propeller basics part 3

8/2/2019 Propeller Basics Part 3

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Propeller Basics Part 3

Propeller designs are examined, with a look at ther various operations of propellers. With illustrations, this topic looks at different

designs and their effects.

Propeller Basics: Bow Lift and Stern LiftThis article is the third in a series that explains the basic operation of propellers. This article simplifies the mysterious operation of

bow-lifting and stern-lifting propellers. (Readers may wish to refer to Part 1 and Part 2of this series article, as well.)

Confusing ClaimsI was fundamentally curious about how a propeller accomplished what seemed to be a rather amazing phenomena. How could it know

it was supposed to lift the bow or the stern? I began a hunt for the answer. As it turned out, I really never found a good answer in my

search (which was was admittedly limited to on-line resources--I am sure there exists much scholarly research on this subject in theliterature of the Naval Architect). Faced with this void of information, I began to consider the problem myself and to develop my own

analysis and explanation. If you are similarly curious about these features of propeller design, I invite you to read on and discover the

answer for yourself.

Propeller DesignsThere are three basic propeller designs that are often used in conjunction with boat propulsion: the "conventional" propeller, the "bow

lifting" propeller, and the "stern lifting" propeller. These last two names refer to the ability of the propeller design to affect the trim of

the boat's bow or stern while underway. These names are in widespread use in the marine industry, and they seem to be accepted by

virtually everyone at face value. It is as if the underlying principle that governs their operation is so widely understood that it never needs mentioning. My experience, however, was just the opposite. Virtually no one could explain to me the actual principles that

produced the lifting effects. What is behind these propeller designs?

Force VectorThere was no argument that propellers called "bow lifting" actually did lift the bow more than propellers called "convetional" or stern

lifting. Since all the propellers would operate in the same location and at the same angles with respect to the boat, something about thethrust vector they produced must be different. But this is the initial conundrum. How could a propeller rotating on a shaft, produce an

asymmetrical thrust vector? In other words, how could the total thrust being produced by the propeller sum into a vector which was

other than in-line with the axis of the propeller shaft? This was the essence of the problem. Without producing some type of

asymmetrical thrust, no bow or stern lift could be achieved from any propeller.

Propeller EnvironmentA propeller's ability to create a thrust vector that is not in-line with the propeller shaft is difficult to understand because we tend to

think of the propeller as operating underwater. Intuition tells us that on a deeply submerged submarine, a spinning propeller is

unlikely to produce any inherent tendency to lift or lower the stern of the submarine. Above, below, on all side, the propeller operates

in an open and symmetrical environment of limitless water. We tend to apply this same model to boat propellers, but that is a crucialmistake.

Unlike a submarine, a boat propeller operates near the surface and is surrounded by significantly different amounts of water. On each

side and below are vast amounts of water, but above the propeller is only a thin layer of water. In addition, above the propeller there isusually a cavitation plate (or anti-cavitation plate), which also changes the symmetry of the environment in which the propeller

rotates. The difference in environment in which a boat propeller operates as compared to a submarine propeller is key to getting

effects like bow lift or stern lift to occur.

Thrust ShapeAnother important concept to understand is the shape of the thrust produced by a propeller. One can think of the thrust produced by a

propeller as being in line with the propeller shaft, but this thrust is produced by accellerated water that is pushed aft by the propeller.

The moving water is directed along the line of the propeller shaft, but it also has a diameter equal or greater than the diameter of the

blades. As the water streams aft, it tends to spread into a larger diameter, so one can think of the moving water coming off the

propeller as a cone. The apex of the cone is at the propeller, and the cone grows outward as it moves aft.

As the accellerated water moves aft and outward, some of it reaches the surface of the water, where it breaks through into the air,

releasing any remaining thrust. We are all familiar with this stream of white water in the center of our boat's wake. The thrust fromthe propeller stream that is lost into the surface affects the total thrust vector coming from the propeller.

Controlling Thrust ShapeThe shape of the cone of accellerated water that comes off a propeller can be controlled by varying the shape of the propeller blades.

One element of the blade that is varied is the rake angle. The rake angle is the angle the blades are tilted back, akin to sweeping the

wings of a plane back for high-speed flight. The "conventional" propeller generally has the blades set without much of a "rake" angle.Propellers which are termed "bow lifting" will have their blades set with a noticeable aft rake. Propellers with "stern lifting" abilities a

usually not raked aft much at all, but have other blade shape characteristics.

Effect of Rake

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The effect of aft rake is to direct the water off the blade inward toward the propeller hub. The net effect of blade raking thus is to vary

the shape of the thrust cone, causing it to become narrower than the conventional propeller's thrust. This narrowing cone of thrust can

travel farther aft before it spreads to the point where it reaches the surface and gives up its thrust to the air. This has several beneficial

effects. First, more thrust is contained in the water and less is lost to the air, making the propeller perform more effectively. In

addition, the propeller has less tendency to cavitate or "blow out", with the result that a raked blade propeller is said to have better "holding" or better grip on the water. Finally, since less of the thust is lost to the air, the total thrust vector from a raked propeller is

more in-line with the propeller shaft.

Propellers designed to be "stern lifting" have little or no rake angle to their blades. This causes their cone of thrust to be rather "fat",with the result that much of the thrust breaks through the surface. Propellers of this style are often used on racing boats, and theroostertail of water that is seen behind them is clear indication of the thrust being lost through the water surface. In exchange, these

propellers lift the entire stern of the boat out of the water, reducing the drag to just that of the propeller and shaft. To enhance this

effect, the blade shape is often thickened at the trailing edge, making the blades resemble a meat cleaver. The "cleaver" blade

generates additional lift because the propeller raises the stern so much that only half of the propeller is in the water at a time. Such

"surface piercing" propellers reduce drag even more. The cleaver blade creates more stern lift. The blade entering the water on thedownward rotation side generates more lift than the blade entering the air on the upward side, with a net result that the stern is lifted.

A series of three illustrations below shows the general shape of the thrust cone coming off the propeller. The darkened arrow shows

the net thrust that results after considering the thrust lost to surface breakthrough.

propeller basics part 3

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