July 31, 2007 Any jet-ski boasting a stroked-out, 1500cc motor straight from the Kawasaki Ninja ZX-12R should be treated with extreme caution – but since the 250-horsepower Jet Ski Ultra 250X was released, owners of the STX-15F have been feeling a little left behind. Not to worry – nothing succeeds like excess in the petrol head world, and the aftermarket has stepped in with a bolt-on supercharger capable of turning the STX-15F into a 330-horsepower aquatic widow-maker that can launch you and a horrified passenger to 60mph nearly as fast as a road-going superbike, leaving your 250X-riding buddies gasping in your wake.

Riva Racing’s supercharger system for the Kawasaki STX-15F is a full kit including all necessary engine components to pimp out a stock 60mph STX-15F into a race-ready, 80mph closed course shoulder-dislocating monster. It contains the same specifications that Team Kawasaki won back-to-back Pro Runabout World Titles with, producing over 300hp using VP MS109 race fuel (Jet X).


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That obscene horsepower figure only has to pull a weight of 337kg, putting the beefed-up STX-15F very close to a 1hp per kilogram power-to-weight ratio that’s up there with fire-breathing superbikes like Yamaha’s R1 and Suzuki’s GSX-R1000. It won’t do the 0-60mph sprint quite as quickly as the bikes – water doesn’t hook up like tyres on tarmac, but we’re still talking low 3-second bracket times.

A Vortech R-5 belt-driven centrifugal supercharger is the heart of the mod kit, which also includes forged racing pistons, bigger injectors, an auxiliary fuel pump, auxiliary fuel pressure regulator and a base gasket spacer. Pressurized air from the supercharger is cooled through a water-cooled intercooler before being forced into the original throttle body and a boost of 13lbs pumps up the stock motor’s original peak output by around 100%.

A free-flow exhaust lets the engine breathe more freely while unleashing a throaty roar, and the intake grate and ride plates are also replaced to improve hook-up and handling. The main pump and impeller are replaced with beefier items for maximum acceleration and there’s several other mods included to make the jet ski handle more like a racer too.

Riva recommend the kit for closed course racing only – and it’s certainly priced for the professional at a total price of US$10,568. For your dollar though, you’re buying a very pure adrenaline hit… if you have the guts to open the throttle!