well too. Glue the feet to the hoops on the ends of the legs using hot glue or Araldite.

9. Slide the top end of the leg into the 3 mm hole in the leg mounting so that it sticks out at one side by about 20 mm. The leg is then fixed in position by the M6 grub screw, which tightens onto the flat at the top of the leg to secure it. Then bend the legs using pliers and/or self-securing grips, so they are at the right angle for running. This can take a bit of trial and error. (Getting the centre line of each foot to be about 80 mm from the centre line of the lizard, and the foot to be flat and level when about 6 mm below the level of the base of the lizard's body seems to work quite well. When level, the centre of each foot is about 60 mm in front of the shaft it is attached to.)

10. Attach the rubber bands by hooking one end between the head of the bolt and the aluminium rod on the drive shaft, and the other at the back of the lizard, underneath a nut at the bottom of the M8 studding. Hook the other band over the nut on the other end of that M6 bolt and then stretch it over the hook over the M8 studding at the other end. This one fixes to the top of the M8 studding and is held there with an M8 nut near the top. The lizard is primed for running by rotating its feet backwards, twisting the rubber bands up until they feel pretty tight – too tight is not necessarily better.

11. Get the lizard to run on water by holding it at water level and letting go of the legs. It should sprint off for at least a few metres.

 

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