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Old 04-25-2017, 05:09 PM   #1
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Default Prop diameter

OK I am old and forgetful so be easy on me!
I am looking for info on what effects prop diameter has on RPM?
I have used Mercruisers Prop Calculator and it states that I need a 15 or 16 inch diameter prop with a 15,16 or 17 inch pitch!
I have the paperwork from the original owner and it states that a 14.5X17p prop was delivered on that boat, 1995 2400SCR 5.7l Alpha 1 Gen 2 with 1.47 ratio!
It is now running a 14.5X19p and as expected it wont reach max RPM!
Looking for some useful feedback from any and all who have knowlege!
Thanks from an old forgetful boater!
Don
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1988 Bayliner 2455 5.0 IO (sold)
1987 Seaswerl 18ft C. Cabin 4.3 IO(lost in fire)
1976 16ft. Tahiti 2.5 IO (sold)
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Old 04-25-2017, 09:54 PM   #2
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Don you absent minded old salt

The diameter results in more blade area. More area more bite in the water plus more load on the engine. I would think the paperwork you have is a good prop for the boat. I ran a similar size on a 79' 22' search I had. My 2400 has a BII and swings a big wheel. The BII was mercs answer to not have the rights to a dual prop for big heavy boats and was an option for our size.
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Old 04-26-2017, 03:26 AM   #3
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Thanks Mike!
I was looking RPM differences.
I know that 1 inch in pitch = about 200 RPMs, but I know that an increase or decrease in diameter will change max RPMs, but how much per inch change?
As I stated merc. is recommending a 15x16p , 15x17p, 15x15p & 16x16p all in a 3 blade alum. prop! Very confusing as Maxum delivered this boat with a 14.5x17p 3blade alum. prop!
With the 19p prop I can only get 3500 to 3800 RPMs!
I only run at 2500 to 2800 RPMs at cruise, I am trying to get the boat to run slower at idle but not have it over rev to much if I give a lot throttle.

Thanks
Don
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1995 Maxum 2400 SCR 5.7 A1 G2 LUNA DE MIEL
1988 Bayliner 2455 5.0 IO (sold)
1987 Seaswerl 18ft C. Cabin 4.3 IO(lost in fire)
1976 16ft. Tahiti 2.5 IO (sold)
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Old 04-26-2017, 11:34 AM   #4
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Impacts of diameter change is harder to gauge as it depends on the blade shape; i.e. more blade area at the tip and changing diameter will have a bigger impact than a design with less area at the tip. Also if it's a cupped prop the impacts are greater.

For slower speed at idle rpm a smaller prop is the way to go. I would think 2500 - 2800 rpm would be the worst place to run this boat as it would be plowing (trying to plane). The bow would be high and fuel consumption the worst as the boat is fighting to plane.
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Old 04-26-2017, 05:54 PM   #5
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Thanks Mike!
That is what I was thinking, but thought I would get some input from others!
I have an old 14.5x17p that I am going to try that is not in the best of shape that I keep as a get home prop!
The boat is just barely on plane! but the First mate has a problem with speed!
We like to fish so trying slow it down as much as possible without dragging several 5gal buckets behind us! As you know on way to mount a kicker motor on a 2400SCR!
Thanks for all the input Mike!
Forgetful Don
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1987 Seaswerl 18ft C. Cabin 4.3 IO(lost in fire)
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Old 04-28-2017, 01:25 PM   #6
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Playing with diameter is very, very uncommon. Usually because you are limited in how big a wheel you can swing before you starting coming in contact with the anti-ventilation plate on the outdrive itself.

I would look to the Mercruiser Prop Calculater for size and pitch. Pitch is really where you're going to modify RPM's.
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Old 04-28-2017, 02:37 PM   #7
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There was a time (way back when!) that I could remember things,just a passing glimmer now!
Anyway I remember that a prop diameter change of 1 inch would relate to x change in RPMs! Just can't remember what it was.
Guess I am have senior moments that last longer!
Any before I forget again Thank You!
Don
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1988 Bayliner 2455 5.0 IO (sold)
1987 Seaswerl 18ft C. Cabin 4.3 IO(lost in fire)
1976 16ft. Tahiti 2.5 IO (sold)
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Old 04-28-2017, 02:40 PM   #8
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Quote:
Originally Posted by donald View Post
There was a time (way back when!) that I could remember things,just a passing glimmer now!
Anyway I remember that a prop diameter change of 1 inch would relate to x change in RPMs! Just can't remember what it was.
Guess I am have senior moments that last longer!
Any before I forget again Thank You!
Don
Again, that applies to Pitch, not diameter. 1" if pitch = 200 RPM.
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Old 04-29-2017, 02:25 AM   #9
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Thanks shrew!
Get'n old sucks!
Yes I know about the change in pitch!
I tried Mercruiser prop selector, but it gave larger diameter and less pitch than what I have now! So was trying to find out what would happen if I went smaller diameter and less pitch than what I have now!
Trying to be able to troll without dragging 5gal buckets. May have to that anyway!
Thanks!
Don
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1995 Maxum 2400 SCR 5.7 A1 G2 LUNA DE MIEL
1988 Bayliner 2455 5.0 IO (sold)
1987 Seaswerl 18ft C. Cabin 4.3 IO(lost in fire)
1976 16ft. Tahiti 2.5 IO (sold)
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