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Engine & Boat Service

By Don Garrison

Second in a series of propeller technology articles: July, 2003

Last month I addressed propeller performance as a function of diameter and pitch. This month, let’s take a look at both the physical and financial performance (and pitfalls) of the most common materials used to make propellers.
The perfect prop is made of a material I call “unobtainium” and it has all the ideal properties any good boater could ask for. What is “unobtanium”? First of all it is lightweight, reducing the stress on your drive train. With a lightweight prop there is less “clunk” when shifting in and out of gear, which translates to less transmission wear. Now, how about balance? Like the tires on your car, rotational balance is important, an out of balance prop will not only cause an annoying vibration but more importantly, higher stress on the bearings in the drive unit. An “unobtainium” prop has razor thin blades that displace less water during rotation making them more efficient. Ever wondered about the erosion at the base of your blades, typically called “cavitation burning”? Even when working correctly, powerboat propellers produce tiny cavitation bubbles, that at some point begin to erode the blade surface. Our perfect prop would be immune to cavitation erosion. What about the dreadful subject of impact-how would our “unobtanium” prop handle that? After a prop strike, it would return to its original shape and it would never transmit destructive energy to your expensive drive train. Instead it would absorb ALL the impact energy protecting your expensive drive components. If you did damage the prop, it would be repairable on the spot and be good as new when done. No more sitting on the shore all weekend for you. Finally and most importantly, how much would you pay for an “unobtainium” prop? Less than anything else out there, of course!

ALUMINUM is a common material for propellers, they are relatively in-expensive to manufacture making them low in purchase price ($75 to $195). Aluminum propellers are manufactured by injecting molten aluminum into reusable die cast molds requiring a significant amount of hand finishing, painting and balancing to complete the product. They are relatively light weight which is good for your lower drive unit; however, aluminum is a soft, electrically conductive material which does not stand up well to abrasion or electrolysis. Many boaters know aluminum is very vulnerable to impact damage. An aluminum prop may return to its original shape after a light impact; this means you stayed within the elastic limit of the material, as if it had a “memory”. A solid strike (like hitting a large log) may cause your aluminum prop to break or permanently bend. If you damage more than 1/3 of any of the blades it is probably not repairable. If it can be saved, you’ll have to send it off to a reputable repair shop that will weld, heat, re-shape, and hopefully rebalance it. You can expect this type of repair to take up to three weeks and cost a minimum of $60 plus pick-up and delivery costs.

STAINLESS STEEL uses a more labor-intensive casting process, which requires a considerable amount of hand finishing. The primary advantage to stainless steel is its stiffness and strength, the stronger material allows for blades 40-50% thinner than aluminum. You can expect to spend $350 to $1,000 for this type of propeller. At Wide Open Throttle you may see a 3-4% improvement over aluminum or composite– that is, an increase from perhaps 50 mph to 52 mph. Stainless is much less susceptible to erosion and wear, but is not entirely immune from the effects of cavitation and corrosion. Stainless steel can withstand an impact much better than aluminum, however, if you do nick or bend your stainless prop the repair can take up to a month and cost well over $100. The downside to having a prop as tough as stainless is it will transmit the energy of an impact directly to your drive. Now, not only do you have to repair the prop but you may end up shelling out as much as $3,500 to repair or replace the drive. A stainless steel propeller is three times the weight of aluminum so your drive must also handle greater inertial forces. Stainless steel is notorious for the “clunk” heard when shifting in and out of gear. Expensive stainless props have also produced a cottage industry for prop locks, as they are frequently the targets of thieves. Is a 4% performance increase worth the costs?

COMPOSITE PROPELLERS are made of extremely strong, high-tech materials, and are well suited for small 6 HP outboards through powerful 280 HP I/O’s. The initial cost is $80 - $175 for a high quality composite.

The injection molding process produces the same high quality propeller every time. This virtually eliminates hand work, which results in well balanced and geometrically correct blades every time.

Composite propellers are roughly half the weight of aluminum and an astonishing 1/6 the weight of stainless steel, resulting in a prop that is much gentler on the engine and drive. I learned first hand about composite propellers while boating with a friend in a very shallow lake. We hit bottom and due to the low weight and damage tolerance of his composite Piranha Propeller we did not discover the problem until the end of the weekend. Five minutes after putting the boat on the trailer we had installed a new blade and the boat was ready for our next trip.

During a hit composites absorb much more energy than metal props. Tap a metal prop with a wrench and it “rings”, while repeating that with a composite it produces only a dull thud. This is because composites internally absorb the vibrational energy, a superior trait when your prop strikes something more substantial than an open-end wrench! This is also why the material is essentiallyimmune to the destructive force of cavitation erosion. Absorbing energy also means you are much less likely to suffer gear train damage after an encounter of the worst kind. Due to the protection composite propellers provide, some insurance companies have offered reduced rates. Talk to your insurance representative and ask about getting a discount.

Should you be unfortunate enough to actually damage a Piranha Propeller blade, just remove it and replace it on the spot. A five-minute repair for $15.00 to $21.00 … now that’s what I’m talking about! You not only save a bunch of money but also save the day of boating fun. What happens in the unlikely case where you damage the hub? Easy! - a Piranha Propeller hub is unconditionally guaranteed for life, just return it!

So, there you have it: aluminum, stainless steel, and composites competing head to head or blade to blade. I can’t tell you which prop best suits your needs, but I can suggest that you take all the facts into consideration and maybe, like me, you’ll lean a lot more towards composites.

Find out more about composites at http://www.piranha.com or call 800-235-7767. My thanks to Piranha Propellers for all the helpful information.

Don Garrison is an aviation and technical writer/engineer. When not in front of a boat propeller, you can find him behind an airplane propeller, some even made from composites.