Balancing an 850-HP 16-bladed Airboat Prop Assembly with DynaVibe GX

​With high performance airboats, the raw power can approach 1000 HP. As with aircraft, an unbalanced airboat propeller will quickly turn some percentage of that raw power into damaging or even catastrophic vibrations. RPX Technologies recently spoke with Steve Boser, Sensenich VP of Engineering, about using the DynaVibe GX to balance an 850 HP airboat. 

Sensenich Propeller Manufacturing Company manufacturers lines of propellers out of carbon fiber, aluminum, wood, or wood composites for aircraft, UAVs, and airboats. Of course, Sensenich propellers are statically balanced during manufacturing and subsequent servicing. However, as with any prop, a dynamic balance after installation can balance the influence of the entire rotating assembly into which that prop is now integrated.

A Sensenich customer that manufacturers airboats was encountering some balancing challenges with their assembly. “My airboat sales manager reached out” says Boser. “He said ‘Hey, is there anything we can do to help these guys out? I don’t think it’s a static balance issue. It’s a pretty novel installation.’”

“There’s a total of 16 blades,” says Boser. “That’s an eight-bladed propeller on the front. Then there’s an eight-bladed propeller on the back. There’s a coaxial shaft. You have a front shaft with a big hollow space in it, and then you’ve got a smaller shaft that sticks through it to the rear prop.  They turn in opposite directions. The marine [airboat] guys call it a counter rotator.  The aircraft guys call them contra rotating.”

Boser’s engineering team already had a DynaVibe GX for use as an engineering tool with spectral analysis. “Also, to help us in refining procedures for static balance,” says Boser.

Sensenich agreed to send Boser, along with the DynaVibe GX, to help troubleshoot this contra-rotating 16-bladed 850-HP power plant.  This power plant can propel the large airboat across the water at up to 80 MPH. However, before this high-performance airboat could be delivered, the vibration issues had to be tamed.

Boser made some initial measurements with the GX, but there wasn’t an application note for this unique setup, so he ended up contacting Steve Sennett at RPX, the makers of DynaVibe. “I reached out to Steve,” says Boser. “and after about 20 minutes I had it all figured out.”

Because of the design of the counter-rotating prop assembly, the rear prop could be removed but it was not practical to remove the front prop. Therefore, Boser first balanced the front prop with the rear prop removed and then balanced the rear prop with the now-balanced front prop in place. In this counter-rotating setup, the props are mechanically locked 1:1 in rotations, so Boser did not have to move the optical tach to balance either prop.

“Half of it was getting into the routine of removing and adding weights,” says Boser. Sensenich Saber Blades are designed to mount on a Warp Drive hub. “There are four bolts that hold each blade into a plate hub.  It’s like two aluminum plates that sandwich the blade pitch blocks. Once I got into the routine of getting them apart and adding the weights it went pretty fast.”

“I’d get it up to cruising RPM because that’s where I wanted to ensure it was going to be nice and smooth,” says Boser. “They don’t go full throttle for a long period of time.  It’s just like an aircraft, you’re going to be trucking around at cruise RPM. It was 1,100 RPM on the prop or so.  It was around 3,000 RPM on the engine with a 2.68 reduction.”
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 “I think I got the front prop in about three runs” says Boser, “and I got the rear prop in two runs.  I ended up with a 0.04 IPS on it. Went very fast.”
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The core business of RPX is aviation -- its passion is making anything with a prop spin more smoothly to minimize damaging and uncomfortable vibrations.  Whether you’re piloting an experimental canard pusher, a traditional tractor-design aircraft that pulls itself through the air, or even a Skymaster that pushes and pulls, RPX can help.  Even if you are skimming around the bayou, or navigating flood waters in water-rescue operations -- DynaVibe can balance your airboat prop too.  If it’s got a prop and it rotates, contact RPX to let us help you smooth out your ride and find out which DynaVibe is right for you.