Ship propellers, or "screws," have been used in marine architecture for quite a long time.
Screws had been suggested throughout the eighteenth century as a means of propulsion for seagoing vessels, but the problem was always how to produce the motive power.
The first patent was granted for a triple-threaded screw in 1794. But it wasn't until the 1830s that Francis Pettit Smith and John Ericsson began experimenting with steam-powered, propeller-driven vessels.
Ever since, the problem of propeller efficiency has continued to beleaguer naval architects and private boaters alike. Propellers have come a long way since the nineteenth century, however, and new designs are increasing efficiency and speed.
So, here are eight tips we've put together that can help improve propeller efficiency.
1. Outline of Propeller Blades
A propeller, it turns out, is just a set of rotating wings.
These wings are called blades, and they have a very important role in propeller efficiency. These blades can come in a variety of different shapes and sizes, but research shows that the smaller these blades are, the greater the boat propeller efficiency.
The reason for this phenomenon is that larger blade sizes mean a greater propensity for frictional drag, which reduces the performance of the propeller. The most efficient propeller shape is, therefore, one that incorporates the smallest blade sizes that are still compatible with the size and power of the propeller.
As researchers have learned how to calculate propeller efficacy, they've discovered that the diameter of the propeller and the speed of the shaft are closely correlated.
So how does this discovery conduce to designing the most efficient boat propeller? It turns out that a low shaft speed, and thus a large diameter of propeller, is the optimal configuration from the standpoint of efficiency.
As a practical consequence, larger diameter propellers are best. Of course, that's easier said than done. A propeller's diameter is ultimately limited by the size of the ship itself. But a high-efficiency propeller is usually one with a greater diameter.
3. Schneekluth Ducts
Another important factor in increasing propeller efficiency is the need to mitigate a ship's wake.
This is where the "Schneekluth ducts" come into the picture. These ducts are designed to redirect water flow around the propeller. This is necessary to counteract the dragging effects of the ship's wake, thereby improving the efficiency of the hull and propeller design.
Schneekluth ducts can also help to accelerate the water flow, thanks to their aerofoil cross-section, which generates a low-pressure area directly in front of the ducts.
4. Angle of Attack
Despite its martial name, the angle of attack actually refers to the angle formed by a propeller's blades with the onrushing water.
Now, a propeller's angle of attack will depend upon the design lift of the propeller, and these are considerations for a naval architect. As an example, a larger angle of attack will generate increased pressure side cavitation. Cavitation, in a nutshell, involves the formation and collapse of vapor-filled bubbles.
And propeller cavitation can have deleterious effects on propeller blades and their overall efficiency. So incorporating these effects into the design of the angle of attack is very important to improving propeller efficiency.
5. Optimum Number of Blades
Fewer blades in a propeller often equate to greater theoretical efficiency. On the other hand, more blades lead to greater propulsive power.
By adding blades to the design of a propeller, it boosts the amount of drag. But it also increases the amount of water that is moved, which makes for a much smoother motion of the propeller blades.
This is why it's important to find the correct number of blades for a specific kind of vessel. The number of blades will correlate to the size, shape, and performance characteristics of the boat in the question.
Check out our prop guides to find the right configuration for your boat.
6. Low RPMs
A further factor to consider in the design of a high-efficiency propeller is to select a low revolutions-per-minute (RPM) range.
This tends to boost the efficiency of a propeller by anywhere from 10-15%. Finding the right RPM range for a propeller will also correlate with its diameter. At the same time, it's important to ensure that the RPMs differ from the resonant frequency of other components—such as the shaft or other propeller parts.
7. The Right Pitch
Finding the optimal pitch is a good way to improve propeller efficiency.
The pitch of a propeller is considered to be "true" if the so-called pitch length between the leading and trailing edges of the blades are identical. On the other hand, it's defined as "progressive" if there's a low pitch for the leading edge and a longer pitch for the trailing edge.
Deciding on the use of a true or progressive pitch is all a part of finding the optimum design for any propeller. Finding the right one can greatly improve propeller efficiency.
8. More Efficient Blade Shape
Another way to increase the efficiency of propellers involves tweaking the shape of their blades.
A common way of doing this entails curving the leading edge of a blade, creating a more asymmetric shape to the blade. This technique manages to extend the sweep of the blade as it rotates.
This nifty little trick reduces the effects of cavitation, minimizing drag and boosting the overall efficiency of the propeller.
Improve Propeller Efficiency With These Tips
Boats are a lot of fun—but by achieving the greatest propeller efficiency, they'll go faster and save you more money on fuel.
Boats are a major investment, so make sure you have the most efficient and effective propellers. Please contact us today, to see how we can help you find the right props for your boat.