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The new X Power Drive is designed to replace Bravo units in big-horsepower applications. Originally published in the August 2009 issue By Matt Trulio There’s a reason the MerCruiser Bravo One XR drive is rated for engines up to 600 horsepower: It simply can’t handle more power. But darn if that hasn’t stopped any number of go-fast powerboat owners from trying to push more power through one. And while the Bravo One XR drive might last awhile in a horsepower application beyond its rating if handled gingerly, failure is inevitable.
Stronger, “upgraded” Bravo units are a better call for such applications, but even those models have been met with limited success. Some folks swear by them. Others curse them. Despite the significant engineering talent that has gone into beefing up the Bravo drives, no company that offers one seems to have developed the perfect formula. Performance-boat enthusiast Bill Auberlen, who does most of his high-performance boating on Lake Havasu in Arizona, has blown up just about every incarnation of a Bravo drive you care to name. A long-time BMW-sponsored endurance automobile racer—he’s won both the 24 Hours of Le Mans and the 24 Hours of Daytona—Auberlen has owned a bunch of big-horsepower sport catamarans. They’ve all had one thing in common: broken Bravo drives and ruined weekends. “It seemed like every weekend I’d have my family and friends on my boat on Havasu, we’d break a drive and the weekend would be over,” he says. “I was getting to know the Vessel Assist guy by first name.” Five years ago, Auberlen decided to do something about it. He tapped his deep list of automotive engineering contacts, gave them a clean sheet of paper and asked them to come up with a reliable, big-horsepower stern drive. The result was the X Power Drive, which was on display at the Los Angeles Boat Show in February. According to Auberlen, the dry-sump unit can handle up to 1,535 hp and 1,295 foot-pounds of torque. “That’s what we’ve tested them to,” he says. “And we haven’t had a single failure.” Among the key components of the unit is its dry-sump oiling system. Internal parts such as bearings and shafts do not sit in an oil “bath.” Instead, oil is delivered to them via precise sprayers and a three-stage pump that returns the oil to an external baffled and filtered oil tank where it is allowed to settle. (Spraying aerates the oil, and that aerated oil must settle before it can be used to effectively lubricate.) The result, according to Auberlen, is a far more efficient—in terms of reduced internal friction and the heat it creates—stern drive than any Bravo One or Bravo-style unit. Exactly how much more efficient, Auberlen says, is proprietary information. He will say, however, that thanks to many hours of hydrodynamic CFD (computational fluid dynamics) modeling in the design phase of the X Power Drive, the unit creates 37 percent less drag in the water than other “comparable” Bravo units. Part of that efficiency stems from the X Power Drive being a surfacing unit, meaning a section of each propeller blade is above the waterline during each revolution of the prop. (The drive reportedly can be set up with the highest X dimension or drive height in the industry.) To adjust the amount of lift or down force for a specific powerboat application, the builder offers optional/removable stainless-steel skegs. As for the X Power Drive housing itself, it is made from “aircraft-certified, hybrid-proprietary” metals, as well as carbon-fiber pieces. Though X Power Designs builds the drives in-house at its Huntington Beach, Calif., plant, all of the drive components such as gears, shafts and bearings are manufactured by outside vendors. “My only requirement was that it be the very best parts built by the very best people,” Auberlen says. Designed as a plug-and-play unit, the X Power Drive fits on a standard Bravo gimbal ring. The only additional pieces of mounting hardware required are stronger stud bolts and nuts, and different trim-ram bushings, which X Power provides. Auberlen says the gimbal ring becomes the “weak link” in the package, which is why his company plans to offer a stainless-steel replacement for it. As a self-contained model, the X Power Drive requires an external water pickup. At present, the unit is offered with 1.28:1, 1.32:1, 1.36:1 and 1.55:1 gear ratios. The price is $35,000. Delivery of the units to customers, most of whom Auberlen says will be individual boat owners rather than boatbuilders, should begin by mid-summer. “We had no idea how much of a market we have for this drive,” he says. “It’s huge. There has already been so much interest.” Contact Information If you like what you're reading, get more by subscribing to Powerboat magazine here. |
But here’s why—against all good advice—people still try it: The nearly bulletproof Mercury Racing No. 6 dry-sump drive doesn’t work in smaller boats with big-horsepower setups. To run a No. 6 drive, you need enough real estate for the drive and a transmission behind the engine, and most catamarans and V-bottoms in the less-than-30-foot range just don’t have it. Plus, for most boats in that size range, the No. 6 unit is just too heavy.