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Selecting the right propeller is an important factor in maximizing your boat's performance, while allowing it to operate in the recommended rpm range. Determining the correct size and style of prop will keep the engine from over-revving, while allowing it to achieve the maximum horsepower at the lowest rpm's.
NOT ALL PROPS ARE THE SAME!
Props are described by referring to diameter and pitch. Diameter is twice the distance from the center of the hub to the tip of any blade. Generally smaller diameter props correspond with smaller engines and boats or higher speeds. Larger diameter props correspond with larger engines and boats or slower speeds. Pitch is the forward movement of a propeller through one complete revolution. Pitch can be thought of as speed. The higher pitch prop the engine can turn, the faster the boat will usually go.
Number of Blades -
Sufficient blade area is needed to properly distribute engine power across blade surface. Too little blade area causes high "blade loading", which means the prop can't absorb all the power transferred by the engine. With increased blade are you can decrease the blade diameter, but often with less efficiency at higher speeds. A 3-blade prop is considered the best combination of blade area and low blade loading. When the number of blades is changed, diameter and pitch may require a minimal adjustment to remain in the safe blade loading range. For most purposes 3 and 4-blade props can be used interchangeably on outboards and sterndrives (I/O) without much of a change in performance.
Propellers are made of either composite, aluminum, or stainless steel. Composite props are good, durable, and inexpensive. Aluminum props are the most common. They are suitable for the widest range of applications. Stainless steel props offer higher performance and better durability.
Cupped Propellers -
Special curved trailing edges enable the prop to cut through the water better. Cupped props allow most boats to achieve a higher top-end speed or at least the same speed at a lower engine rpm. They also promote more efficient fuel consumption.
Wide Open Throttle (WOT) rpm Range -
When selecting a prop, the goal is to choose one that allows the engine to reach its optimal WOT. This is generally between 5000 and 5500 rpm, depending on engine type. This information is usually included in the owner's manual of a new boat or engine.
If your current prop's performance is within manufacturer's WOT guidelines,
choose a replacement prop which duplicates the diameter and pitch of your current prop. You might consider upgrading to a different material such as stainless steel.
If your current prop is unsatisfactory -
What if your engine operates at the wrong rpm at WOT? Pitch and rpm have an inverse relationship. Increasing pitch reduces rpm and reducing pitch increases rpm. A 1" change in pitch will result in a 200 rpm change in engine speed. Therefore, if your engine operates below the proper rpm, consider a propeller with less pitch. If your engine over-revs, consider increasing the pitch.
You might also consider changing the propeller size to affect a specific performance attribute. A lower-pitch power prop makes it easier to pop skiers out of the water. Tournament bass boats may need more top end speed and should use a prop with a higher pitch. Houseboats and cruisers care more about efficiency at displacement speeds, therefore they require a lower pitch to achieve low-end power and the largest diameter their lower unit can handle.