Dale
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Gard, I think you have it backward. A backspin causes the projectile to fly farther in air. This is because the spin draws air backward over the top faster, while retarding the flow of air across the bottom, thus producing lift. A topspin, where the ball/stone/whatever is spinning the same way as it would were it rolling along the ground, produces negative lift, and it dives into the ground.
As a stone or ball hits water, you get odd effects like you observed. With a backspin, the bottom of the stone grabs some water and pushes it ahead, thus trying to pile up some water in front of it. This keeps the stone on top of the water, and it skips along. But with a topspin, the stone grabs water and pushes it backward and upward, and the stone dives into the water.
Slinger_Man_Dan, The traditional Balearic test was to hit the slats of a gate. Nine slats, nine casts, but I do not know the range. I believe one might have been required to hit them in a particular order, or perhaps to announce beforehand which slat would be hit (a la billiards: "Cue ball over the rim, land on right foot of spectator wearing the fedora").
Your description sounds exactly like how I do an Apache cast, when I'm trying for a backspin. But usually I try for a football/rifle spin, so I want the sling cords side-by-each at release, instead of fore-and-aft. Painting a ball or stone does help. Casting at very low power also helps, because the stone stays close enough to see, longer.
Thomas, Tennis balls are great for practicing various spins. The results are quite obvious. Long ago, I slung a tennis ball underhand and it just scooted along a foot above the ground for 60 or 70 feet before dropping ... marvelous! I've never been able to repeat that, but it is one of the reasons I keep on slinging. Someday before I die, maybe I will be good enough I can do something like that on purpose!
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