It All Comes Down to This:

 

Horsepower does you no good if you can't effectively transfer it to the water. That's why the right propeller is so important. Propellers can be likened to automotive tires. There are different designs, sizes and shapes to allow proper selection of the type and model that meets the specific performance requirements of the particular application.

Likewise for propellers; 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. 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. Although 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 give maximum performance in 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 ability to carry a load is less important. Certain designs and styles of propellers offer a "middle ground" between these two extremes.


 

The Basic "Whats & Whys" of Propellers

 

Ventilation

Ventilation occurs when air is drawn in around the propeller blades from the water's surface or from the engine's exhaust gases. Normally, this causes a gain in RPM, but a loss of speed, since the propeller blades are not biting "clean" water and become increasingly inefficient. Other times ventilation can be beneficial by helping the propeller have a "controlled slip" on initial acceleration, thus generating more RPM at a lower engine speed, allowing horsepower to be developed more quickly and aid in acceleration in certain applications. This is intentionally done through the strategic placement of exhaust vents or by using an "over and thru-hub" style propeller design. These systems are normally found only on high-performance props.

Cavitation

Cavitation is caused when a propeller, under certain conditions, is passing through the water at a sufficient speed to cause a low pressure area to form on the blade surface. The resulting low pressure can cause the water to boil in that specific spot, a condition known as cavitation, because of the lower boiling point of water at lower pressure. When this occurs, water vapor is formed and the water vapor bubbles move across the propeller blade surface. As the water vapor slows while passing across the blade, it will enter an area of higher pressure. If the pressure is no longer low enough to support "boiling" of the water the water vapor bubbles "pop" and return to water. This action can cause a "cavitation burn" on the propeller's surface at the point on the blade where the low pressure and the higher pressure areas meet. This action, if left unchecked, can erode the metal, discolor the propeller blade surface, chip away at any propeller paint and actually deteriorate the blade's surface. Cavitation is most often caused by imperfections on the leading edges of a propeller's blades, such as a dent or a ding, a poor blade design for the application, or any thing that causes a continuous disturbance in the water flow ahead of the propeller or gear case.

Pitch

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 greater acceleration or pushing power but with a lower top speed, while a higher pitch propeller will provide less acceleration or pushing power but a greater potential for higher top speeds.

Too much pitch can cause an excess load on the engine and prevent it from reaching its ideal wide-open throttle (WOT) RPM range as specified by the engine manufacturer, thus reducing both top speed and performance. Too little pitch will result in the engine regularly exceeding its manufacturer-specified WOT RPM (if the engine WOT RPM is below it's stated range, it's referred to as "lugging" the engine; if above it, it's referred to as "over-revving" the engine). Sustained operation in either of these situations can cause catastrophic damage to the engine.

In short, you want a propeller that allows your engine to reach the upper portion of the WOT range specified by the manufacturer, with a normal-to-heavy load.

You many also hear the terms "flat pitch" and "progressive pitch". Flat pitch means just that the water leaves the blade at the same pitch (or distance) measurement as when it first came in contact with it (for example 19" of pitch from the leading edge of the prop blade to the trailing edge). Conversely, progressive pitch means that the pitch may measure lower at the leading edge of the blade (say 19", which would be better for acceleration) and have a higher pitch angle toward the trailing edge (say 23", which would be better for top speed). The pitch of these type propeller blades are progressively curved, hence the term "progressive pitch".

Slip

"Slip" is expressed as a percentage, and represents the amount of "wasted" energy a particular prop generates. Technically, it represents the difference in the actual distance traveled in one full propeller revolution verses the expected distance traveled in one propeller revolution, and is expressed as a "percentage of inefficiency" of the propeller in the particular application in which its measured. Slip can be caused by an incorrect propeller blade design, too small a propeller diameter, to much positive trim of the engine while underway or an incorrect engine mounting height. Excessive slip causes the propeller to operate less efficiently and performance will suffer, as will fuel economy and top speed.

Hole Shot

"Hole shot" refers to rapid acceleration of the boat, from a standing rest or very slow speed until the boat is "on-plane" and riding on top of the water. This is when the engine works its hardest, so it is important to have the right propeller for the job. A propeller that offers too fast of a hole shot will typically yield low top speed and may cause the engine to exceed its specified WOT RPM range, while a propeller that has too slow of a hole shot typically has poor acceleration performance and may not bring the engine up to its desired WOT RPM range. Either of these scenarios cause undue strain on the engine and reduce its overall performance and fuel efficiency.

Diameter

Propeller diameter means the width of the circle as defined by the blades as the propeller spins. The diameter of a propeller is dictated by its design and intended use as well as the size of the gear case and overall speed considerations. Higher engine mounting heights and/or heavier boat weights typically call for larger diameter propellers. Smaller diameter propellers are typically used for lighter-weight boats, engine mounting heights where the propeller is lower in the water, or where an increase in engine RPM is desired. Propeller diameter varies between models and is specifically designed into each. The optimum diameter is determined by the manufacturer during the initial design, development, and testing phase of each propeller model.

