This section gives you a bit more 'in-depth' information about propellers, and why one size doesn’t fit all.
Horsepower does you no good if you can't effectively transfer it to the water. That's why the right propeller is so important. From their size, shape and the number of blades to a host of other variable criteria, no one type or style of propeller is perfect for all boats and boating applications. Propellers can be likened to automotive tires. There are different designs, sizes and shapes to allow selection of the type and model that best meets the specific performance requirements of the particular application.
A propeller represents a system of trade-offs to find the right prop for your particular application, where the phrase "too much of a good thing can hurt you" rings true. There's no "magic bullet" when it comes to choosing the correct propeller. The information presented here will help get you closer to maximizing the performance of your particular boat and your typical use of that boat.
Keep in mind that there's an important distinction between "high-performance" and "high-speed" propellers. High-performance propellers have certain characteristics that allow the propeller and boat to provide maximum performance in multiple areas, i.e. acceleration, mid range and top speed. A high-speed propeller is designed to perform and operate most effectively in those applications where top speed is the priority and acceleration and the ability to carry a load is less important.
Pitch is the distance (in inches) a particular prop would theoretically travel in one full revolution, as if traveling through a solid. A lower pitch will have better hole shot and “pushing power” but a lower top speed, while a higher pitch prop will provide less acceleration, but a greater potential for higher top speeds. The correct propeller will allow your engine to reach the upper portion of the WOT range speci!ed by the manufacturer with a normal-to-heavy load (without exceeding it). Each inch of pitch is equal to approximately 150 +/- 50 RPM.
Diameter means the total width of the “circle” at the blade tips as the propeller spins. A larger diameter pushes more water and reaches deeper into the water, so they’re typically used on large, heavy boats or ones with high engine mounting heights. A smaller diameter is usually used on lighter weight boats, where the prop operates lower in the water or when a gain in engine RPM is desired.
Three-bladed propellers are the most common, o!ering good overall performance, top speed, and e"ciency for most applications. Four blades characteristically provide increased acceleration, enhanced bow- and stern-lift, and reduced ventilation. However, four blades typically mean more drag on the engine, resulting in lower top speeds and di!erent handling characteristics.
Blade surface area refers to the total surface of the blades. The more blade surface area a prop has the more water it pushes, for better hole shot and increased planing e!ciency. Too much can create signi"cantly more drag, however, potentially restricting engine RPM and causing negative boat-handling issues.
Ventilation is when air is drawn in around the propeller blades. Normally, this causes a gain in rpm, but a loss of speed, since the propeller blades are not biting clean water. This most usually occurs during hard cornering or in certain water conditions, such as following seas.
Controlled ventilation can be bene!cial, though, in helping the engine gain rpm during hard acceleration. This is engineered into certain propellers, and is most-usually used on two stroke outboards. An example of this is the small holes in the side of the propeller barrel which allow exhaust to intentionally be drawn in around the blades at hole shot. This helps two stroke engines generate the higher rpm they need for proper hole shot performance. Four stroke engines typically don’t need them.
Cavitation occurs when pressure on the water across the blade’s surface is reduced to the point of becoming water vapor, forming bubbles. If these bubbles burst, they can cause a cavitation burn which can deteriorate the propeller’s surface and cause negative performance issues. As this condition can cause an increase in engine rpm, it’s often confused with ventilation.
Slip is the amount of wasted energy a particular prop generates, meaning that the actual distance traveled in one full propeller revolution is less than its pitch measurement. It is normally expressed as a percentage of ine!ciency. A certain amount of slip is engineered into each line of propellers to create di"erent performance characteristics.
“Hole shot” refers to rapid acceleration of the boat, from a standing rest or very slow speed until just on-plane. This is when the engine and the propeller work their hardest.