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Propelled To Success

Long fiber tough enough for interchangeable propeller blades

A holiday conversation between Brad Stahl and his brother led to the development of an innovative motorboat propeller designed to reduce the cost of damage from strikes with underwater obstacles by employing interchangeable blades and a new material to replace metal.

At the time, Stahl was an aerospace engineer working on designing propeller systems for unmanned aerial vehicles (UAV) who had begun working with long fiber reinforced thermoplastic composites.

As his brother, a motorboat enthusiast, told stories of damaged propellers and the expense of replacement, Stahl knew he could translate his work with UAVs to motorboats and provide an alternative to one-piece solid aluminum and stainless steel props.

Interchangeable Design

Composite Motorboat PropellerPiranha’s unique composite propeller design consists of a center hub with three or four slots for inserting interchangeable blades of varying size and pitch.

Easily changing a damaged or broken blade instead of the entire prop lowers ownership cost; interchangeability also allows tuning of propulsion thrust to meet a boats intended use, for either speed or transporting loads.

When a metal propeller blade strikes a submersed obstacle, it can permanently bend or break, with severe impact forces often causing additional costly damage to the motors lower unit.

With a Piranha composite propeller, the blades flex and absorb the energy from minor impacts.  At higher impacts, the blades, which have lower shear strength than metal, sacrifice themselves by breaking and releasing energy before transferring damaging forces to the motors lower unit.

Lower unit repairs can cost up to several thousand dollars, making a $25 composite replacement blade a considerable cost saver, according to Stahl.

Long Fiber Composites

Stahl’s first propeller designs used a 60% long glass fiber reinforced nylon 6 composite and testing on higher horsepower motors was successful.  “When I dropped back to lower horsepower two-stroke motors, we started seeing failures,” he said.  “It came down to higher harmonics and vibration in the two-stroke motors.”

The solution was adding a toughening agent to the long fiber composite that better dampened vibration energy, according to Stahl.

“You really have to understand the flexural modulus properties of long fiber composites when designing a propeller,” said Stahl.  “Our designs have to be slightly distorted when molded so that in use, when the blade is experiencing propulsion forces, it has the proper shape to maximize thrust.”

Maximum Toughness

Today, Piranha uses a Complēt® MT series long glass fiber reinforced nylon 6 composite from PlastiComp for its propellers.  PlastiComp formulated its Complēt MT composites for maximum toughness to provide up to 40% more impact resistance than standard long fiber reinforced materials.  The extra durability provided by Complēt MT long fiber composites makes them ideal for demanding applications like Piranha’s propellers.

“I originally started working with PlastiComp because I was looking for a second material source – they quickly became my primary source,” said Stahl.  “Other long fiber materials I tried looked bad when molded.  Complēt MT easily provided the surface finish I wanted for our propellers.”

“I really enjoy the relationship we have with PlastiComp,” he said.  “Every time I have a question PlastiComp is right on the ball.”

Hydrothrust Propeller

Piranha’s latest propeller iteration for houseboat and service barges improves reverse thrust.  “When the wind kicks up, a houseboat can quickly become difficult to maneuver,” said Stahl.  “We do a lot of sales to the rental market and it doesn’t take much for inexperienced people to have difficulty because reverse thrust performance is significantly lower.  Propellers are designed to push, not pull.”

Composite Motorboat Propeller BladeThe innovative blade design of Piranha’s Hydrothrust propeller provides 400% more reverse thrust than standard propellers and twice as much as competing metal props.  “Our Hydrothrust propeller produced 650 pounds of thrust in reverse with virtually no change in forward performance,” said Stahl.

“Whether backing a houseboat off a beach or maneuvering around docks, especially when there is wind in the picture, Hydrothrust provides a real boost in safety by increasing controllability,” he said.

Other enhancements Piranha is able to offer include Hydrobites, small flow fences on propeller blades that control water flow and reduce turbulence vibration.  “Injection molding with long fiber composites makes it easy to add feature details like our Hydrobites that would be difficult and expensive to produce in metal,” said Stahl.

Piranha Propellers, based in Jackson, California, is the leader in composite marine propellers serving the recreational boating market with propellers for all major brands of outboard and inboard motors.  For more information, visit their website at

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