Propellers
A propeller or "prop" is the final piece of the engine and its drive train. It is the part of the boat that transfers the engine horsepower to the water. The transfer is termed as thrust. Thrust is created as the propeller pulls water into the front of it, (the boat side) and pushes it out on the back side. This momentum change is caused by a pull push affect of the blades and the pressure differential from low to high and is the basis for the creation of thrust. All propellers have the same basic parts. However, there are many variables within those basic parts. The descriptions and picture listed below will describe those basic parts, and help you to understand some of the distinctive characteristics of those propeller parts that can vary, causing different reactions.
Pitch
Pitch is the theoretical forward travel of any given propeller. This forward travel is measured in inches. The pitch stamped on the propeller tells you how far forward the propeller would move in inches if it was turned or screwed into a solid media such as cork or the ground.
Pitch can be "True" or "Progressive". True pitch describes a propeller that measures the same pitch from the leading or beginning edge to the trailing or following edge. Progressive pitch describes a propeller that starts low in pitch at the leading edge and increases its pitch as the water travels towards the trailing edge. Both styles of pitch offer benefits depending on the type of boat and engine package or set up a person uses.
Pitch controls the engine revolutions per minute or "RPM". Adjusting pitch for any given situation can change a boats performance anywhere in the rpm range and give the boater the desired performance they need while keeping the propulsion system working as the manufacturer engineered.
Diameter
Diameter is the total outer circumference of the propeller measured across the center of the prop. In other words it is the distance across the circle the blade tips make as you rotate the propeller.
Diameters are limitless. Most propellers have a diameter pre-determined by the manufacturer as correct for the pitch, horsepower and application it was designed for. Diameter changes can affect the "attitude" of the boat or the way the boat carries it self. Generally, the lower the pitch of a propeller, the larger the diameter will be. Propellers running at deeper depths can be smaller in diameter while performance applications that surface the propellers while running can use larger diameters. Trained custom propeller shops can adjust diameter to aid in the operation of your specific vessel and its set-up.
Rake
Rake angle is measured in degrees. Rake is the amount of degrees the propeller blades angle perpendicular to the propeller hub. Rake can be slightly negative (leaning towards the boat), or positive (leaning away from the boat). The range of degrees could vary from -5 to +30 degrees. An average rake angle for most outboard propellers is 15 degrees. Rake can be progressive meaning it increases as you move out from the propeller hub to the blade tips. Or rake can be flat maintaining its degree of angle continuously from propeller hub to blade tip. Higher rake angles help to improve some boats ability to operate in aerated water situations by causing the water to adhere to the blades better than a prop with lower rake angles. The centrifugal force the propeller creates, throws the water outward and the higher and/or progressively raked propellers can hold and control that water better, increasing the thrust rearward. Higher rake angles can also help to hold a boats bow higher in the air, reducing hull drag and increasing speeds. However, low rake propellers can be a better choice for light weight fast hulls that carry the bow on their own since they direct thrust with less drag and can increase efficiency.
Cupping
When the final trailing edge of a propeller blade has a "curl" away from the boat, it is referred to as being "Cupped." Cupping on propellers was not always used as a standard propeller design. When cupping was first used, it was done to propellers as an easy way to increase the pitch or rake without bending the entire blade a blade. Once propeller shops began to do this, they realized other benefits that came along too. One of those benefits was improved performance. Because cupping can increase pitch, rpm will generally decrease. Depending on where and how much of a cup is added, determines the rpm and the boat personality changes.
Rotation
Just as the word suggests, rotation determines the direction a propeller revolves around the axis of the propeller shaft. Rotation is determined by looking at the propeller from the rear of the boat. A right hand propeller will turn clockwise therefore moving down to the right hand side. Contrary, a left hand propeller will rotate counterclockwise or down to the left while looking at the propeller from the rear of the vessel. Changing rotations on single, twin, or triple engine applications can affect the boats performance too.
Number of Blades
A propeller with one blade would be the fastest and most efficient propeller if it weren't for the fact that the vibration could not be contained. The reason for its great speed would be the lack of blade drag caused by metal in the water. As blades are added to propeller hubs, the drag is increased. However, the ability to grab more water and decrease the time frame between load and unload is shortened. This means the propeller will "feel" smoother as it grabs water or loads the blades. The number of blades does many other things to the performance and control of a vessel. Performance type hulls tend to show the greatest response from adding and subtracting blades. Propellers typically come with 2 to 6 blades.
