Hey you! I have this great new device to improve the efficiency of your propeller. I even have a completely new propeller design! Twice as efficient as anything else you can buy! Buy today!!
. . . Can you believe my claims? Probably not. On the other hand, small improvements to propeller efficiency seem likely. But will they work? Can you combine multiple devices together? To evaluate these claims, we need a framework to understand how they improve the propeller efficiency. We need to understand the underlying physics that go into designing propeller blades. Armed with this knowledge, we appreciate the tradeoffs of blade design and quickly discern which efficiency improvements are possible.
To generate forward thrust, the propeller blade works like a wing, only rotating. (Figure 2‑1) We use this same principle on airplane propellers, ship propellers, or helicopter rotors. All propellers follow the same basic principles of a rotating wing.
Figure 2‑1: Spinning Wings of Propellers
Before we get into the design of propeller wings, we must focus on the direction of water velocity that the blade sees. (Figure 2‑2) First, we have the velocity due to the ship traveling forward (V0). But remember that at the same time, the propeller blade spins very fast. If we look at a section along the propeller blade, we can find the added velocity due to the spinning motion (V2). Add these two together and you have the velocity that the blade sections sees (V1). That combined velocity is the flow direction that we must design to.
But the diagram does have a minor complication. This particular direction of water velocity only applied at one section of the propeller blade. As you move out along the blade radius, the spinning velocity (V2) increases. So we need to put twist in the propeller blade and keep changing the shape to match flow conditions along the entire radius of the blade. This blade twists shows the first of many complexities and tradeoffs in propeller blade design.
The design for a propeller blade consists of a series of blade sections (Figure 3‑1) stacked up to form the whole blade. For each section, the designer must consider a tradeoff between three main elements.
Figure 3‑1: Example of Typical Wing Section Shape
The game is this: for each blade section, try to get the most forward thrust, with as little drag as possible. The designer starts by picking a blade section shape. These are usually standard section shapes that are thoroughly tested to yield predictable performance. But a shape is useless without answering the question of section size.
The blade section shape determine the lift for a given blade size. So now the designer scales that section shape up to get the desired amount of lift from that section. In a perfect propeller, we would select a very large section size and pick thin section shapes. That would be ideal for a very efficient propeller. But pesky reality bothers us with two problems. First, the blade section must still be small enough to fit inside the propeller diameter. And second, larger sections mean more surface area for skin friction. Skin friction slows down the blade motion and increases the propeller torque. We prefer to minimize that. The propeller designer must balance these demands to get an optimum blade section.
Part of that section optimization includes selecting the angle of attack. The higher the angle, more lift is generated for a selected section shape. But at the expense of greater drag. Designers typically prefer low angles of attack (5 – 10 deg). The wing sections are most efficient at these angles. We also prefer low angles of attack to get better use out of a blade section. Take a second look at the propeller velocity diagram (Figure 3‑2) and this time focus on the resultant force vectors in the bottom left corner.
That propeller blade generates lift at a right angle to the zero lift line for the section. Lift always points at a right angle to the zero lift line. But lift is not the ultimate goal of a propeller. We want forward thrust. If we increase the angle of attack for this blade section, the lift force points further to the side, not forward. So higher angles of attack may generate more lift, but less of that lift goes towards creating thrust. This trade-off is another crux of propeller design. Getting the most force possible, provided it points in a useful direction.
All the tradeoffs of blade design hint at a recurring theme: you can’t get something for nothing. In propeller blades, if we want lift, the price is drag. But how does that drag show up in the overall propeller. We see it in two ways: the torque required to spin the propeller and the swirl of water leaving the propeller.
We knew the propeller wouldn’t spin by magic. It requires a torque applied to the propeller shaft. This is where we pay for the propeller thrust. The torque comes from the drag of the propeller blade. But an equally important component is the propeller lift. Due to the angle of attack, part of the propeller lift points backwards, fighting the rotation of the propeller blade. Drag and the lift component add up to a huge amount of torque. In fact, you put more energy into spinning the propeller than you get out in terms of thrust.
If we put more energy in than we get out, where did the rest go? It went into the swirl motion of the water. (Figure 4‑1) When the water first enters the propeller, it moves straight. After the propeller, the water has a strong swirling pattern. This swirl does nothing to help us. It is just wasted energy.
You might think all this theory is only useful for propeller designers. But it also gives you a sound background to evaluate any new device that promises to improve propeller efficiency. Based on this knowledge of blade physics, there are only five major ways to improve propeller efficiency.
Options one and two are unlikely. Fluid dynamics experts have studied blade section shapes for years. They are very well understood. We can hope for a genius breakthrough, but I wouldn’t hold my breath for it. And the blade section size gets driven by the required propeller thrust. Same for option three. The angle of attack is largely driven by the required propeller thrust.
Most of the current efforts are focused on option four. If we can lower the velocity angle, more blade lift points in the direction of forward thrust. Always a good thing. There are two ways to do this. First is increase the propeller spin velocity component (V2 in Figure 3‑2). That reduces the velocity angle, but it also increased the drag on the propeller blades. The other option is to reduce the forward velocity that the propeller sees (V0 in Figure 3‑2). This reduces the velocity angle and reduces drag from skin friction on the propeller blades. All good things.
The last option is to somehow recover the lost energy from the propeller swirl. Recovery devices usually look like some form of smaller propeller downstream of the main propeller. Don’t forget that you still have to pay a drag penalty to pull that extra device through the water. As with all tradeoffs in propeller design, you can’t get something without paying a penalty.
Propeller design is all about tradeoffs. The designer tries to create the most efficient propeller possible. The principle challenge to this is the direction of water velocity. Then the tradeoffs of propeller blade design are complex, with many choices along the way. Ironically, the most efficient propeller is one that produces no thrust. A perfectly thin blade section that is super tiny, with a zero angle of attack. Of course, that’s useless to us. To go from useless to a practical propeller, everything is a tradeoff and loss of efficiency.
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