Choosing the Proper Propeller

By Jim Daly

Prop Selection

 

When you bought your first boat, propeller selection was easy—whatever came with the boat, usually made of aluminum, was fine. Eventually your props stack up some miles and show significant wear. Would changing the propeller material significantly affect boat performance? Of course, the answer is yes. A stainless steel prop is likely to deliver the most speed from your motor because it has thin blades (to reduce drag going through the water) that can be forged into complex shapes to optimize performance. That should also translate into better fuel economy. Stainless props also absorb minor impacts and spring back into shape—most dings can be banged out easily. However, stainless props do have a couple of downsides. Stainless is typically the most expensive prop material and, if you keep your boat in salt water, is most likely to cause corrosion in an outboard or sterndrive leg.

 

Aluminum props are the common alternative to stainless. They are less expensive and reasonably durable. However, significant dings in aluminum props often cannot be banged back into shape without becoming excessively brittle. Compared to stainless wheels, aluminum props have a lower tensile strength, so they are cast with thicker blades, and they are typically 2 mph slower than stainless at wide-open throttle. For my money, however, top-end speed is less important overall than cruise speed, where the two materials will often perform equally. Since aluminum is similar in material to your outboard or sterndrive’s lower unit, corrosion problems are minimized.

 

Composite propellers (typically carbon-fiber composites) have some unique advantages. In terms of cost and performance, they rank somewhere between aluminum and stainless. There is no chance they will cause corrosion problems and they bounce back to their original shape even after significant impact. Composite props generally have a multipart hub section that can be fitted with replacement blades. This means you can carry spare blades rather than spare props, or carry one set of blades for heavy loading and another for running light. Composite props are remarkably durable and have the added benefit of being extremely light—so light that if one blade breaks off, you can continue home without causing the kind of vibration that can damage your engine. Cheaper plastic propellers are also available, but they are meant to be used primarily as emergency replacements.

 

The key to choosing the correct propeller size for your engine is rpm. If your motor is turning to its top-rated rpm at full throttle under normal load, then the prop is probably the right size. If it doesn’t turn all the way up, a smaller prop is required. If the engine over revs, the existing prop could be too big. However, if it does turn above rated rpm, make sure you’ve loaded the boat as you would normally before switching wheels.

 

For more information on prop selection, visit the Web sites of propeller companies. There are helpful comparisons of propeller options available for your specific outboard or sterndrive.

 

This article appeared in Boating Life magazine July/August 2003.