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Relationship Between Prop Diameter & Pitch

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Relationship Between Prop Diameter & Pitch

Old 08-19-2020, 08:08 AM
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Default Relationship Between Prop Diameter & Pitch

Hey! Is there a general rule of thumb on how diameter of a prop will impact your WOT RPM? I've been messing with a 1980s Glastron Carlson CVX16 with a 135 Blackmax (did a build thread on here!) all summer. Right now it is running a 13x21 3 blade aluminum and hits about 5500 RPM at WOT. Great all around prop for hole shot, cruise and top end. Picked up a 12x21 stainlee and it kept blowing it out. Having a hard time finding a 13x21 in stainless. What would happen if I stepped up to a 14x19 or 14x21? How many RPMs can I expect to lose stepping up ever inch in diameter?
Thanks!
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Old 08-19-2020, 01:20 PM
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Iíve been reading thread after thread about this and seems like biggest problem is you can have three different 14 x 21 props behave differently because of rake and cup in blades too. Itís enough to make your head pop lmao .i have a 14.5 x 21 on mine right now that I havenít read of anyone using . I like the cruising speed except when it gets to rough on the river. I donít see any other way around having multiple props for when you have more weight in boat or just crappie conditions
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Old 08-21-2020, 05:10 PM
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For very rough estimation, each increase or decrease in pitch by 1" accounts for 200-400 rpm loss or gain ASSUMING the exact same series of props. For example going from a 19" Laser II to a 21" Laser II would decrease WOT rpm by approximately 200-400 rpm. You could not make the same comparison between say a Laser II and Mirage Plus, although the estimate would be closer than a comparison between a Laser II and say a Bravo 1 4 blade.

For diameter the rough rule of thumb is for every increase in prop diameter WOT rpm will be reduced by 50-100 rpm. The problem with this is that even in the same line or series of props the diameter usually changes a small amount as pitch increases or decreases. As another example you can't get a Laser II 19" prop in both a 14 and 14.5" diameter.

Start with the basics.

What is the max WOT rpm for your particular motor? Get a prop that will allow the motor to achieve that rpm with normal loads. Stainless props perform better due to better and more consistent blade geometry and they flex less under load, thus maintaining their pitch under load. Downside is they are more costly.

It is difficult to find the perfect prop that does everything well. If max speed is your preference typically 3 blade stainless would be my first choice. Going up to a 4 blade may offer better midrange and holeshot performance. Sometimes you just have to try a bunch of props.
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Old 08-21-2020, 09:34 PM
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Originally Posted by Trash View Post
For very rough estimation, each increase or decrease in pitch by 1" accounts for 200-400 rpm loss or gain ASSUMING the exact same series of props. For example going from a 19" Laser II to a 21" Laser II would decrease WOT rpm by approximately 200-400 rpm. You could not make the same comparison between say a Laser II and Mirage Plus, although the estimate would be closer than a comparison between a Laser II and say a Bravo 1 4 blade.

For diameter the rough rule of thumb is for every increase in prop diameter WOT rpm will be reduced by 50-100 rpm. The problem with this is that even in the same line or series of props the diameter usually changes a small amount as pitch increases or decreases. As another example you can't get a Laser II 19" prop in both a 14 and 14.5" diameter.

Start with the basics.

What is the max WOT rpm for your particular motor? Get a prop that will allow the motor to achieve that rpm with normal loads. Stainless props perform better due to better and more consistent blade geometry and they flex less under load, thus maintaining their pitch under load. Downside is they are more costly.

It is difficult to find the perfect prop that does everything well. If max speed is your preference typically 3 blade stainless would be my first choice. Going up to a 4 blade may offer better midrange and holeshot performance. Sometimes you just have to try a bunch of props.
trash funny you mention the laser II. I just got a 21p laser II today that Iím swapping for a solas 14.5 x 21p. I couldnít find any info about the solas, other then itís cheap,which we all no what that means. I bought the laser because Iíve read my boat came with one and a mirage from the factory. I can tell you one thing for sure the solas is noticeably heavier. I should be out on the water tomorrow,so Iíll get to see how it behaves 👍🏻
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Old 08-22-2020, 12:11 AM
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A Laser II would be a good prop to try on that hull. I ran a few 16' shallow v outboards in the day and I had a Marlin 16 with a 150 Black Max that ran low 60's when lightly loaded with a 24 pitch Merc chopper. Your hull is similar, and will run well with a bow-lifting prop. The Laser II has a similar blade to a small-ear Chopper.
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Old 08-23-2020, 01:52 PM
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Rule of thumb for diameter is each 1/4" increase is about 100rpm decrease but that is comparing apples to apples. Results will change when changing the model of prop.
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