A propeller is a device that converts rotational energy into thrust. The basic structure of the propellers is driven by the engine’s pistons, which spin the rotor blades inside the propeller housing. These spinning propeller blades create an air draft to propel boats forward and generate their power through friction with the surrounding fluid (air or water).
Propeller technology has changed dramatically since World War II when first introduced on Royal Caribbean’s Navigator of the Seas cruise line over 40 years ago! The Oasis-class ships are the largest and most advanced in class. With three 20 feet tall propellers, these vessels can move up to 28 knots (35 km/h).
One thing about propellers is that you can’t see them working unless there happens to be something moving behind them, like a boat. If you’re standing outside your neighbor’s house looking at his car, you won’t know if it has any propulsion system attached to its back end until they get in and drive away.
When you look out over the lake toward where his little sailboat might be floating, you’ll notice it moved slightly closer to shore than before. How did it do that without him doing anything special? All those shiny metallic things sticking out from under the boat’s hull were propelling it along! They’re called “propulsors,” and they use one word to describe themselves – propellers.
So let’s start our journey down the path of understanding propels! In this article, we will take apart exactly what goes into making a propeller and how it works so that you understand why your neighbor’s boat seems to float higher above the water after he leaves.
We will also examine how large these devices get to help us appreciate their size compared to other common man-made structures such as buildings and cars. Finally, we will discuss some interesting designs used today and how they differ from traditional ones. At last, we’ll learn what the term “foil” means.
The Main Parts Of A Propel
As mentioned earlier, propellers are mechanical mechanisms designed to convert rotational energy into thrust. This conversion occurs because of friction between the rotating blade(s) and the surrounding fluid (usually air), causing the fluid particles to travel faster past each propeller’s side. As the fluid travels faster, it pushes against the blade’s outer surface, creating a force known as thrust.
Thrust causes the entire drive unit (the motor, transmission, and sometimes additional accessories) to rotate more quickly, increasing the rotational energy available to the propeller structure. When the blades spin fast enough, they begin to generate lift instead of pushing the boat directly downward. Lift increases the overall stability of the craft and prevents it from tipping too far off balance.
Since most modern-day cruise ships rely heavily on hydrodynamic forces generated by propulsion systems rather than being pushed by the wind, most cruising occurs within oceans and seas. Because of this, most people probably don’t think much about propellers once they’ve boarded the ship.
However, many commercial vessels still include complete sets of props near the vessel’s bow, just ahead of the superstructure (such as the bridge or radar mast), allowing the crew to maintain control while navigating rough waters.
So now that we understand what propels do let’s take a close look at what makes up the main components of a typical propeller assembly.
When someone mentions a propeller, chances are pretty good that you automatically picture a long thin object with two opposite flat surfaces facing outward and perpendicular to the direction of motion. You may even remember seeing one of these objects sitting on top of a ship’s wheel during World War II movies. That would be correct but not entirely accurate.
The actual shape of the propeller varies depending on whether it’s mounted horizontally or vertically. Most horizontal-type propellers, commonly called screw propellers since they resemble the threads found on garden hoses, face back toward the stern of the ship, pointing straight up into the sky.
Vertical propellers, however, point straight down, parallel to the boat’s deck, generating thrust only below the ship’s body. Both propellers have three major sections: a hub, flanges, and blades. Let’s break down these sections individually.
Hub: The hub connects the shaft, bearings, and gears to the rest of the propeller mechanism. While the exact design differs among manufacturers, the fundamental purpose of hubs remains constant: To provide support for everything else connected to it. Some larger models may incorporate multiple hubs stacked on top of one another.
Flanges: Flanges extend radially outward from the center of the hub and connect the blades to the hub itself. Due to strength requirements, the flange material usually consists of light metal, such as aluminum.
Blades/Rotor: Each blade extends axially from the hub in a radial fashion. On smaller propellers, the number of blades ranges from 4 to 12 per inch in diameter (for reference purposes, 1 foot = 30 inches). Larger propellers often contain dozens of blades arranged in groups of 3 or 5 around the circumference of the hub.
Blade materials vary widely based on function, construction method, and cost constraints. Steel blades offer excellent longevity and durability, although fiberglass composite blades tend to weigh less and require fewer repairs due to more outstanding toughness. Several methods are used to attach individual blades to the hub, including welding, bolting, and splicing.
Now that we’ve taken a detailed look at a propeller’s major components, let’s talk about how these pieces fit together.
Some engineers claim that a well-designed propeller should produce no noise whatsoever. Others say that sound coming from a propeller must be loud enough to wake the dead. Either way, propellers indeed make quite a bit of racket. For starters, propellers operate using dynamic pressure (pressure exerted on an immersed solid object) rather than static pressure (a force acting across a liquid occupying a given space.)
According to Newtonian mechanics, a dynamic tension is created when mass moves relative to stationary fluids. As an object moves through the medium, the molecules closest to it collide with it first, exerting a tremendous force on it. Moving further away, the collisions become weaker and farther away, resulting in lower pressures.
As we saw previously, dynamic pressure creates a low-pressure area at the front side of the propeller blades, forcing fluid particles to flow past. Sounds produced by propellers come from the high-speed rotation of the blades. These sounds range from squeaks and squeals to screams and shrieks, typically becoming louder as the rpm rises.
While propellers are certainly noisy creatures, they aren’t the biggest machines. Next time you go for a ride in a convertible, keep an eye peeled for big box stores such as Lowe’s Home Improvement Center, Wal-Mart Supercenter, or Target.
Driveways to houses, garages, and apartment complexes often feature huge industrial-sized vehicles parked next to small motorcycles. What could account for such massive vehicle sizes? If you look closely, you will discover that these gigantic trucks and tractors are giant versions of the same machine you find on the average household kitchen counter: the blender.
Yes, America’s favorite kitchen appliance holds the record for the most extensive consumer product ever sold. Except for a few very specialized items like nuclear reactors, blenders occupy the absolute bottom rung of the world’s heaviest machinery list.
If you imagine yourself atop a skyscraper, perhaps you’d agree with me when I tell you that the tallest building in New York City, One World Trade, stands nearly twice as tall as the Eiffel Tower. The tower was built to accommodate the new headquarters of financial services company Morgan Stanley, which recently relocated from Manhattan to Midtown East.
The building features a total height of 320 feet, seven stories, and is currently the third tallest building globally.
If you thought the propeller blades discussed earlier looked complicated, wait until you hear about a foiler’s internal workings.
Inside And Outside Blading Structure
Foilers refer specifically to vertical-type propellers situated beneath the superstructures of ships. Foilers are almost always made of lightweight metals such as stainless steel or titanium due to their strength requirements.
Their blades are manufactured with extremely sharp edges to prevent ice buildup and damage caused by contact with underwater obstacles. Even though foilers generally sit relatively deep within the water column, the extreme edge of their blades remains exposed to the atmosphere.
Therefore, foilers can experience corrosion problems similar to those experienced by aircraft wings. Corrosion is the destructive chemical reaction that occurs when oxygen reacts chemically with a material’s surface layer, breaking bonds and leaving negatively charged ions free to attack base metals.
Over time, corrosive reactions cause the loss of structural integrity, eventually rendering the propeller unfit for service. Fortunately, foilers have been coated internally with highly conductive coatings to minimize corrosion. Foilers also carry a protective fin extending out from the rear side of the blade, providing added protection from damaging currents of saltwater.
Additionally, foilers employ numerous safety measures to ensure safe operation, including redundant backups and interlocks.