A lot of people try to spread out their arms and legs, because they think that the largest area of impact with the water is the best. Nope! They certainly can make LOUD impacts with the water, but not much of a splash. The actual Physics involved is very different than that.
The trick is to capture the largest amount of air, and to push it down two or three feet below the water surface. When that large air bubble rises back up, which it does very rapidly, it pushes the water above it upward, at good velocity, so a LOT of water and air zooms up to splash everyone nearby!
When I REALLY did this well, it made a strange sound 'ka-WUMP" as the large bubble of air broke the surface (above me). I could hear this sound, even down under water, and the kid-witnesses LOVED the 'ka-wump' sound! If you try this, your best clue if you did it well is if you can create that sound.
I discovered this in 1964, when I visited my half-brother's home in Florida, which had a nice backyard swimming pool. I watched as some of his many kids tried to do Cannonball or bellyflop dives, and noticed that they all made pitiful efforts! My brother and his wife often invited friends over for backyard parties. As an 18-year-old kid with a new toy, I tended to do Cannonballs or bellyflops which drenched all the adults. My brother's wife often asked me to stop, that her guests were all getting drenched, but of course, I was just a kid with an interesting new toy so I tended to keep diving.
I made sure that my back was horizontal when I hit the water. I would tilt my head downward to partially create a Front Wall for an airbox. I would bend my hips to get my thighs to be vertical, to create a Rear Wall for the airbox. (knees together) Then I bent my arms tightly, and made it so both upper arms partially blocked the upper sides of the airbox, and the lower (fore) arms were just below that in extending the sidewalls downward.
When I hit the water, I therefore had created a fairly decent airbox to trap more than two cubic feet of air under my chest and belly. The airbox obviously leaked a lot, but in a fraction of a second, not much air could get out. I now had about two or more cubic feet of air trapped as deep as three feet below the water level. Simple hydrodynamics shows that an air bubble that deep contains air at about 2 PSI air pressure. Since my trapped bubble was about two square feet area, or 288 square inches, that means that the water would force the air bubble UPWARD with around 576 pounds of vertical force!
That causes an upward acceleration of the maybe 128 pounds of water above the bubble of more than FOUR Gs of acceleration. Some witnesses said that water from my bellyflops would sometimes go 20 feet high.
Generally, older people sitting at a nearby table or on nearby pool chairs, would get drenched, and they were NEVER happy with me for that! But that was sort of the idea! But the people around the pool were usually cheering when I broke the surface! (So it was worth it!)
(Obviously, as a College Physics student I had figured all this out before I first tried to do it!) All I later tried to enhance was to hold my arms to best block off the sides of my airbox. I also later experimented with slightly tilting my back forward, because my head was not wide enough to fully block the Front of the airbox, to try to better trap the water so I could maintain my airbox to a deeper depth. But that was not particularly successful. I was already drenching everyone in the yard around a swimming pool, so I guess that was not necessary anyway.
I also experimented with wearing a really baggy sweatshirt, and trying to use the extra cloth to help block areas between my arms on the sides.
I will let YOU figure out how to cause most of the splash to go toward one side of the pool. If you followed the logic of everything described above, you probably will think up at least two ways that you can do that, one of which works a LOT better than the other! When I would show up at my half-brother's house, four to six small kids would line up along one side of the pool, to encourage me to try to drown them! Of course, I never hesitated at the opportunity! The kids rarely wanted to even let me go into the house to visit with my half-brother!
Sometimes, the adult visitors to my half-brother would sit maybe 20 feet away from the pool, which I thought his wife thought up to keep her guests from getting drenched. But I just considered that to be an additional challenge, in more directing the main splash in a more specific direction, to 'get them anyway.'
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