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.
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.
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.
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.