Prop School

What you need to know about propellers

Published online: Jul 06, 2009 Feature PDB Staff
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What is the most important thing when choosing a propeller? Pitch. No matter what size engine or what kind of boat, if the propeller isn't the right pitch, the boat isn't going to live up to its potential.
Pitch is the theoretical distance, in inches, a propeller moves forward every time it turns (i.e. a 19-inch prop should move forward 19 inches with each revolution; in reality, it's a bit less, because the propeller isn't 100 percent efficient).
Here's the skinny: You want to be able to run the engine at, or near, the manufacturer's recommended maximum RPM at full throttle, trimmed up for speed, with a typical load in the boat-and if you can, your propeller is the right pitch.
Too much pitch (over propped)-The engine won't reach its max RPM, is sluggish getting on plane, and has poor throttle response. Not enough pitch (under-propped)-the boat pops on plane and accelerates with confidence, easily exceeding the recommended full throttle RPM. Neither condition is good for engine longevity or fuel economy.
The cure? Since every inch of pitch is worth about 150-200 RPM, decreasing pitch should provide a proportionate increase in RPM-drop pitch an inch, gain a couple of hundred RPM. Conversely, increasing pitch usually results in an RPM decrease-go up an inch, lose a couple of hundred RPM.
After you have the pitch part of the equation figured out, then the propeller selection process can continue. Steve McLelland, sales and marketing manager of Precision Propeller Industries says, "Different styles of propellers offer unique performance characteristics-more bow lift, quicker hole shots, and the like. Once we've determined the correct pitch range for a given boat, then we can start talking about what your performance goals are and what propeller will help you get there."





Aluminum vs. Stainless Steel
Aluminum propellers aren't expensive, they work well, and are relatively easy to repair, but aluminum isn't particularly durable, making aluminum props susceptible to wear and damage. Stainless steel propellers cost more up front; however, because stainless steel is stronger than aluminum, a stainless propeller's blades are usually thinner and can be made in a wider variety of styles and shapes than an aluminum prop, thus the potential for better performance and increased service life.
Remember, it's stainless steel, not stain-proof-a stainless propeller can still rust, so you need to keep it clean, just like the rest of your boat.

Three Blades vs. Four Blades
In broad terms, three-blade propellers offer good overall performance; however, a three blade prop may lose its grip in turns and may not be the best handling propeller on high-horsepower rigs. Four-blade propellers can get a boat on plane faster, they're less likely to lose traction in turns, and help many boats handle better, although a four-blade propeller is often a bit slower
(1-2 MPH) at top speed than a three-blade prop.

Propeller Resources
Yamaha Marine offers propellers to suit nearly any application, and its website, www.yamaha-motor.com, is a great place to start searching for the perfect prop. Here, you can check out Yamaha's propeller charts, as well as over a thousand performance bulletins that document real-world results on almost any boat imaginable. We also suggest looking into Precision Propeller Industries, Inc. (www.precisionpropeller.com, 800-922-9955), which functions as part of Yamaha Marine Group. Precision Propeller makes Turbo Prop stainless steel propellers, if you're seeking something a little different than an original equipment prop.
When contacting Yamaha or Precision Propeller, make sure to have all the information about your current setup (boat length, style, engine, propeller, full-throttle RPM, and other performance data) available to discuss with the prop pros-it'll help them help you.

Finally
Try as many different propellers as possible before shelling out hard-earned dollars on a new prop. Your local dealer might let you borrow a propeller or two if you ask nicely-there is no substitute for on-water testing. 

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