How does diameter affect your prop?

[UtahBoating]

Been looking for an answer on how (if any) the diameter affects your prop. I am currently running a 21* 14 1/4 diameter prop. When looking at other props I notice there are 21* props in 13 3/4 for my alpha one. The 21 pitch seems dead on perfect for my boat. Just wondering if I buy the smaller diameter if it makes a difference. There is a used one for cheap in my area was going to pick up as an extra.

 

[tpenfield]

The diameter determines how much water the prop will move. The pitch determines how far it will move it. Each of those factors has an affect on the overall result. A smaller diameter prop of the same pitch will move less water and therefore require less power.

So, if you go down in diameter, the motor will probably reach a higher RPM. As to the resulting boat performance, it is hard to say and often you just have to try it and see. By my calculations, the smaller prop of the same pitch will move about 8% less water per revolution.

Generally speaking larger/heavier boats do better with large diameter props. Smaller/lighter boats are fine with smaller diameter props.