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How do I know what diameter prop I need?
Propeller size is expressed with two numbers, diameter and pitch, with diameter always stated first. Diameter is two times the distance from the center of the hub to the tip of any blade. Smaller prop diameters generally go with smaller engines, or with fast high performing boats.
Does the diameter of a prop matter?
Propeller diameter increases with the size of the boat. As the diameter increases, the blades of the propeller offer more resistance to travel through the water. This creates potentially damaging drag on smaller engines and boats. For larger boats, a larger diameter provides more propulsion power for the same reason.
What are the dimensions of a boat propeller?
All props will be described by two numbers, such as 13 x 21. Those numbers are the diameter and pitch. Diameter is the overall size of the blade as measured from the tips. A large diameter propeller can push more water than a small diameter propeller, creating more power.
What are the factors to consider when choosing a propeller?
Your propeller decision-making process will include the following factors: pitch, diameter, aluminum or steel, blade number, rake, cupping, and rotation. Pitch is a critically important factor when selecting a new propeller. Pitch is the distance the prop would travel forward after one complete rotation, assuming no resistance or slip.
How is the pitch of a propeller calculated?
The second number is the pitch. The pitch is the theoretical distance in inches that the propeller would travel forward in one single revolution. For example, a 21-pitch propeller would move forward 21 inches in one revolution. Think of your pitch as the gears on your car.
How does the size of a propeller affect thrust?
Roughly speaking, diameter influences the amount of thrust generated but an ever-increasing and non-performance related issue these days, linked to prop diameter, is that of noise. A faster turning propeller (and props can easily turn in excess of 10,000 RPM) generates a lot of noise as the tips cut through the air.