Of the current boats, they're all running old Vietnam-era helicopter engines. They're turbine engines.
If you really want to know, they are usually the Lycoming T-55, L-7 engine. That's a gas-powered turbine engine - 2,650 HP.
They run on jet fuel and hold about 80 gallons, running through nearly four and a half gallons a minute at top speeds.
The Lycomings were used most commonly in the CH-47 Chinook military helicopters, a twin engine helicopter used for heavy lifting. The Chinook was used primarily for troop movement, placing artillery and resupplying batefields.
The turbine engines are fast. They can propel an unlimited hydroplane at speeds of more than 200 mph on the straightaways. Dave Villwock set a speed record of more than 220 mph using a turbine engine in a test run in 2004.
Accidents have been rare in recent years - at least due to engine problems. Perhaps the most dramatic was in 2006 at Seattle's Seafair when driver Kevin Aylesworth had to jump to safety when his boat caught fire. It ws actually vapor from an oil leak that caught the turbine engine on fire.
Most teams travel with a backup engine that can be swapped out during the course of a race weekend. It's not something teams look forward to, but they can take one out and put a new one in in a remarkably short amount of time, sometimes between heat races.
In recent years, there has been only one piston-powered engine on the Unlimited Hydroplane racing circuit.
The piston-power Allison V-12 engine is inside the U3 from Cooper Racing. It's always a crowd favorite because it's incredibly loud.
Before the 2005 season, the piston powered boat was a novelty act with a huge fan base, but not competitive. That changed with new tweaks to the engine and suddenly the U-3 was competitive and winning some heats.