Science of the Aquaglider.
I don’t understand all the physics of the Aquaglider, and I won’t pretend to. I imagine it will be some time—possibly a long time—before anyone truly understands all that’s going on with these things. I’ve spent hundreds of hours watching and pondering the action of Aquagliders, looking for the smallest details of difference, and I still don’t get why they’ll sometimes go in circles, and sometimes in a straight line. Maybe you’ll be the one to finally figure all this out and let the rest of us know!
Meanwhile, I’ll tell you what I do know. The wings and body of this toy create turbulent currents as they glide through the water. It will probably take slow motion photography and particles suspended in the water to carefully observe these currents, and of course it is careful observation that is the basis of science.
What is it that causes this movement of water, which we call currents? The foam body is lighter than water, and thus it floats. As Archimedes noted some 2,250 years ago, objects submerged in water displace an amount of water equal to their own volume. If that volume of water is heavier than the object, then the water works to push the object up to the surface. If the object is heavier than the water it displaces, then the object sinks to the bottom. The current around the Aquaglider is thus the rushing in of water to displace it.
If you’ve ever tried to hold a swimboard underwater, you know it takes a large force to displace that water. It isn’t a huge volume, but it sure is heavy—just as heavy as the force you must exert to hold the swimboard under. When you let go, you see how readily that board wants to come to the surface. This is the same kind of force that acts on the foam body of the Aquaglider. You can observe it with a small object like a cork or piece of foam, or any other floating object that you might put in the bathtub or a sink.
But if you look at the Aquaglider from the top, you’ll see it has a lot of surface area. It’s that area that pushes against the water in an upward direction, when you release the toy in a flat position. Now look at the Aquaglider from the front, and you’ll see there’s not much surface area to keep it from moving in a forward direction. Also, if you set the Aquaglider on the surface of the water, you’ll observe that it floats with the front end slightly raised, giving it what is called an “attitude,” and meaning it’s aimed upward few degrees.
If you fasten a string to the bottom of a Aquaglider, for example with a paperclip, you can find the center of buoyancy of the toy. This is the point at which the foam and wings will float in a flat position while the toy is held still underwater. Doing this, you’ll see that the front-up attitude is about the same underwater as it is on the surface. In other words, this is the natural angle that the Aquaglider will float at. And it is the same angle as the path that the Aquaglider takes when you release it.
The greatest single difficulty in designing the Aquaglider was finding a way to keep the back end of the toy from floating up and becoming level with the front end. When that happens, the toy goes into a stall—it stops forward movement and begins to back up. If you carve a piece of foam into a saucer shape, then release it from the bottom of a pool, you’ll see this happen rapidly, while the foam zigzags back and forth in an erratic dash to the surface.
You’d think that tapering the saucer to a guitar-pic shape would be all that’s needed to prevent this, and to keep the object on a straight course. But not so. The guitar-pic shape alone still zig-zags back-and-forth, and won’t continue in a straight line.
I made countless shapes and found several that that did move laterally, but they did so slow and awkwardly. I found other shapes that simply glided up rapidly, at a slight angle, to the surface. Not much fun to play with, and it always seemed like there could be a better shape, one that could glide flatter and longer and keep going in a straight line.
When I eventually added fins to the shaped foam, I thought for sure this would solve the problem and steer the contraption at a shallow angle, yet still allow it to move at a good speed. Again, not so. I added several sets of fins, elongated the guitar pic, tried triangular, rectangular, oval, and other shapes, but still the potential toy went back and forth, back and forth, defying what seemed to be common sense and the laws of physics.
In fact, it was almost by accident that I finally tested a set of very large fins, which might now be called wings, that finally corrected the course of the flotation. I had some long pieces of wing-shaped formica in my “dufflebag of parts,” and was about to call it quits one day, when, I thought, “Well, I came all the way down here to get in the pool, and I paid to get in, so I may as well try these longer shapes.”
To my extreme surprise, when I released a guitar-pic shape with wings, which I was fairly certain wouldn’t work, about three feet below the surface, it picked up and took off across the pool! A new species of floating object came to life before my eyes–the first Aquaglider! This was after testing and “tinkering” with over 5,000 models!
I believe it’s correct to say that the wings on the Aquaglider prevent the back of the toy from rising higher than the front of the toy, when released underwater, and they help it maintain the proper attitude, while it glides forward through the water. With this attitude, a path of least resistance is taken, while the buoyancy, or the force of the displaced water, drives the Aquaglider forward along this line.
Also, as you’ll see if you look at the Aquaglider underwater, the wings bend down a bit, creating a sort of arc, when you release it. This arc appears to help the Aquaglider glide along the path of least resistance, apparently by creating a twisting effect that helps keep the body at the same attitude. I discovered “flexing wings” somewhat by chance, using a soft plastic which was all that was available when I went to the plastic supply store one day.
There are other ways to consider or imagine what is going on with a moving Aquaglider, and I think one way is to consider a small, plastic package of ketchup. Most of us have seen one of these little containers get squished one way or another. And we’ve seen the ketchup squirt seemingly everywhere, with its special affinity for white shirts. Picture the water “squishing” the Aquaglider forward, as it presses on the Aquaglider from both the top and bottom. The “squished Aquaglider” has nowhere to go but forward! And it does so about as fast as the ketchup seems to move.
Many people, including myself, think the Aquaglider goes faster in deeper water than in shallow water, though it’s difficult to measure precisely, with nothing more than a stopwatch. A friend of mine took a Aquaglider scuba diving, and released it from something like thirty feet underwater. He said it took off so fast he couldn’t possibly catch it, and he thought he’d never see it again! He did find it later, on a beach about a quarter mile away, but doesn’t recommend releasing it with scuba gear unless you really don’t want to see it again.
To summarize the physics of the Aquaglider: It is the large, flat surface area that holds the toy underwater, while the buoyancy of the foam body works to push it toward the surface. The shape of the foam and the extended wings hold the toy in an attitude that is the same as the glide path. The water appears to squish the Aquaglider from above and below, driving it forward. The energy that you impart to the Aquaglider while pushing it underwater is the energy that is released as it glides forward toward the surface.
