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What Is a Prop Shaft?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 20 April 2014
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A propeller shaft is a shaft which is used to spin a propeller on a craft such as a boat or airplane. The prop shaft, as it is also known, takes the rotation and torque from the engine and brings it to the propeller, causing it to spin at a rate which can be controlled by the operator of the craft. Prop shafts vary widely in length, width, and composition, depending on where and how they are being used. Stores which stock replacement parts and engine equipment for craft such as boats tend to carry prop shafts and can also order specialty shafts by request from customers.

When designing a prop shaft, engineers want to create a shaft which is strong and durable, but as lightweight as possible, because weight on the shaft can impede its efficiency. The need to balance weight with strength can be tricky with some materials, especially when one considers the added weight which can be created by attachments to the prop shaft, since the device may need attachments to allow the propeller to work at an angle.

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Another consideration is the environments where it will be used. On aircraft, a prop shaft can get extremely cold, and these can be dangerous with some metals. The metal may contract in the cold, which could interfere with function, and cold can make some metals brittle, which could lead to a break in the prop shaft. Obviously, one does not want a prop shaft to break while in the air.

On boats, corrosion is a concern. Boats designed for saltwater navigation need to have prop shafts which can resist corrosion, and the design should be easy to inspect and service, as well. Boats used in freshwater are less subject to corrosion, which means that they can be made out of a wider array of metals, including metals which are cheaper and easier to work with.

The movement of the prop shaft, when transmitted to the propeller, allows the propeller to rotate, with the rotation creating thrust. The amount of thrust can be influenced by the speed and angle of the propeller, both of which may be adjustable. On a boat, a propeller acts similar to a rowing crew on steroids to move the boat through the water. The physics behind flight with propellers are rather interesting, with the propeller generating enough thrust to make a heavier-than-air object literally take wing.

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winslo2004
Post 3

@MaPa - It is a safe bet with either a plane or a boat of any size that it is going to cost you a bunch of money at the best of times. Like the old saying, a boat (plane) is a bit hole in the water (sky) into which you throw money.

MaPa
Post 2

@bigjim - You are right about the precision of the shaft. It has be be engineered and manufactured with a lot of precision. Which likely has a lot to do with why it's so ridiculously expensive.

On even a modest high-performance airplane, the prop and hardware can be tens of thousands of dollars. On a ship, it can be millions of dollars. The shaft and its hardware is only a portion of that, but it is still significant.

bigjim
Post 1

This looks like a pretty simple piece of gear, just a metal rod, really, but in reality it is very critical that the driveshaft (or prop shaft) be precisely made and balanced.

When you are spinning something with enough force to pull an airplane or push a boat through the water, if it is even a little bit off it can set up vibrations that can start tearing things apart. Whether in the ocean or up in the air, you don't want a big hole where your propeller used to be.

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