Prop diameter
05-31-2002, 04:55 PM
#1
Registered
Thread Starter
Prop diameter
How does prop diameter affect top speed? If you have two identical props with identical pitch with the same number of blades but one of them is 14 5/8" and the other is 14 3/8". Which will go faster at top speed. One school might say the bigger one will go faster since it can push more water. Another school might say the smaller one will go faster since it has less to move thru the water.
Which school is correct?
Which school is correct?
05-31-2002, 05:13 PM
#2
Platinum Member
Platinum Member
Well, theoretically...Power in should equal power out.
That being said, you would think that the bigger one will turn slower, yet pushing more water/rev and the smaller one will turn faster pushing less water/rev.
However, in reality, the difference is that the bigger prop will resist with more torque upsetting the balance of your boat's list to one side and the larger blade area will induce more drag resulting in greater slip numbers.
The idea is to go with the smallest prop possible to do the job of converting as much of the engine's power into thrust. In an extreme case, imagine mating a big-block to a trolling motor propellor(6")...The prop will froth the water, becoming turbulent and will not do a whole lot of good. Now take that same engine and mate it to 36' diameter mini-ship prop...Very efficient in moving water, but the motor will be struggling to turn it very slowly while most of the power is eaten up in drag.
Bottom line...it's not a simple answer and trial and error often provide the quickest results...theoretically, the smaller the better so long as all the power is utilized.
That being said, you would think that the bigger one will turn slower, yet pushing more water/rev and the smaller one will turn faster pushing less water/rev.
However, in reality, the difference is that the bigger prop will resist with more torque upsetting the balance of your boat's list to one side and the larger blade area will induce more drag resulting in greater slip numbers.
The idea is to go with the smallest prop possible to do the job of converting as much of the engine's power into thrust. In an extreme case, imagine mating a big-block to a trolling motor propellor(6")...The prop will froth the water, becoming turbulent and will not do a whole lot of good. Now take that same engine and mate it to 36' diameter mini-ship prop...Very efficient in moving water, but the motor will be struggling to turn it very slowly while most of the power is eaten up in drag.
Bottom line...it's not a simple answer and trial and error often provide the quickest results...theoretically, the smaller the better so long as all the power is utilized.
