Prop diameter & it's affect

[ob15]

What affect does a change in prop diameter have on performance? I don't mean a big change, maybe 1/2" either way. I need to get a new prop and don't think I can get the same diameter & a smaller pitch.

 

[Dhadley]

Re: Prop diameter & it's affect

It really depends on the technology built into a given prop. As a very general rule of thumb, the larger the diameter -- the higher you can mount the motor. But thats if the prop is built with that in mind, like a Turbo or even a Stiletto. Also blade area come into play.<br /><br />Again, a very loose general rule of thumb is that diameter plays a big role in holeshot and pitch is the main factor in top rpm. Each affects the other but you get the idea.<br /><br />BTW -- a difference of 1/2" is quite a lot. Also remember to compare 3 blade and 4 blade props to each other. <br /><br />Good luck!

 

[JasonJ]

Re: Prop diameter & it's affect

I had a 13x17 three blade on my 125 horse Merc, and just for giggles I put my spare prop on, which is a 14x17. With the increased bite, it caused the boat to plow in a bow down position, so trimmed it out one hole (I have manual trim). That in turn brought my rpms up, and returned the boat to its proper running attitude. The end result was that I gained speed and rpms because I was able to trim the motor out due to the increased bite of the prop. I can't say that will happen to you, every boat situation is different, but you will notice a small drop in rpms with the bigger prop and a small increase in rpms with the smaller prop until you change other apsects of your setup. Good luck...

 

[ob15]

Re: Prop diameter & it's affect

Thanks!

 

[walleyehed]

Re: Prop diameter & it's affect

Dave, usually an increase in diameter does not increase speed, unless it was trimmed down (the prop) or the wrong prop to begin with...diameter usually goes up with lower pitch to hold better at higher RPM, but also increases hole shot.<br />JasonJ's case is to me, unusual because the 14X17 vs. the 13X17, should help with hole shot, but rob top end "slightly"..In this case, I'm going to say they were 2 different blade designs.<br />As I said, if the proper match of Diam.+ pitch is in use now, just an increase in diameter will lower top end due to larger blade area being a more efficient "Load-pusher", but also more drag at high RPM.<br />The 13, which "should" actually be 13-1/4" diameter, has rounded ear blades, and the 14" series has longer, tapered ears...for OMC engines..

 

[ob15]

Re: Prop diameter & it's affect

JasonJ<br /><br />Nice job on the refit. Have done something similar but a little more involved. Check it out.<br /><br /> http://bellsouthpwp.net/o/b/ob15/

 

[JasonJ]

Re: Prop diameter & it's affect

Walleye, my deal was weird I agree, and not what I expected either, but since I was able to do other setup tweaking (trim, motor height, etc) due to the better bite, I think that is how I ended up with better performance. I forgot to mention in my last post that I also raised the motor 1 inch on the transom as well since I had the better bite. The props were both aluminum, but the 14x17 had the vent holes drilled at the base of the blades. That may have helped as well. I am not a prop expert by any means, I was just experimenting.<br /><br />Dave, I checked out your site, very cool. There was a guy on here that built an 18 ft Bataeu center console, it was Civilengineer or something like that. He was my inspiration.

 

[walleyehed]

Re: Prop diameter & it's affect

Jason, you got your foot thru the door when you raised the engine...It DOES make a difference.<br />Efficiency is the name of the game!