Cup

Cup is the small curved lip along the trailing edge of each propellers blade and extending to each blade's tip. Different prop models have different amounts of cup, depending on their intended application and the performance characteristics they're designed to provide. Cupping keeps water on each blade longer, allowing better 'bite'. As cup increases it helps reduce ventilation and propeller slippage, can allow for higher mounting heights, and helps provide greater bow lift. Too much cup on a propeller, however, can cause excessive steering torque and too much bow lift. This scenario can limit the engine's ability to develop and maintain proper RPM for a certain pitch in specific applications. Size Propeller sizing is characteristically expressed in two numbers, diameter and pitch, and each is expressed in inches. The diameter of the propeller is the first number and the theoretical pitch measurement is the second number. So a propeller that says 14" x 17" is 14" in diameter and 17" of pitch. This same propeller may also be described as a 14x17x3 which would be a 14" diameter x 17" pitch in a 3 blade design.

Rake

Rake is the angle of the blades in relation to the propeller's barrel (the center tube), and is expressed in degrees. A high-rake propeller is best-suited for high speed applications, particularly those utilizing high engine mounting heights where ventilation or cavitation is more likely to occur. Higher rake can also increase bow lift and propeller efficiency, resulting in better performance. Too much rake, however, strains the engine, decreases hole shot, and can produce negative performance and handling results. Basic propellers usually have 5-20 degrees of rake. Higher performance progressive pitch propellers will usually have 20 to 30 degrees of rake. Propellers typically operating in ventilated or disturbed water can enjoy performance benefits from having higher rake.

Gear Ratio

Gear ratio on a marine engine refers to the gears used in the lower unit, and is always expressed in reference to one (1), such as 2:1. In this instance, it means that for every 2 revolutions of the drive shaft, the lower unit will produce 1 revolution of the prop shaft. The gear ratio for a certain engine is determined by the manufacturer at the time of the engine's development, specific to its intended use and application. It's important to choose a propeller that's correct for the specific gear ratio your engine has and the application in which you use it. The correct propeller should allow the engine to operate within the manufacturers WOT RPM specifications under the loads and conditions in which the boat is being used. Pitch is the most, but not only, influential factor in determining the correct propeller for the application and gear ratio.

Blade Geometry

Blade geometry refers to the actual shape of the blade (or 'ear'). By manipulating the blade's shape, diameter, and pitch progression; different performance characteristics are created for each different type and style of propeller. That's why no one prop is perfect for every application. By design, different prop styles and models create different performance characteristics. Choosing the best one for the job allows a boat and engine combination to achieve the desired performance results.

Blade Surface Area

Blade surface area refers to the total surface of the propeller blade. This is important because the more blade surface area a prop has, the more water it pushes, but it can also create more drag on the engine. More blade area can give you better hole shot and allow a boat to remain on plane at lower engine speeds, but too much blade area can restrict the RPM that the engine can develop and cause boat handling issues. As mentioned elsewhere in this document, operating the engine outside its recommended specifications makes the engine work harder or faster than it has to; reducing efficiency, fuel economy and possibly damaging the engine.

Lower Unit (Gearcase) Size

Different engine models have different sized and shaped lower units. The "size" of your engine's lower unit is the measurement of the diameter of the gearcase "bullet" that being the round portion facing the rear of the boat. Propeller models are designed with gear case dimensions and requirements in mind during the manufacturers' propeller development process. Propeller diameter remains a key dimension in proper performance of any propeller. Most propellers are sized to fit the gearcase diameter precisely so that all of the engine's exhaust gases are passed directly through the propeller, while some specialty props are designed have a smaller 'barrel tube' which allows the exhaust gases to flow over and through the "barrel", thus generating beneficial RPM lower in the engine's power curve. These two design scenarios are specifically for performance purposes.

Lower Unit (Gearcase) Shape

Gearcase shape is normally only considered in very high-speed and/or racing applications, but it demonstrates that the shape of your lower unit is important in selecting the correct propeller, too. The more 'blunt' the leading edge of a gearcase is, and the faster the unit (boat) is traveling through the water, the farther out water spreads when it hits the leading edge of the gearcase. If the water is spread out beyond the tips of the blades, the prop will experience extreme ventilation causing a drastic loss of performance. This is commonly known as 'gearcase blowout'. Conversely, too much angularity to the leading edge of the lower unit will make the lower unit into too much of a 'rudder' and cause the engine and boat to 'dart' excessively, leading to improper high-speed running and handling characteristics. Again, this is not normally a concern to most boaters.

Number of Blades

Three-bladed is by far the most common propeller design for today's boat engines. The three blade design offers good performance and efficiency with the best "cost vs. benefit ratio" for most boating applications. Four-blade propellers are used to enhance performance where increased engine heights are required, faster acceleration is needed or where the boat is being operated in rough water conditions. Additionally, four-bladed propellers can increase overall performance in applications where heavier boats and/or loads are typically being experienced. The four-bladed design can also enhance the ability to use more positive trim angle without "slip" and cause the boat to have increased bow and/or stern lift. However, four blades also mean increased drag on the engine, lower top speeds, and different handling characteristics. It's beneficial to note that if a three blade propeller is operating within the proper WOT RPM specification, a four-bladed propeller must normally be one size pitch less in order to accurately maintain the correct WOT RPM.