what size aluminum prop

[redlinj]

Just banged up my 14 x 19 ss prop. It's on a Grady White Tournament 223. Boat weighs a hefty 3600 lbs. Rather than spend big bucks on a new ss prop or trying to get the old one repaired, I'm thinking of replacing it with an aluminum prop. Should the size and pitch of the aluminum prop be the same as the ss. Thanks

 

[jestor68]

Re: what size aluminum prop

Solas aluminum props are highly regarded for their excellent performance ,and deliver the best bang for the buck.

Provided your motor was/is operating within it's recommended WOT rpm range, simply replacing it with a Solas Rubex 19" pitch will get the job done.

What sort of motor are you using?

 

[Johny25]

Re: what size aluminum prop

Stainless tends to run a higher RPM in the same pitch as aluminum. If you go to aluminum you may want to go down 1 pitch depending on your RPM readings. Need far more info than what you gave to be able to give good advice though.

 

[redlinj]

Re: what size aluminum prop

OK, lets see! On calm water, rpm's top out at 4350 and 30 mph on the gps with 2 people and half tank of gas. This is with the 14 x 19 ss prop. Today, I replaced the ss prop with an aluminum 17" pitch with a diameter of 15.25. The boat actually seem to run better, but as Lake Erie is running two and a half to 3 foot swells, I couldn't really test it. Maybe this evening if things calm down a bit.

 

[jestor68]

Re: what size aluminum prop

Stainless tends to run a higher RPM in the same pitch as aluminum.

You have that backwards. SS of the same size will run a little lower rpm due to it's greater efficiency.

This thread is a waste of time until we know what sort of power plant we're talking about.

 

[Johny25]

Re: what size aluminum prop

You have that backwards. SS of the same size will run a little lower rpm due to it's greater efficiency.

This thread is a waste of time until we know what sort of power plant we're talking about.

No I do NOT have this backwards, stainless runs a higher RPM because it is more efficient. Rule of thumb is whatever you turn with aluminum you can go one pitch higher in stainless. STAINLESS WILL NOT RUN A LOWER RPM IN THE SAME PITCH

 

[jestor68]

Re: what size aluminum prop

No I do NOT have this backwards, stainless runs a higher RPM because it is more efficient. Rule of thumb is whatever you turn with aluminum you can go one pitch higher in stainless. STAINLESS WILL NOT RUN A LOWER RPM IN THE SAME PITCH

The recommendation IS to reduce 1 inch of pitch when switching from aluminum to stainless when dealing with props of similar design.

The 1 inch reduction is to partially compensate for the cupping on the stainless prop, which adds effective pitch. More aggressive cupping can add 2(or more) inches of effective pitch to a prop.

When switching from aluminum to stainless(for example; 17" pitch), the cupping on the stainless prop can cause it to act like an 18" or 19" pitch, loading the motor and dropping rpm. Stainless blades(being 5 times stronger) don't flex as much; maintaining a better "bite", which further places more load on the motor.

If you stop and think about it, it all makes sense.

Hopefully, we can get back on topic now.

 

[jestor68]

Re: what size aluminum prop

OK, lets see! On calm water, rpm's top out at 4350 and 30 mph on the gps with 2 people and half tank of gas. This is with the 14 x 19 ss prop. Today, I replaced the ss prop with an aluminum 17" pitch with a diameter of 15.25. The boat actually seem to run better, but as Lake Erie is running two and a half to 3 foot swells, I couldn't really test it. Maybe this evening if things calm down a bit.

Dropping to a 17 certainly sounds like a move in the right direction. But we really need to know what sort of power package you have.

4350 WOT rpm could mean vastly different things depending on the type of power plant.

 

[Johny25]

Re: what size aluminum prop

The recommendation IS to reduce 1 inch of pitch when switching from aluminum to stainless when dealing with props of similar design.

The 1 inch reduction is to partially compensate for the cupping on the stainless prop, which adds effective pitch. More aggressive cupping can add 2(or more) inches of effective pitch to a prop.

When switching from aluminum to stainless(for example; 17" pitch), the cupping on the stainless prop can cause it to act like an 18" or 19" pitch, loading the motor and dropping rpm. Stainless blades(being 5 times stronger) don't flex as much; maintaining a better "bite", which further places more load on the motor.

If you stop and think about it, it all makes sense.

Hopefully, we can get back on topic now.

