A hydroplane (or hydro, or thunderboat) is a type of motorboat used exclusively for racing.
One of the unique things about these boats is that they only use the water they're on for propulsion and steering (not for flotation)—when going at full speed they are primarily held aloft by a principle of fluid dynamics known as "planing", with only a tiny fraction of their hull actually touching the water.
The American Boat Racing Association (ABRA) is the primary unlimited hydroplane boat racing league.
The basic hull design of most hydroplanes has remained relatively unchanged since the 1950s: two sponsons in front, one on either side of the bow; behind the wide bow, is a narrower, mostly rectangular section housing the driver, engine, and steering equipment. The aft part of the vessel is supported in the water by the lower half of the propeller, which is designed to operate semi-submerged at all times. The goal is to keep as little of the boat in contact with the water as possible, as water is much denser than air, and so exerts more drag on the vehicle than air does. Essentially the boat 'flies' over the surface of the water rather than actually traveling through it.
One of the few significant attempts at a radically different design since the three-point propriding design was introduced was referred to as Canard. It reversed the width properties, having a very narrow bow that only touched the water in one place, and two small outrigger sponsons in the back.
Early hydroplanes had mostly straight lines and flat surfaces aside from the uniformly curved bow and sponsons. The curved bow was eventually replaced by what is known as a pickle fork bow, where a space is left between the front few feet of the sponsons. Also, the centered single, vertical tail (similar to the ones on most modern airplanes) was gradually replaced by a horizontal stabilizer supported by vertical tails on either side of the boat and as of 2006 the horizontal stabilizer was mostly abandoned. Later, as fine-tuning the aerodynamics became more important, the bottoms of the main hull have subtle curves to give the best lift.
Donald Campbell attempted world speed records in the jet engined hydroplane, Bluebird in the early 1950s. The Ted Jones-designed Slo-Mo-Shun IV three-point, Allison-powered hydroplane set the water speed record (160.323 mph) in Lake Washington, off Seattle, Washington's Sand Point, on June 26, 1950, breaking the previous (ten-plus-year-old) record (141.740 mph/228.1 km/h) by almost 20 mph (32 km/h).
Starting in 1980, they have increasingly used Vietnam War-era turboshaft engines from helicopters (in 1973–1974, one hydroplane, U-95, used turbine engines in races to test the technology). The most commonly used turbine is the Lycoming T55, L-7C, used in the CH-47 Chinook.
Efforts have occasionally been made to use automotive engines, but they generally have not proven competitive.
The "limited" classes of inboard hydroplane racing are organized under the name Inboard Powerboat Circuit. These classes utilize automotive power, as well as two-stroke power. There are races throughout the country from April to October. Many Unlimited drivers got their start in the "limited" classes.
Until November 20, 1977, every official water speed record had been set by an American or Briton. That day Australian Ken Warby broke the Anglo-American domination when he piloted his Spirit of Australia to 464.5 km/h (290.313 mph) to beat Lee Taylor’s record. Warby, who had built the craft in his back yard, used the publicity to find sponsorship to pay for improvements to the Spirit. On October 8, 1978 Warby travelled to Blowering Dam, Australia, and broke both the 480 km/h (300 mph) and 500 km/h barriers with an average speed of 510 km/h (318.75 mph).
As of 2005, Warby’s record still stands, and there have only been two official attempts to break it.