When you start looking for a new boat prop you go on-line and search boat propellers to see what is available for your engine. If you have never bought a boat propeller before you will see that the sizes are shown with two numbers not just one. You will see 10 1/8 x 14 or 14.5 x 19 and a bunch of other sizes that make you wonder what do these numbers mean? Well it is really very simple. The first number is the diameter of the boat propeller and the second number is the pitch. First I’ll give a brief definition of pitch and the rest of this blog will focus on diameter.
The next blog will be about pitch. The easiest explanation about pitch is that it is how far a propeller will travel forward through a solid one revolution in terms of inches. A 14 pitch prop will go 14 inches forward in one turn and a 19 pitch prop will go 19 inches forward in one turn.The definition of a boat propellers diameter is a measurement from the center of the propeller shaft hole in the center of the prop out to one blade tip then multiplied by two. For example, if you measured from the center out to one tip and got a measurement of five and one sixteenth inches then the diameter of you propeller is 10 1/8 inches. If you measured seven and one quarter inches then you have a 14. 5 diameter prop.
The smaller the outboard motor in terms of horsepower, the smaller the diameter of the boat prop is. And the bigger the motor is the bigger the diameter of the prop is. The reason for this is that you cannot use a huge diameter prop on say a twenty horsepower engine because it would be trying to bite way too much water for the little horsepower engine to handle and it would bog down the engine so much that the boat would hardly move. The opposite effect would happen if you put a real small diameter boat propeller on a large horsepower engine. The prop would just pretty much free spin at a high rate of speed because it’s not biting enough water to push the boat and the engine would rev way up and the boat would hardly move.
What diameters work best for what engines and for different pitches is determined by the propeller manufacturer through research and development witch means just trial and error on what diameter gives the best performance for a certain pitch.
For more information on boat propellers and to place orders please check out our website. Or call us a 1-800-573-5533