larger diameter vs. higher pitch

[joshkegg]

2000 bayliner capri, 305 mercruiser, alpha one drive...looking to lower RPMs at WOT. i know if i increase prop pitch from 21 to 23 i can reduce RPMs, but if I use a larger diameter prop (increase by 1") will that also reduce RPMs?
--if so, will it keep top speed the same?
--will the time to get to plane be reduced?

 

[steelespike]

Re: larger diameter vs. higher pitch

Welcome,if you read the stickies at the top of the page and the helps on the prop store page
it will help you understand props a little better.We'll be glad to try to help you out.
We could be a lot more help if you gave us your present performance numbers max wot rpm and speed and what you would like to accomplish.
Diameter is seldom considered directly when trying to change rpm/performance.
Yes a large increase in diameter could reduce rpm.Generally it would slow hole shot, speed change would depend on the present performance.Generally an increase in pitch would reduce rpm
and slow hole shot, speed change again would depend on the present performance.

 

[joshkegg]

Re: larger diameter vs. higher pitch

I understand the basics of props from this site and some experience. I don't want to sacarifice the ability to pull wakeboarder/skiier/tuber out quickly, but I am looking for a little more top end speed.

more details (w/~1000 lbs of people/equip/fuel):
CURRENT...
- prop = 14.3" dia., 21" pitch
- WOT = 4800 RPM @ ~45 mph
OLD...
- prop = 14" dia., 19" pitch
- WOT = 5500 RPM @ ?? mph

Any suggestions...????

 

[Robbabob]

Re: larger diameter vs. higher pitch

more details (w/~1000 lbs of people/equip/fuel):
CURRENT...
- prop = 14.3" dia., 21" pitch
- WOT = 4800 RPM @ ~45 mph
OLD...
- prop = 14" dia., 19" pitch
- WOT = 5500 RPM @ ?? mph

Any suggestions...????

Current motor max rating rpm is 4600 or 4800, right? With the 21P, you've maximized your set up for optimum performance. Want to go faster? Get a bigger boat with more HP.

Questions of your prop testing.....
#1 Guessing you tested one is SS and the other is aluminum.
#2 What were the manufacturers and models.

If you want sound advice, you must provide a higher degree of information.

 

[joshkegg]

Re: larger diameter vs. higher pitch

Current motor max rating rpm is 4600 or 4800, right? With the 21P, you've maximized your set up for optimum performance. Want to go faster? Get a bigger boat with more HP.

Questions of your prop testing.....
#1 Guessing you tested one is SS and the other is aluminum.
#2 What were the manufacturers and models.

If you want sound advice, you must provide a higher degree of information.

Rating on motor is 4200-4600 RPMs. I get the more HP/bigger boat...it will come eventually, but for now I own this boat.
Both props were Solas Aluminum. Always run in freshwater.

 

[Robbabob]

Re: larger diameter vs. higher pitch

Wow,

Never heard of a 700 rpm change when adjusting by 2" of pitch. I realize you also changed diameter a little, but that .3" is very small change in rpm (maybe 25).

Well, with the 21, you are still over proped as you are still running above your engine's rating. However, with water sports being your desire, I see no real issues except when you are running empty.

 

[Bondo]

Re: larger diameter vs. higher pitch

Wow,

Never heard of a 700 rpm change when adjusting by 2" of pitch. I realize you also changed diameter a little, but that .3" is very small change in rpm (maybe 25).

Well, with the 21, you are still over proped as you are still running above your engine's rating. However, with water sports being your desire, I see no real issues except when you are running empty.

Ayuh,.... Don'tcha mean Under propped,..??

josh,...
It appears you might be able to pull a 23" prop, 'n possibly not be Over propped, luggin' the motor...
It might be too much for water sports though...

The diameter follows the blade design more than anything else...
Shop by Pitch, 'n blade design,... diameter will sort itself out...

 

[sschefer]

Re: larger diameter vs. higher pitch

Ayuh,.... Don'tcha mean Under propped,..??

josh,...
It appears you might be able to pull a 23" prop, 'n possibly not be Over propped, luggin' the motor...
It might be too much for water sports though...

The diameter follows the blade design more than anything else...
Shop by Pitch, 'n blade design,... diameter will sort itself out...

Yup, all we know is the brand name and not the blade type. The basic rule of 200 RPM per inch is for general purpose props. It's always best to use the manufacturers prop selector software if they offer it. If they don't then a phone call is in order.

 

[QC]

Re: larger diameter vs. higher pitch

There is something a little odd as I am getting 23% slip. How you getting speed? You may be able to achieve an improvement in top speed AND maintain your watersports performance with the perfect prop. Problem is that it will take really good data (Tach has to be right and speed has to be right, no speculation) and then some testing to find it. It's just the way it is . . .

I am thinking a 4 blade Stainless at 20", but that assumes your data is right, and I don't think it is

 

[joshkegg]

Re: larger diameter vs. higher pitch

Thanks for the input. I will check prop models this week and rerun my speed/RPM tests with both props.

 

[joshkegg]

Re: larger diameter vs. higher pitch

Some more details about the props:
CURRENT:
- Solas Amita 3-blade Aluminum
- 14.3" dia. x 21" pitch SR
OLD:
- Michigan Wheel 3-blade Aluminum
- 14.5" dia. x 19" pitch

Will have updated speed and RPM test done this weekend.

Another question, how do I know the gear ratio of my drive? (Transom Serial #OL842163, Drive Serial #OL704991)