Marine Propellers 101

Being as how we are a manufacturer of propellers, we thought you might enjoy a brief introduction to marine propellers! We think this stuff is fascinating, so hopefully you’ll find something interesting here.

As you probably know, a propeller is the way most boats generate momentum. Propellers turn in the water, moving it and causing a force to act on the ship. The ideal surface to do this would be an actuator disc. That’s the shape that helicopter blades make when it’s hovering. However, this is theoretical—in reality, a propeller uses sections (blades) of helicoidal surfaces that act together to “screw” through the water. (You can see where the common reference of propellers as screws is apt.) The most common designs have 3, 4, or 5 blades. The rule there is that the more blades, the less noise and the smoother the action of the prop. Each of the blades meets the boss (which is the hub) in the center. Ideally, this boss is as small as it can be while remaining structurally strong.

An alternative design is the controllable pitch propeller (CPP). This design uses machinery to rotate propeller blades to control speed and direction with a great degree of precision. A CPP allows the drive machinery to operate at a constant speed, which eliminates the need for a reversing gear and makes rapid changes in thrust possible. You see this kind of propeller on boats such as tugboats, where great amounts of torque are needed to do the job. As we mentioned above, a boss is ideally small and this kind of propeller causes some downsides as it requires a relatively large boss, or hub.

Self-pitching propellers are popular for use with small motors. Here, the blades move freely at right angles to the shaft coming down from the motor. This freedom allows the forces of the water and movement to set the angle and pitch of the propeller.

Sometimes it’s good to talk about the basics. Anything you can think of to add to complete the picture?

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