A well-designed hydroplane (from the early 1950s to date) is mostly supported by air, the relative wind forced under the hull as it gains speed after it has "climbed over the hump" and gotten on-plane. The late Ron Jones, an engineer and life-long professional designer-builder of race-winning outboard, inboard, and unlimited hydros, stated that, "a hydroplane is an airfoil in ground effect." Ground effect has been noted by airplane pilots since the earliest days. As an airplane nears the ground when landing, a cushioning effect takes place as the as the wing gets close enough that its downwash effectively begins to compress the air between itself and the ground . . . the happy result being, as any student pilot will attest, a smoother landing. This is more noticeable in low-wing airplanes like Beechcrafts than in high-wing machines like Cessnas. Hydroplanes (and this also applies to tunnel boats) don't have wings, and act more like the experimental "ram-wing" watercraft such as the Russian ekranoplans, which had very short wings and could not climb out of the surface effect or ground effect. At top speed, a racing hydroplane is almost fully supported by the ram-air, and its "3-points" carry little weight and mostly serve to stabilize the boat. It's awfully small, but maybe you can tell from the photo of my 10'2" 1966 Karelsen B Stock hydro, the sponsons run a couple of inches out of the water all the way down the straightaways, just tapping the top of the chop. In the corners (customerily left-hand turns) a hydro leans slightly to its right side, supported by the right sponson, while the skidfin affixed to the back of the left-side sponson acts to reduce side-slip so you can carve a nice tight fast turn.
To enhance the ram-air ground effect, all racing hydros have used airtraps which since the very early 1960s have extended from the back inside edge of each sponson aft to the back of the boat, tapering down to maybe 1/4" deep at the back edge of the bottom. Carrying the ram-air effect all the way back to the transom, rather than letting it spill out after it gets past the sponsons, is what really allows a racing hydro to fly on the surface of the water, with the ground effects carrying almost the whole weight of the hull/engine/driver. A well-set-up hydro at speed may draw remarks like, "nice and loose" or "really aired-out." Airtraps may also serve as a hull-tuning device: if a boat is running excessively loose and wants to blow over even after you have tried adjusting engine kick-out, moving the fuel tank forward, etc., you can cut down the depth of the airtraps, say 3/8" at a time until the boat settles down to your liking. Going the other direction, if you own a simple flea-size hydro with no airtraps, and want to see if you can get some ram-air action to help pick up your heavy engine and/or driver, well, get a 1/2" thick spruce or mahogany plank and saw out some airtraps!
