Information

A lot has been written about marine propellers on vessels of varying design. Nevertheless, in practice it becomes apparent that misinformation, incorrect assumptions and overlooked variables are rather common. This alone should warrant reservations in dealing with theoretical performance predictions. Though important for calculation purposes, such predictions have limitations and can be misleading or misinterpreted.

No two vessels are exactly alike. Therefore, over-generalizing performance expectations carries with it a good deal of risk. Add to this the fact that no two hand-made/hand-finished props are the same, and things can become even more confusing.

The surest way to determine the best propeller for a particular vessel is through sea trials. Again, it should be emphasized that during sea trials - whether it be testing a small ski boat on an inland lake or a large sport fishing yacht on the open ocean — many variables do influence the outcome. Reliable engine performance data is critical to final analysis. In addition, internal combustion engines operate on an air/fuel mixture, which means air temperature, humidity and elevation also impact the final results. Some other factors that affect the outcome of sea trials are vessel load and distribution, appendages (i.e., rudders, struts, nozzles, shafts and towers, etc.), shaft angle, and wetted surface area of the hull, etc.. For sea trials to be valuable and conclusive it is vital to isolate variables, and to maintain as much consistency as possible in surrounding factors.

Propeller Technology

There are a variety of terms used to describe propeller characteristics as well as performance attributes. It is important that you have a good understanding of them for the best communication, as detailed here.

Angle of Attack Blade Contour Blade Thickness Cavitation Cupping Diameter Number of Blades Pitch Rake Rotation ("Hand") Skew Slip Ventilation

What does the propeller do?

How Propellers Work

The "Push/Pull" Concept

To understand this concept, let us freeze a propeller just at the point where one of the blades is projecting directly out of the page (below). This is a right-hand rotation propeller, whose projecting blade is rotating from top to bottom and is moving from left to right. As the blade in this discussion rotates or moves downward, it pushes water down and back as is done by your hand when swimming. At the same time, water must rush in behind the blade to fill the space left by the downward moving blade. This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect, on the underside and a negative pressure, or pulling effect, on the top side. This action, of course, occurs on all the blades around the full circle of rotation as the engine rotates the propeller. So the propeller is both pushing and being pulled through the water.

Thrust/Momentum

These pressures cause water to be drawn into the propeller from in front and accelerated out the back, just as a household fan pulls air in from behind it and blows it out toward you (Figure 3-2 below).
The marine propeller draws or pulls water in from its front end through an imaginary cylinder a little larger than the propeller diameter (Figure 3-3 below). The front end of the propeller is the end that faces the boat. As the propeller spins, water accelerates through it, creating a jet stream of higher-velocity water behind the propeller. This exiting water jet is smaller in diameter than the actual diameter of the propeller.
This water jet action of pulling water in and pushing it out at a higher velocity adds momentum to the water. This change in momentum or acceleration of the water results in a force which we can call thrust.


A propeller converts the rotational energy generated by the engine into thrust, pushing or screwing the boat through the water. Discounting the propeller's blade design, the larger the propeller, the more efficient it will be in pushing the boat forward. The problem is balancing the propeller's size with the motor's ability to spin it and your personal performance goals.


Propellers are often referred to as screws, this is a result of the way they work in water. Each turn of the propeller moves it linearly forward by the amount of its pitch assuming it was running in a solid medium. This is similar to turning a screw into a block of wood. Of course the propeller is not 100% efficient, nor is water a solid medium. Under actual operating conditions, slip occurs as the propeller rotates resulting in the absolute forward movement (actual pitch) being less than theoretical pitch. Slip will differ depending on hull design, configuration and intended use.


Cavitation occurs when the pressure on the forward face of the propeller blade becomes low enough that vapor bubbles form and the water boils. As the vapor bubbles pass over the blade face and move away from the low pressure area, they collapse. The collapsing of the vapor bubbles might seem trivial, but it is a very violent event which can result in the pitting of the propeller surface. Cavitation is a major source of propeller damage, vibration, noise, and loss of performance. Cavitation can be caused by nicks in the leading edge, bent blades, too much cup or simply high boat speed.


Ventilation or aeration occurs when surface air is drawn into the propeller blades. When this happens, boat speed is lost and engine RPM climbs rapidly. This can result from a hull error, excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine. Ventilation is most often confused with Cavitation.


Surface-piercing propeller is a propeller that is positioned so that when the boat is at full speed the waterline passes through the propeller's hub. This is accomplished by extending the drive shaft out through the very bottom of the transom. When running properly only one blade of a two bladed propeller is actually in the water. The surface propeller is very efficient at minimizing or eliminating cavitation by replacing it with ventilation. With each stroke, the propeller blade brings a bubble of air into what would otherwise be the vacuum cavity region.