Have i wasted my money?

[davis7210]

Recieved new johnson alum. 13.25 x 17 prop today, put it on as soon as i got home. Heres my question thought i was replacing 12.75 x 21 alum . WRONG!!!! Turns out its stainless part# 389512 looks like letter M with U around it. 21 was turning 5000 rpm WOT, trying to get 5800 rpm. What do you think new prop will turn at? I have learned a lesson from this, Magnets don't lie!!!Thanks for any input.

 

[Gary H NC]

Re: Have i wasted my money?

The M with the U looking markings around it is the OMC logo.
Dropping from a 21 to a 17 should get you 800 more rpms...
On average about 200 rpms per pitch size..

 

[davis7210]

Re: Have i wasted my money?

What about going from stainless to alum? Will it add any rpm, it sure is alot lighter!!

 

[Silvertip]

Re: Have i wasted my money?

Weight won't have a significant effect if any on rpm. Stainless to aluminum or aluminum to stainless can have an affect on RPM but the amount is difficult to determine since the prop design has a bearing on that as well. In other words two aluminum props of the same diameter and pitch from two different manufacturers may not and generally do not perform the same on any given boat. Close - yes! But not identically.

 

[MikDee]

Re: Have i wasted my money?

Dropping down 2 increments in pitch, because props are usually made this way, lets say a 19" prop of the same material & design should net you 200rpm more, then to a 17" pitch would add another 200rpm, with a total of 400 rpm more net. That would put you at 5400rpm, if they were both stainless Finally being an aluminum prop, I think you'll run 100 to 200 rpm more, so I'd say, you'd net maybe 5500-5600 rpm That should be fine for your motor! Even If your motor woke up more from the swap, I doubt you'd get more then the 5800rpm Don recommended, IMO from my experience, it'll be just right.

 

[davis7210]

Re: Have i wasted my money?

OK glad to here i should be pretty close. Want to get it in the water soon and test , i have no patience.

 

[Silvertip]

Re: Have i wasted my money?

Actually, the rule of thumb is one inch of pitch up or down = 150 - 200 RPM up or down. So a four inch drop in pitch would net about 600 - 800 RPM not 400. But again, all of that depends on the design of the existing prop and the design of the replacement. Pitch is only one factor. You can also gain rpm by playing with engine mouting height. Just don't go so high that you lose water pressure before the prop begins to lose bite.

 

[davis7210]

Re: Have i wasted my money?

So i am still close on pitch. I posted pics of motor height on johnson/evinrude motor questions on iboats forum, under SET ME UP thread. Got some replies that i was close. Since i have found this forum seems like i can not get on the water so i can get real data on everything. Might just have to call in sick one day!

 

[MikDee]

Re: Have i wasted my money?

Actually, the rule of thumb is one inch of pitch up or down = 150 - 200 RPM up or down. So a four inch drop in pitch would net about 600 - 800 RPM not 400. But again, all of that depends on the design of the existing prop and the design of the replacement. Pitch is only one factor. You can also gain rpm by playing with engine mouting height. Just don't go so high that you lose water pressure before the prop begins to lose bite.

So you're saying going from a 17"P to a 19"P, or vice versa, 2" difference in pitch, yields 400rpm's difference? have you physically done this? I have, more then a few times, on different boats, with different props, and found only about a 200rpm difference, but that's my opinion.

 

[gotboostedvr6]

Re: Have i wasted my money?

Instead of replacing the prop why dont you try mounting the motor one hole higher? 5800 rpm is alot of rpm plus your low speed will now suffer worse fuel consumption with the 17 pitch prop.

Yes you wasted your money, but that aluminum prop can be a good spare.

You would never catch me using an aluminum prop unless i just threw a blade or just plain lost my stainless one. There is NO comparison between the 2.

If you really want to know what to do post up your speed (GPS ONLY) and exact rpm. do this at different trim angles.

 

[Texasmark]

Re: Have i wasted my money?

On similarities, alum has thicker blades and usually not cupped which SS have/are. SS can raise rpms because of better efficiency with thinner blades, while the cup, can pull them back down because your prop gets more bite. So like the guys said only way to know is to run it.

Mark

 

[Silvertip]

Re: Have i wasted my money?

MikDee -- 200 rpm per inch of pitch up or down is as I said "rule of thumb" and generally accepted. How much that change is depends on what two props you are comparing. If you compare two aluminim 3 blades of say 15P each from two different manufacturers, chances are the RPM will differ between them. If you compare a 15P aluminum to 17P stainless from different manufacturers the rpm may be slightly more or less than 200 rpm. The next factor is some prop designs work better on a given boat than others. What works well on one boat may not work worth a darn on another. Anyway -- Below is just part of a discussion on prop pitch from Caravelle Powerboats (and you will find this number anywhere you look when reading about propeller theory). Yes -- I have done this and in it is a valid rule of thumb.

Here's a simple rule of thumb to follow when experimenting with prop pitch. Remember that at wide-open RPM increasing the prop pitch reduces RPM levels by roughly 200 rpm's per inch of pitch. In other words, when switching from a 23- to a 25-pitch prop, the maximum RPM level will drop approximately 400 rpm's. The reverse is true when going down in pitch size.

