The 2004 Edinburgh International Science Festival ran a competition called Walking on Water. Competitors had to create a pair of floating "shoes" in which they would race the 50 metres of the Commonwealth Pool in Edinburgh.
I was part of a team from Glasgow Science Centre that entered the competition. I was chief designer and builder of the shoes, and by mysterious logic that I never understood, was appointed pilot also. During the race my swimalong partner was Heather "FOCUS, BEN! FOCUS!" Sinclair. Onur Orkut provided the artwork on our shoes - I think that we put everyone else to shame in this respect. Tara Cochrane, Jillian Boag, Jon Davies, Robin Hoyle and Joe McGrath glued, varnished, drove vans, worked at testing sessions, took photographs...
...and it all paid off. We won Gold in our category of the race (for shoes costing more than £50) and we won bronze in the Design Excellence competition, judged by Professor Heinz Wolff. We returned to the science centre proudly bearing a cheque for £1000, two bottles of champagne, and two plastic ducks.
Details of the design of our shoes
Is anything worn under Ben's T-shirt?
No. It's all in perfect working order!
Focus, Ben! Focus!
I cross the line in the nick of time, as one of my shoes has filled up and is on the point of sinking.
The winners! I'm on the left, with Tara, Heather and Joe. Note trophies standing on champagne boxes in front of our shoes.
Design of our shoes
First prototype (made out of an ice-cream box, string and sticky tape). The two flaps on the bottom (one is hard to see) let the shoe slide easily one way through the water, but not the other. The twin keels are an attempt to make the flaps trap water more effectively.
This picture is the third prototype. The second prototype used the same foam hull as this one, but had a row of flaps in place of the two flaps of the first prototype. The trouble was that even with the shoe going in the backwards direction, there wasn't much resistance, because the water flowed easily round the edges of the flaps. We tried attaching fabric webs between the flap sides and the hull, to trap the water, but it didn't help.
The thing that made the third prototype different was the box on the bottom of the hull. This was open fore and aft, and it had a set of louvres at the front. The louvres opened as the shoe moved forward, allowing the box to move easily through the water. When the shoe moved backwards, the louvres closed, so that a boxful of water was now attached to the shoe. In effect, the shoe was heavier in the backward direction than it was in the forward direction. The addition of the box and louvres made a big difference, and we kept this design in the full-size shoes.
Making the shoes seaworthy
The volumes of the shoes were calculated to be big enough to support the entire weight of the pilot on one shoe, with some freeboard to prevent them shipping water.
The pilot's foot was right in the bottom of the shoes. The foot must be low enough so that the combined force of the shoe's weight and pilot's weight acts at a point below the centre of buoyancy. If this is not so, the shoe will want to float the wrong way up and will be unstable.
Details of the design...
We used several small flaps in the louvre rather than one big one because the shoe has to move backwards a distance roughly equal to the height of the flaps in order to close them. The smaller the flaps, the faster the closure. But too many flaps means too much drag when they are open. It's a compromise.
In the full-size shoes the flaps were pivoted at the bottom, so that their buoyancy would cause them naturally to close the right way up.
The swivelling rods on which the flaps were mounted were a very loose fit in the holes in the sides of the box, to avoid any possible problems with the wood swelling when it got wet.
The pilot's foot was restrained in a close-fitting footwell cut out of stiff foam. A strap kept the toes in place. The pilot's heel was free to rise (as in cross-country skis), and the footwell was made high at the back so that the pilot could push sideways with his heel to apply a twisting force to the shoe to steer it.
During testing we found that the buoyancy of the box gave the shoe a tendency to capsize, so we bolted in a steel weight at the bottom of the shoe, which fixed the problem but increased inertia.
...and things we'd do differently next time
The shoes were excellent in terms of allowing quick progress along the pool, but the box gave the shoes a lot of keel, which made them very unmanoeuvrable. With the both the pilot's foot and the box halfway along the boat, it was impossible for the pilot to apply a moment to the hull except by twisting his foot, which wasn't very effective. Manoeuvrability is important because you have to be able to avoid obstacles, like ladders on the pool side. Our most dangerous rivals in the Final (with a very similar design) fell foul of just such a ladder. It might be better to move the box further aft. The pilot could then apply a moment by simply pushing sideways with their foot.
In the Final, one of the shoes shipped some water. This made it sit lower in the water, making it ship more water, and so on until I crossed the line on the point of sinking. Instead of giving the shoes some freeboard, we could (using no extra wood) have given them a closed top. The construction would have been made harder, but shipping water would not have been a problem.
The shoes were rather heavy.
Which were the best shoes in the competition?
The under £50 race was won by a pair of cigar-shaped shoes made of expanded polystyrene, covered with rubber flaps glued on in such a way as to "grip" the water going backwards, but lie smoothly against the side as the shoe went forward. These shoes did a faster time than we did in the "senior" race. I tried them out afterwards, and they were light, easy to use, and easily steerable.