Marine propellers come in several different types of metal and alloys, and each has its pros and cons, as we learned from Larry Kindberg of AccuTech Marine Propeller in Dover, New Hampshire. AccuTech is the largest propeller tuning and diagnostics center in the Northeast, and they repair and tune every type of prop imaginable using the latest high-tech computer equipment and software.
When it comes to prop failure, Kindberg has seen it all, including frequent examples involving “demetalization” in lower-cost bronze propellers. He explains that many of today’s manganese-bronze props contain high amounts of zinc, which leads to electrolysis in salt water and eventually causes the metal to fail or corrode. He has seen examples of catastrophic failure in manganese-bronze props that are just a year old.
“Don’t buy a manganese-bronze propeller!” Kindberg warns. “Manganese-bronze props have about 30% zinc content in the alloy. Through metallurgical analysis of several manganese-bronze propellers we conducted several years ago, it was found that the zinc content in the newer propellers was about 3% higher than the ones manufactured about 10 years prior. Many things affect metal in salt water, but raising the zinc content in the alloy accelerates the demetalization process.
“Water salinity, acidity and temperature are all factors that can accelerate the process, but there is no way to guard against this cancerous process. That’s why we educate our customers about the benefits of NiBral [nickel-aluminum-bronze]—which has no zinc content. Unfortunately, price usually dictates what a boat owner chooses for a propeller.”
NiBral may cost more, but the alloy is stronger than bronze, and resists the effects of electrolysis. Plus, it is easier to repair and refinish through grinding and welding. That said, even NiBral props need to be inspected each season and properly tuned to give the best performance.
Another benefit is weight. NiBral weighs approximately 10 percent less than manganese-bronze, and it can be made into thinner sections because of its higher strength. Loading stresses on the tailshaft and bearings are reduced, permitting the use of smaller shafts.
Stainless is the metal of choice among fishermen and performance-boat enthusiasts who use outboard or I/O power, but it too can fail without the prop striking a hard object. In the accompanying video, Kindberg describes a case in which the blade of a stainless prop sheared off and sailed 150 yards through the plate-glass window of a lake house. Luckily, no one was hurt, but the incident showed the importance of inspecting the prop blades for stress cracks.