Kevin Shaw: I Swear Guys, I’m Right About This One


It was late 2006 when Kawasaki introduced the world to its new full-sized runabout, the Ultra 250X. The sharply chiseled JetSki touted the biggest engine producing the most horsepower of any personal watercraft before it; 35HP over the closest Sea-Doo and an astounding 70HP ahead of Yamaha. Equally, the 250X unseated the brand’s STX-15F as its flagship craft, and rightly fixed Kawasaki back atop the heap of performance PWC.

It was also the first watercraft I ever rode as an editor for a jet ski magazine. It was also the first supercharged 4-stroke PWC ski I had ever ridden despite the RXT and RXP being introduced a couple of years earlier. The staccato whistle of the roots-style blower shrieked angrily in my ears with each swell I leapt over. The deep V-hull split the undulating surf like a Coast Guard Cruiser. It was a surreal experience, uprooting everything I knew a jet ski to be. It was transformative.

But that year’s IJSBA World Finals, beneath the shade of the Kawasaki racing rig’s awning, were a line up of heavily modded STXs. Why? I queried, when the Ultra was so impressive? Sure, master craftsman Les Cook had grafted the rough-water Ultra hull to the smaller 15F in perfect Frankenstein fashion, and Skip Holmes seemed to be the only tuner who could make the 250X a closed course contender; but it was only after several changes were made (bracing, weight distribution, etc.) over the following year that the Ultra could serve as a worthwhile buoy course competitor, completely making the STX-15F obsolete.


Racers have been using the STX-15F as a racing platform since it’s early days as the STX 1100. It’s time for a refresh.

Today, the STX is less of an addendum to Kawasaki’s lineup than the SuperJet is for Yamaha (rather, rumors of a new SuperJet hull and a variety of engines are being considered). As it stands, the STX is 13 years old with the only major change being the graduation from the 12F to the 15F when the first Ultra 250X was released (making the 15F seven years old). Once the gilded crown of Kawasaki’s runabouts, the STX remains as the company’s deep-discounted entry level unit sitting quietly in the background as new 310-horsepower Ultras soak in the spotlight.

But it doesn’t have to be this way.

A year ago this January, I sat with some of the biggest names in Kawasaki’s watercraft division discussing this very topic. Soaking in the balmy night air of Florida’s Duck Key resort, I pitched the possibility of a Ultra-powered STX dubbed the “STX-310R.”

Beneath the two-piece bench seat of the STX is the real estate necessary to fit the current 1.5-liter Ultra 310X powerplant. Only the slightest of changes would be necessary to shoehorn the supercharged powerhouse into the lowly ski, namely moving the ECU bulkhead forward into the front stowage bin area and reconfiguring the waterbox/intercooler/pump configuration. The latter point requires the only major modification to the powertrain.


Unchanged since swapping from the 12F to the 15F over seven years ago, the STX hull has remained the same for a dozen years.

I suggested replacing the factory’s massive vertical intercooler with a smaller-yet-equally high volume cooler from RIVA Racing (as an example), and a single high performance waterbox placed opposite the intercooler. Similarly, the STX’s 155mm pump replaced with the Ultra’s 160mm axial-flow, single stage jet pump. And, a fixed-position, exposed motocross-style neck and handlebar (identical to the 310R) could replace the staid steering system.

Besides a woefully necessary redesign of the hood and top deck, the SMC fiberglass hull could effectively remain untouched if the hull was paired with an aggressive top-loader intake grate, ride plate and adjustable sponsons. The STX is small, nimble and now touting the most horsepower in the industry. What more could you want? One featured I begged for was launch control:

Similar to the Launch Control Mode on the high tech KX450F dirt bike, the system activates a second engine map for more efficient off-the-line starts. Like the KX450F, the ski’s Launch Control Mode is activated by a button on the handlebar, where the new map (tune) slightly retards ignition timing to dial back the engine’s torque and reduce cavitation off the start, disengaging and returning to the standard engine map automatically once the rider once reaching a specified MPH.


IJSBA World Champion Craig Warner’s Votech-supercharged STX-15F is a living example of how the STX can be used to outperform Yamahas and Sea-Doos handily.

I outlined a price and availability structure that would ensure the requesat interest, hype and demand: Kawasaki would pattern the STX-310R’s release similar to the recent H2 and H2R superbike releases, teasing the ski with viral videos, slowly leaking information over several months building up to a large-scale reveal. Dealers would be encouraged to take deposits on the ski early as delivery of the units wouldn’t be for several months out.

For 2016, only 505 units would be built to meet racing homologation standards. Priced at slightly above the luxury 310LX, the STX-310R would be Kawasaki’s most advanced, aggressive and potent personal watercraft ever built. I dreamed of a small “hit squad” of hand-picked and vetted racers to campaign the new STX-310R throughout the US National Tour, patterning after Yamaha’s masterful execution of their 2014 season, clinching four National Titles.

Effectively, Kawasaki could convert their wallflower STX-15F into a race-ready, top performance machine equal (if not superior) to Yamaha’s SVHO-powered FZR and Sea-Doo’s RXP-X 260 overnight. And considering the equipment the STX-310R would come stock with, championships in closed course and HydroDrag Pro Runabout Stock classes and the AquaX series are almost a given. Of course, we fully expect the same Launch Control Mode to be made available at the very least for the Ultra 310R, as it has seen so much use in competition already.

Again, this is what I shared that one night. I think people would want it. I know I would. Wouldn’t you?

Go Get Wet,

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Kevin Shaw

Kevin Shaw

Editor-in-Chief – Kevin Shaw is a decade-long powersports and automotive journalist whose love for things that go too fast has led him to launching The Watercraft Journal. Almost always found with stained hands and dirt under his fingernails, Kevin has an eye for the technical while keeping a eye out for beautiful photography and a great story.

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