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Get two pieces of BLUE building insulation slabs, 2" thick by 24" by 96". Using a power saw, cut one-inch deep slots across one of the large sides, 20" long, so these slots will leave 2" undisturbed near each edge. Make a slot like that every four inches along the length of the entire 6 feet length. Then, a severely sloping cut needs to be made for each of these slots. This sloping cut will loosen and remove a triangular shaped piece of foam. That piece removed will be 20 inches long, 1" high and 3" wide.
After all of those triangular pieces are removed, the remaining slab will look like a giant version of a file! That surface will have a sawtooth contour.
This concept was invented and Engineered by July 1995, and I built a very crude version of it then to confirm that it worked, which it did. This presentation was first placed on the Internet in February 2000.
The very forward-most edge of each slab is then beveled like the bottom of the front of any boat.
When that slab is later placed on a lake or river, with weight on it, it will partially submerge. Those sawtooth surfaces would then be in contact with the water. The result is fairly easy movement on one direction but substantial resistance in the other. In one case, the ramped surface of the sawtooth slides fairly easily through the water. In the other, the vertical surface of the sawtooth notch tends to catch the water, which therefore resists movement.
Now imagine using a power source to alternately slide the slab forward and backward on the water. During the rearward stroke, there is forward thrust generated. During the forward stroke, the slab slides relatively freely over the water.
Now consider having two such slabs attached to the feet of a standing person on top of them. As the person did an exaggerated walking motion, as each slab was pushed rearward in the walking motion, a forward thrust would push the person forward, which would also push the OTHER slab forward, sliding over the water.
This mechanism is an EXTREMELY efficient method of creating forward motion on water. It is actually much more efficient than using a rowboat!
As an exercise method, it also has great value. The motion is almost exactly the same as that of cross-country snow skiing. It would even be possible to add arm levers to add to both the exercising value and the speed of movement across the water.
Those boot attachments cannot be centered (laterally) on the slabs, because that would be an uncomfortable position for the rider. But, since those weight centers are necessarily toward the inner edge, the two slabs would tend to tilt inward, thus dumping the rider!
To avoid this, the two slabs must be securely attached together, while still allowing a sliding action between them. There are quite a number of mechanisms that can accomplish this. There are two conditions that must be arranged. (1) The two main slabs must be able to slide edge-to-edge at least equal to the length of a person's slide, probably about 36". (2) The top surfaces of the two slabs must be kept flat with each other. Generally, this usually involves rigid arms on each slab that reach across ABOVE the other slab, with provision to move smoothly and easily.
One was that is pretty effective at doing both involves having 3" wide aluminum or stainless strips mounted to the inner edge of each slab. That strip extends 1" above the top surface of the slabs. Several long slots would be in these metal objects. Two slots (one for each slab) would trap a short stud mounted to the other slab. This would keep the inner edges together but still allow the sliding motion. Two more slots would be in the metal above the top surface of the slabs. These slots (near the very front and very rear of the slabs) would contain a 24" long rod that is firmly mounted to the other slab. This would keep the top surfaces flat.
Let's consider a person who weights 200 pounds. the device only weighs around 5 pounds, so the total weight involved is about 205 pounds. The area of each of the 2 foot by 8 foot slabs is 16 square feet, for 32 square feet total. Enough water must be displaced to equal the total weight. Fresh water weighs about 62 pounds per cubic foot, so we have to displace 205/62 or about 3.3 cubic feet of water. This results in the slabs being about 1.2 inches under the water, with only around 3/4 of an inch above the water. Depending on waves and splashing, it may be desirable to add gunwales around the edges to minimize water on top. Without that, enough water weight on top can sometimes submerge everything!
There are several obvious enhancements to this description. If there is a single 25" rod mounted 18" from one end of each slab, and a slotted metal rail is provided on both inside and outside edge of each slab, all of the necessary functions can easily be combined.
It DOESN'T seem to work very well in surf, with breaking waves! In all other environments tested, it has worked great. No rowboat has been able to keep up with it. It is believed that a refined model will be able to outperform canoes or kayaks, although such tests have not yet been done.
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago