Propeller Rake Explained: It’s all About the Angle
Apart from the diameter and the pitch, there are other characteristics of propellers that impact overall performance. They include the number of blades, their thickness and the propeller material. One design element that has been playing a bigger role in the past few years is the rake angle. Let’s take a closer look at it.
The term “rake” refers to the angle of the blades relative to the propeller hub. It must not be confused with the pitch. The rake angle of the blades refers to their forward or aft inclination in relation with the propeller hub.
Right Angle
The rake angle is measured on the propeller's hub and it is formed vertically with the straight line that connects the base of the blade to its tip. When the rake angle is zero degrees, the blades are upright on the propeller's hub. The more backwards the blades lean, the higher the rake the angle that is formed.
Rake is negative when the blades face the gearbox. It’s positive when the blades angle aft away from the gearbox.
Most outboard-engine propellers have positive rake with blades angling aft between 5 and 15 degrees. High-performance props can have a rake angle as high as 30 degrees.
Trim out the engine and the boat’s bow is raised, resulting in reduced drag and increased efficiency. Higher rake angle improves performance because the water stays between the blades and the propeller's hub longer. This limits ventilation at higher trim angles. When our boat rides higher out of the water, high rake helps the propeller trim the bow more effectively. The reduced wetted surfaces result in higher top speed.
Raised Engine Heights
A propeller with a higher rake angle can let a boat owner or technician mount the outboard higher. Hydrodynamic resistance on the lower unit is reduced because there’s less of the gearcase in the water.
Propellers with high rake angle perform better near the water’s surface compared to those with low rake. This happens because the former “bite” better in conditions that cause propeller ventilation like sharp turns or in heavier seas.
But there is a limit that we reach when the propeller ventilates. The propeller starts biting more air than water and because air is less dense than water, less thrust is produced. Engine rpm increase, but the speed drops.
The higher rake angle lets the propeller take advantage of the limited amount of water it’s operating in at the elevated height. It holds the water longer so it still produces thrust when a prop with less rake would ventilate.