Prop pitch conversion from 3 blade to 4 blade

[jurgenscraft]

I posted a topic some while ago regarding converting from a three blade prop to a four blade proplellor , well what I have found out is as follows :the pitch remains the same but the diamater, has become smaller eg,11 1/2 x 12 becomes 10 1/2 x 12 pitch.( four blade) and a 11 1/2 x 13 becomes 10 1/2 x 13 pitch( four blade) I wonder if any body else has found this to be the case, the motor I am using is a Yamaha 55 with the following specs,wot is 4500 to 5500 ( normaly run at 4000 rpm )gear ratio 13:24 (1.85)<br /> I have not bought the propellors yet so some comments would be appreciated, kind regards William Wright.

 

[DJ]

Re: Prop pitch conversion from 3 blade to 4 blade

4 or 3, doesn't matter. What are you trying to do?<br /><br />Run the engine, light load, at the top of its recommended RPM range, at wide open throttle.<br /><br />Outboards DO NOT like to lug, as you are doing at 4K.

 

[Silvertip]

Re: Prop pitch conversion from 3 blade to 4 blade

Propeller diameter changes slightly with each change in pitch up or down. How much change depends on the propeller design. Generally, as pitch increases, diameter decreases by 1/4 to 1/2 inch. As pitch goes down, diameter increases by roughly that amount. Diameter changes in those amounts does not affect performance as much as pitch changes. Before changing props, do wide open throttle testing to check RPM and speed with existing prop. Then and only then can you make new prop selection and valid comparisons.

 

[jurgenscraft]

Re: Prop pitch conversion from 3 blade to 4 blade

DJ I want to get going faster ( Have a better hole shot get onto the plane quicker) and I do not want to run the motor flat out 5000 - 5500rpm all day long,<br />I am able to cruise at 4000 - 4500rpm rpm at the moment without any difficulty, economy is ok, maybe a four blade prop will help a little with economy, what I really want to know is if two props have the same pitch but have four blades instead of three should they perform in the same way? (see original post for comparison propellors )if I run at wot with the 11 1/2 x 12 pitch 3 blade, I run max speed at 5000 rpm.the extra 500 at 5500 rpm make no visible difference the acceleration is good, when I run the 11 1/2 x 13 3 blade pitch top speed is a little better but acceleration is slightly less and the revs are slightly lower for the same speed ( gps measured)kind regards William Wright.

 

[walleyehed]

Re: Prop pitch conversion from 3 blade to 4 blade

You don't "have" to run at or above 5500 but you better have it proped so it "will" run at or above 5500 at WOT...Then, and only then, will your cruise be at it's highest efficiency at 4000.

 

[umblecumbuz]

Re: Prop pitch conversion from 3 blade to 4 blade

William,<br /><br />From your posts it seems that you can run at 5500 if you want to, so I question the conclusion by DJ that you are 'lugging' at 4000. That would only be true if 4000 was the max you could hit.<br /><br />I can run 5800, but I choose to run less for a whole raft of reasons - comfort, economy, etc. I'm sure many here do the same.<br /><br />As Kenny says - you don't have to run 5500, but you need to be able to hit it with your prop choice.