Rake is the forward or aft slant of the propeller blade with respect to a line perpendicular to the propeller axis of rotation.
The word propeller rake has several definitions and can describe the indicated horsepower and related application.
In the next few paragraphs, I will explain what it does and how it affects the performance of your boat.
• Positive rake, or also known as (Aft Rake) - is when the propeller blade slants towards the aft end of the propeller hub.
The application in which a positive rake propeller would be used can be a variety of applications. Most pleasure craft dealing with an I/O or outboard propeller application would typically be in the range of 8 degrees to 20 degrees of positive rake. When referring to rake we can assume that the higher the degree of rake, the more bow lift we will have. That is if the horsepower is high enough to spin the propeller at the rated RPM of the engine. And the boat hull has lifting characteristics designed into it. When we achieve bow lift, we lessen the amount of hull surface that is in contact with the water. Resulting in higher MPH/ Knots.
On racing applications, it is not uncommon to see rake angles exceed 30 degrees. This is pretty extreme So I do not recommend running out and purchasing a hi rake propeller, just to try to get more MPH. Because it probably will not work in a pleasure boat application. You should always consult a National Marine Propeller Association certified shop for advice on this issue.
• Negative Rake, or also referred to as (Forward rake) - The propeller blade slants towards the forward end of the propeller hub.
In most applications of forward or negative rake, we will typically see these types of propellers used on houseboats, large cruisers, and tows.
We could also relate it to larger displacement applications, where these larger craft have the propellers buried deep in the water.
Lower propeller rake can achieve more bollard trust and become more efficient when attempting to propel a larger mass through the water. Again, hull design plays a big factor on how the propeller will function depending on the application.
Larger cruisers and houseboat applications typically run a zero to 3 degree rake.
As we speak of rake, it is also very important to understand track.
• Track The deviation for rake in a propeller blade relative to the design rake.
In solving propeller vibration problems of cruisers and houseboat applications, I usually find that there are inconsistent rake readings from blade to blade. This results in the blade tracking being off causing vibrations that can drive you absolutely crazy. Therefore, as you can understand from the explained definitions of rake, why rake is so important for proper efficient propeller function.