I think you need to stop and think about it because there are far more dynamics at work than you seem to comprehend. What you are not and DO NOT account for is the efficiency of stainless. Actually you are making my point without knowing it lol. Stainless has thinner blades, smoother blade surface causing less drag, better cupping (as you stated) causing less slip which INCREASES the speed of the boat HENCE the higher RPM. Now obviously each manufacturer has different blade designs, cupping, rake, blade thickness....etc. But a comparable aluminum to a stainless the SS with turn a higher RPM in the same pitch. And I know this from first hand test data I have collected running aluminums vs stainless.
This is not a given though, depending on hull design, weight being pushed, horsepower of the motor, quality of the aluminum and or stainless, manufacturer and on and on. There are tons of variables that can change this but apples to apples the stainless will run a higher RPM pitch for pitch do to it's efficiency thank you very much. Now we can get back on topic

 

[jestor68]

Re: what size aluminum prop

If you google"effect of rpm when changing from aluminum to stainless", you'll find lots of discussions. 100% of the "experts" recommend dropping one inch of pitch to maintain rpm. Invariably, there seems to always be a couple of folks who "heard" it was the other way around.

 

[Tail_Gunner]

Re: what size aluminum prop

I think you need to stop and think about it because there are far more dynamics at work than you seem to comprehend. What you are not and DO NOT account for is the efficiency of stainless. Actually you are making my point without knowing it lol. Stainless has thinner blades, smoother blade surface causing less drag, better cupping (as you stated) causing less slip which INCREASES the speed of the boat HENCE the higher RPM. Now obviously each manufacturer has different blade designs, cupping, rake, blade thickness....etc. But a comparable aluminum to a stainless the SS with turn a higher RPM in the same pitch. And I know this from first hand test data I have collected running aluminums vs stainless.
This is not a given though, depending on hull design, weight being pushed, horsepower of the motor, quality of the aluminum and or stainless, manufacturer and on and on. There are tons of variables that can change this but apples to apples the stainless will run a higher RPM pitch for pitch do to it's efficiency thank you very much. Now we can get back on topic [/QUO

Johnny your backing yourself into a corner. Jestor is correct in stating SS in the same pitch will lower rpm by approx 200 rpm. The dynamic's are simple...SS allows for more complex blade design..big words huh..Its called rake and cupping. Have you ever seen a highly raked alum prop...The blades would deform quite quickly...



http://www.boattest.com/resources/view_news.aspx?newsid=4226

While boaters wanting top speeds usually select stainless steel over aluminum, industry experts tell us that the material of the prop accounts for only about 10% of the prop's performance- the other 90% comes from the prop's geometry

 

[Johny25]

Re: what size aluminum prop

Thanks for the link but it provides NO evidence to support what your claiming. I am backed into no corner FYI. I can produce just as much evidence to support my theory as yours and probably more. I am speaking from first hand user experience also, not merely a web page that shows the dynamics of propellers. I have read and studied all that info many times before thank you. Reminds me of the kid that gets out of college and thinks he knows everything because he read a book, then when he tries to take it into the real world he realizes he doesn't know jack squat. Sounds great on paper but real life is far different.
Your best argument is to imply we are talking about double cupped or aggressive high performance props which we are not.

 

[jestor68]

Re: what size aluminum prop

Thanks for the link but it provides NO evidence to support what your claiming. I am backed into no corner FYI. I can produce just as much evidence to support my theory as yours and probably more. I am speaking from first hand user experience also, not merely a web page that shows the dynamics of propellers. I have read and studied all that info many times before thank you. Reminds me of the kid that gets out of college and thinks he knows everything because he read a book, then when he tries to take it into the real world he realizes he doesn't know jack squat. Sounds great on paper but real life is far different.
Your best argument is to imply we are talking about double cupped or aggressive high performance props which we are not.

Can we give this a rest. If the OP puts a 17" on there, should be fine.

 

[Johny25]

Re: what size aluminum prop

Retired service manager? :facepalm: When I need to learn how to file receipts away I will look you up

 

[Tail_Gunner]

Re: what size aluminum prop

Retired service manager? :facepalm: When I need to learn how to file receipts away I will look you up

John start a new thread and let this one go . The OP has asked a serious question.

 

[redlinj]

Re: what size aluminum prop

Since my last post I have still not been able to get back out on Lake Erie to do a WOT test run with the 15.25 x 17 aluminum prop due to rough seas. The engine in the boat is a mercruiser 485. RPM range is supposed to be 4200-4600. Compression is pretty even give or take 5 psi at 155. The boat topped out at 31 mph (gps) on calm seas with the 14 x 19 ss prop. Now according to the Mercruiser 485 power package installation manual (which came with the boat) I should be running a 14 x 19 ss prop which I had been til I bent it up. The replacement in aluminum is stated to be 15.5 x 19----which I don't have. Thats why I put on the 15.25 x 17(and also to see if I could raise the rpms up a little.) Not to pose another question, but I read somewhere that diameter doesn't have that much affect. true/false?

 

[Johny25]

Re: what size aluminum prop

true..... as long as it is not a huge difference in diameter.