Plastic propellers on RC boats break for several common reasons, with impact being the most frequent culprit. When a boat is running at high speed, the plastic prop is quite brittle compared to metal, so striking a submerged rock, stick, clump of grass, or even scraping the bottom in shallow water can instantly shatter one or more blades. Collisions with docks or pool walls have the same effect.
Another major cause is the propeller losing its grip on the water, leading to a phenomenon known as cavitation or ventilation. When the prop spins in aerated water or pulls air down from the surface, it suddenly encounters far less resistance. This causes the motor to over-rev violently, and the resulting spike in RPMs can easily tear a plastic blade off or explode the hub. This often happens if the hull design creates turbulence or if the boat is running too shallow in the water.
Boats that jump wakes or waves face a specific risk known as prop strike. When the boat comes down hard from a jump and the spinning blades hit the water surface, the force of that impact is tremendous. The water acts almost like concrete at that speed, and the sudden deceleration can snap the blades clean off.
Sometimes the damage is cumulative. A prop might have suffered a minor, invisible crack from a previous minor impact. Under normal running, that hairline fracture will gradually grow due to the constant high-speed spinning until it finally gives way. Similarly, a bent propeller shaft, even a slight wobble, will cause the prop to vibrate excessively at high RPM, placing cyclical stress on the plastic hub until it cracks and separates.
Upgrading a boat’s power system can also lead to breakage. If a stock plastic prop is paired with a more powerful aftermarket motor or higher voltage battery, it may simply not be strong enough to handle the increased torque. The motor can literally twist the prop hub apart or rip the blades off under acceleration.
Finally, improper installation plays a role. If the prop nut is too loose, the prop can slide forward on the shaft and then slam back against the drive dog when throttle is applied, creating a hammering effect that shatters the hub. Conversely, overtightening the nut can crush the plastic hub or create stress points, especially as temperatures change.