 

[MikDee]

Re: Have i wasted my money?

MikDee -- 200 rpm per inch of pitch up or down is as I said "rule of thumb" and generally accepted.

Not my rule of thumb from what I've found.

How much that change is depends on what two props you are comparing. If you compare two aluminim 3 blades of say 15P each from two different manufacturers, chances are the RPM will differ between them. If you compare a 15P aluminum to 17P stainless from different manufacturers the rpm may be slightly more or less than 200 rpm. The next factor is some prop designs work better on a given boat than others. What works well on one boat may not work worth a darn on another.

This is all a moot point anyway, I've done it all these ways, and still come to my same conclusion 200rpm between prop pitch changes, which happen to be in increments of 2", not 1". Unless you're talking about 4blade, or multiblade props, of which some do come in increments of 1".

Anyway -- Below is just part of a discussion on prop pitch from Caravelle Powerboats (and you will find this number anywhere you look when reading about propeller theory). Yes -- I have done this and in it is a valid rule of thumb.

Here's a simple rule of thumb to follow when experimenting with prop pitch. Remember that at wide-open RPM increasing the prop pitch reduces RPM levels by roughly 200 rpm's per inch of pitch. In other words, when switching from a 23- to a 25-pitch prop, the maximum RPM level will drop approximately 400 rpm's. The reverse is true when going down in pitch size.

Your repeating yourself here, that doesn't make it necessarily so, In due respect, you believe what you want, and I'll believe what works for me, and the bottom line is, I've never physically seen a 400 rpm raise, or drop, by changing one normal increment of pitch, hence 2"

 

[Silvertip]

Re: Have i wasted my money?

Wow -- I didn't realize someone could get so opinionated over a "rule of thumb". My gosh it's a rule of thumb, not the eleventh commandment. First, I wasn't repeating myself -- that last quote was from Caravelle Power Boats but I suppose they don't know what they're talking about either. Here's another:
marine-outboards.com/choosing-propeller.htm.

I agree with you that three blade props "generally" change pitch in 2-inch increments. But that's not ALWAYS true either! Look up this number 5034754_5034755 on iBoats and there are other examples as well if want to take the time to look. You'll find it's a 22-1/2 inch pitch three blade. And this one (345041) is a rather common Ballistic 22P three blade. Even the prop manufacturers use the 150 - 200 rpm per inch (not 2-inches) number in their prop recommendations. It the rule of thumb doesn't work for you I can't help that. It seems to work for the majority of us.

 

[davis7210]

Re: Have i wasted my money?

Alright, so i still have my 12.75 X 21p stainless. What can be done with it, can i send it to a prop shop and have them work it over? Can they change the pitch, rake, cup? Or is it to expensive vs just buying another ss prop. I really haven't seen to many 12.75 x 21ss props, ebay or anywhere else. Not much call for them?

 

[Silvertip]

Re: Have i wasted my money?

A prop shop can change the pitch and they can add or remove cup. How much depends on the prop. Before you spend more money, do a good deal of testing to make sure you have a good base line (WOT RPM and speed) with an average load. Armed with that information, visit a prop rebuilder and see what they say. Give these folks a call: Formula Propeller & Marine or contact them at forumlapropeller and you can add the .com to keep things legal for this forum. Phone number is 763.535.8011. They are located in Minnesota but I'm sure they will discuss your issue with you and offer advice. They are very fair.

 

[davis7210]

Re: Have i wasted my money?

Thanks for the info. Now it's time to get on the lake and put all this good knowledge that i now have to work. A friend has a handheld GPS i can use, tach works, new plugs i can hardly wait!!!!!

 

[Frank Acampora]

Re: Have i wasted my money?

Well, Mik! MY experience with MY Force and Chrysler engines is that a 2 inch pitch change usually gives me around a 400 RPM difference---But that's with my engines, and with props---there are a lot of variables. But for now, I'll go with the rule of thumb when giving advice on Force engines.

Chill out! Don't get your shorts in a twist. We're all here to help and have fun.

 

[jay_merrill]

Re: Have i wasted my money?

I had a 17" aluminum wheel on my motor and it was only turning 4,600 rpm at WOT, which is way too low. I was looking for about 5,000 rpm so I went down to a 15" aluminum wheel. My tach indicates that I ended up exactly where I wanted to be - 5,000 rpm. The rule of thumb of 200 rpm per inch has become one for a reason - when comparing props of a like material, design and number of blades, its usually pretty close.

 

[Texasmark]

Re: Have i wasted my money?

Well, Mik! MY experience with MY Force and Chrysler engines is that a 2 inch pitch change usually gives me around a 400 RPM difference---But that's with my engines, and with props---there are a lot of variables. But for now, I'll go with the rule of thumb when giving advice on Force engines.

Chill out! Don't get your shorts in a twist. We're all here to help and have fun.

Frank, man I don't want to rain on your parade and you will probably reply that I haven't (great) but personally, after having a significant investment in Chrysler, West Bend, Lone Star and the like, these guys totally suck. Have no idea how they lasted (through as many hands as they went through) for as long as they did. Yucho.

Mark