Propeller Education

A - Blade Tip: the point on the propeller blade that is the farthest from the center of the propeller hub.

B - Leading edge: the section of the propeller blade that initially enters the water. This spans from the hub of the propeller to the blade tip.

C - Trailing edge: the section of the propeller blade that is the last to enter the water. This also spans from the hub of the propeller to the blade tip.

D - Cup: This is a slight curve that increases the pitch at the trailing edge of the blade. This curvature helps the propeller maintain a better grip on the water.

E - Blade Face: This is the side of the propeller blade that is oriented away from the boat.

F - Blade Back: the side of the blade that faces towards the boat.

G - Blade Root: this is where the propeller blade joins the outer hub.

H - Inner Hub: this is the portion of the propeller that contains the hub system allowing the propeller to be connected to the propshaft.

I - Propeller Barrel: the outside of the barrel is the portion that connects to the propeller blade. The interior of the barrel makes contact with the exhaust passage.

J - Ribs: This is what connects the barrel to the inner hub. A propeller will have between two and five ribs parallel to the propshaft or the blades.

K - Flo-Torq Hub: this shifts thrust from the propshaft into the propeller.

L - Exhaust Passage: This is the channel between the inner hub and the barrel of the propeller which allows exhaust gases to pass through the propeller and into the water.

M - PVS Holes: this allows for better acceleration by allowing exhaust to pass into the blades at lower RPM’s.

N - Diffuser Ring: This prevents exhaust gases from returning to the propeller blades and aids in reducing pressure.

O - Labyrinth Seal: This lessens the amount of exhaust gases that can become trapped between an outboard engine and the propeller.

Terminology

Rotation: Propellers are manufactured in both right-hand and left-hand rotations. A right-hand rotation propeller turns clockwise to move the boat forwards whereas a left-hand propeller turns counterclockwise. For most single-engine outboards and sterndrive powered boats, right-hand rotation propellers are standard. Number of Blades: The most common propellers used in pleasure boating are either equipped with three or four blades. The propellers with three blades are known for their efficiency and vibration reduction, whereas four-blade propellers are preferred for enhancing acceleration while increasing the amount of blade contact with the water.

Diameter: Regarding propellers, diameter refers to the width across the circle formed by the blade tips during rotation. Appropriate diameter can be determined by the resulting RPM’s and power. Diameter is also influenced by the application as knowing the extent to which the propeller is submerged in the water is key. If a propeller is partially surfaced, a larger diameter propeller is necessary to ensure sufficient power can be generated. Occasionally, physical constraints can limit diameter. Situations in which this may occur include drive type, how close the engines are together, and if engines are staggered. Within a specific propeller style, diameter increases for slower boats and decreases for faster boats, and engines with lower maximum speeds generally require larger diameter propellers. However, as the blade surface area increases (for the same engine power and resulting RPM’s), diameter should decrease. For example, when changing out a three-blade propeller with a four-blade propeller of the same pitch, the diameter should be decreased.

Pitch: Pitch refers to the distance the propeller would move forward in one complete revolution, in inches. This is measured across the face of a propeller blade. However, the actual pitch of the propeller may vary from the number stamped on the inner hub if modifications have been made by a prop shop or if the propeller has sustained damage from objects under the water.

There are two main types of propeller pitch: constant and progressive. Constant pitch means that it remains the same from the leading edge to the trailing edge. In contrast, progressive pitch means it changes over the blade surface. In instances of progressive pitch, it starts lower at the leading edge and increases as you move over the surface of the blade towards the trailing edge, and the pitch number represents the average pitch over the entire blade. Pitch acts like a set of gears for your engine. Proper pitch selection ensures your engine operates within its recommended RPM range, thereby prolonging its life. A lower pitch increases engine RPM, which can provide better acceleration for water sports applications, but potentially sacrifices top speed and fuel efficiency. Conversely, selecting a propeller with too high of a pitch can cause the engine to operate below recommended RPM’s which may reduce acceleration and cause engine strain, ultimately reducing the lifespan of your engine. Mercury propellers are designed to allow for a one-inch pitch change to result in a 150 RPM difference. However, heavy loads, high heat, and high humidity can all affect the resulting RPM and speed. By carefully considering pitch in selecting your propeller, optimal engine performance can be achieved and result in a satisfying boating experience.

Blade Rake... a science and an art

In terms of a propeller, the rake is the angle in which a propeller blade faces relative to the barrel. A blade that is perpendicular to the barrel is considered to have 0° rake. As the blade slants away from the barrel, the blade rake increases. Rake can either be a straight or a progressive angle. Usually, propellers meant for higher horsepower engines have a higher rake, whether it be flat or progressive. Increasing the rake of a propeller generally improves its ability to ventilate properly. On light boats, higher rake angles can improve performance as it lifts the bow of the boat higher. However, this can be a delicate balance as it can potentially create too much bow lift, making the boat less stable.

Blade Cup

The cup in a propeller is the curve cast into the trailing edge of the blade. The propeller blade should be entirely concave when this is done properly, and the location of the cup in the blade can affect how much it improves performance. Cupping can also reduce the risk of blow outs and prevent cavitation