Whether it’s a tournament tow boat or a Downeaster, boats powered by single inboard engines require more technique and skill to operate in the close quarters of a dock, marina or launch ramp than boats powered by other sources.
Enter Project Disruption from ZF Marine’s (zf.com) Special Driveline Technology unit. Introduced as a concept last fall, sources say that Project Disruption testing and development are currently underway aboard an undisclosed make of test boat at an undisclosed location. For now, this is what you need to know about this new propulsion system.
Project Disruption interfaces with a standard-issue ZF Marine transmission and is comprised of a strut-housed underwater gear box that transmits power to two contra- rotating propellers.
“For many years there has been little innovation in inboard shaftline technology. ZF saw the opportunity to bring all the benefits of contra-rotating propellers to this most traditional of propulsion systems. We started with tow sports because it presented a unique opportunity to investigate the concept at full scale while subjecting it to a wide variation of loading conditions,” says Keith Stanley, product line manager for Pleasurecraft.
Know that Project Disruption addresses more than just maneuverability. Its twin contra-rotating propellers deliver more blade area without an increase in overall diameter. That can result in superior acceleration and the ability to hold plane at a lower speed and provides more options with respect to optimizing the torque and power characteristics of a marine engine for the mission of the boat it’s intended to be installed aboard. Because prop diameter is limited, the means to deliver optimized power and thrust to the water have been limited too. But the increased blade area of contra-rotating props gives propulsion engineers a way to more precisely harness a marine engine’s output for the desired performance characteristic.
Since docking is a hot topic for most boaters, we’ll consider the potential slow-speed maneuverability gains boaters might expect from Project Disruption in detail.
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An inboard-powered boat lacks the directional thrust of steerable drivelines, like outboards, sterndrives, water jets and pods. Instead of a steerable prop or nozzle, they rely on deflection of the propeller stream by a rudder. Compounding things, when in reverse, there is no stream of water from the prop flowing over the rudder at all. This couples with a propeller’s tendency to induce a sideways motion to the boat that kicks the transom to the side. Thus, when trying to go astern, the uninitiated skipper can suddenly find the boat going in circles. Even shifting into reverse to slow down may cause the boat to go sideways enough that all it takes is an errant gust of wind to put the boat at the mercy of the elements.
We’ve written extensively about prop torque in the past, and you can learn more about the phenomenon by visiting boatingmag.com/how-to/how-to-use-prop-torque.
Contra-rotating props, like those of Project Disruption, eliminate the kick caused by prop torque. Since each prop spins in a different direction, each negates the sideways thrust created by the other.
Contra-rotating propellers have been in use since torpedoes fitted with them in World War II ran straighter than their single-prop counterparts, and they were popularized in the recreational marine arena by numerous sterndrive and outboard offerings. Additionally, ZF maintains that while Project Disruption is initially sized for tow-sports boats, the concept could be scaled to larger propulsion packages or hybrid systems for a multitude of vessel applications.
I’ll be visiting the ZF Marine headquarters in Italy later this spring to meet with engineering staff. Following that trip and (hopefully) a sea trial of Project Disruption, I’ll file a follow-up report.