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Determining Pitch

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Yesterday i went to a boat show. Attended a seminar entitled Props 101. Moderator gave a mathematical formula to help determine pitch; he stressed that trying different props was the ultimate determination method. Thought I would post to help others and see the comments from the knowledgable:

Current RPM ÷ Desired RPM × Current Pitch = Proper Pitch

I.E. At WOT and your current pitch is 15; if you are running at 5200 rpm’s and you feel that you should be running at 5800 rpm the math formula is as follows:

5200 ÷ 5800 × 15 = 13.49 [therefore try a 13 pitch]
 
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Seems like a reasonable calculation. The one thing it doesn't take into account is the prop diameter, he assumes the diameter remains the same.

Steve
 
Not defending the math formula but wanted to admit it was me that asked the question of how to determine the right pitch if my boat wasn't at the proper rpm at WOT. The prop expert did say that the best was to trial run a prop but did offer this math formula as an indicator to where it should be.

I also see SteveL response, how do I know what diameter to choose?

With the amount of discussion I have seen on power and props I do agree and also LOL because there is no easy answer.
 
Link said:
I also see SteveL response, how do I know what diameter to choose?
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Great question; each manufacturer makes their pitch with their diameter :( I haven't spent much time looking but I believe the Yamaha aluminum 15 pitch is 13.5 inches in diameter; the 13 pitch is 13.625 inches in diameter. I don't think that the 1/8 inch difference in diameter would mean much, but a different manufacturer could offer a fairly large difference (1/2 inch or more). So answer to your question is "I don't know" other than trying a few props and see what works best for you.

Things that will make a difference in props:

  • Diameter
  • Pitch
  • How many blades
  • Blade design (profile & cup)
  • Material prop is made from
  • Hub diameter
  • Exhaust blade porting (you can find all kinds of pros & cons on this one)
  • Hull design of watercraft
  • Load
  • Depth of the prop (means much more to high performance applications, ie: chopper props, surface props)

I don't know the high tech stuff on boat props, but airplane propellers diameter will affect rpm more than pitch (talking about one diameter change vs. a pitch change).

I currently have aluminum 13.5 x 15 on my Yamaha F115, for my boat & load it's the wrong prop. I will be hunting soon.

Steve
 
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Here's some new eye candy for ya. A new one-off 4 blade Stinger IV that was hand tuned just for me by Stinger Custom Racing Propellers. There is definitely more to it than simple math.
 
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How does it perform? Looks mean as hell. :)
 
Sweet looking prop... if it's that bad send it my way and I'll dispose of it properly for you :)

Steve
 
I ran 4 blades on my center consoles. Had heavy boat with twins and needed the surface area of 4 blade. Robbed some top end, but was great for acceleration and fuel economy. Not sure how it would perform on a light pontoon boat.
 
Each boat/prop is different. Ya gotta test test test. The Stinger IV will stay with me. I wouldn't feel right about letting such a terrible prop get into the hands of anyone else.
 
So what's so wrong with it??????
 
Here's one of his promotional shirts that I've had from back in the day when we used to race together. He claims he's retired now.....we'll see.

5kotar.jpg
 
Newaygo we camp there 4-5 times a yr and play on hardy pond. We only live about 45min from there. maybe we will see u on the pond. First trip planed for up there is the weekend of June 6th
 
bassn429 said:
Newaygo we camp there 4-5 times a yr and play on hardy pond. We only live about 45min from there. maybe we will see u on the pond. First trip planed for up there is the weekend of June 6th
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That's where we hang out most of the time. You'll find us at Consumers beach or normally up at the sandbar by US 131. That's our favorite spot.
 
Both excellent spots. We also love those spots or at the island if u get there in time to get a spot. Hey will that prop fit on my merc 60hp bf. And for your sake i hope those idiots arent back around with there scuba gear stealing ss props like they did a few yrs back
 
bassn429 said:
Both excellent spots. We also love those spots or at the island if u get there in time to get a spot. Hey will that prop fit on my merc 60hp bf. And for your sake i hope those idiots arent back around with there scuba gear stealing ss props like they did a few yrs back
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Sorry, but it's a 16" diameter. Do you mean weekend of July 6th?
 
Link said:
Yesterday i went to a boat show. Attended a seminar entitled Props 101. Moderator gave a mathematical formula to help determine pitch; he stressed that trying different props was the ultimate determination method. Thought I would post to help others and see the comments from the knowledgable:

Current RPM ÷ Desired RPM × Current Pitch = Proper Pitch

I.E. At WOT and your current pitch is 15; if you are running at 5200 rpm’s and you feel that you should be running at 5800 rpm the math formula is as follows:

5200 ÷ 5800 × 15 = 13.49 [therefore try a 13 pitch]
Click to expand...
This formula makes no sense. The numbers don't work at all
 
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