"Prop-er" etiquette?

[rbser]

Can someone explain the relationship of diameter change AND pitch change for a boat running same weight and HP? like how will the RPM's change??

I always remembered it to be an increase of 200 RPM for each inch LESS pitch for same prop material..

AND I remembered it to be a decrease of about 200 RPM for each inch INCREASE in diameter for same prop material

I can't find a prop calculator that adds diameter into the mix of calculations...

I also know that when you change from aluminium to stainless, you generally will DECREASE pitch... is there a specific amount as a guideline (like 2" LESS pitch over the aluminium prop of same diameter)

This stuff is confusing enough without adding another variable to the equation...

HELP??

Rich

 

[tpenfield]

Re: "Prop-er" etiquette?

Diameter and pitch combine to determine how much water the propeller will try to move per revolution. I have not seen a calculator that takes both into considerations either . . .

I did make my own a while back using a spreadsheet, etc. It basically compares props by the volume of water that they try to move.

The RPM rules of thumb are just that, rules of thumb, and good for estimating the impact of small changes in diameter or pitch. But like you said when you start changing both, things get confusing.

 

[rbser]

Re: "Prop-er" etiquette?

I am going to see what a 1/2" increase in diameter and a 2" decrease in pitch AND a switch from aluminium to stainless steel will do for my 16' long 800 Lb. 1983 Baja with a 70 HP Evinrude on it...

Should prove interesting...

Rich

 

[steelespike]

Re: "Prop-er" etiquette?

If you check the diameter of props basically as pitch goes up diameter goes down. There are exceptions probanly props designed for a specific application ie: pontoon boats.
Adding a blade generally reduces diameter and it does all seem logical.
It does seem logical that a inch difference in diameter may affect rpm 1/2" not so much. Extremes in diameter are usually aimed at a specific application,wake boarding, houseboat power, extremes in hull design or performance criteria.
In order to tell the affect you will need to make the changes one at a time whose to know if there isn't some subtle design tweak by
Joe Blow prop company isn't the difference. Prop design is kind of like a private secret recipe, it's different but how do you tell why.

 

[Frank Acampora]

Re: "Prop-er" etiquette?

Well. it is your engine and you can do what you want. However, with one prop replacement you are changing three variables. Thus, you can not tell what effect each variable has on performance.

If you are serious, you must compare apples to apples by changing only one variable at a time.

 

[tpenfield]

Re: "Prop-er" etiquette?

If you check the diameter of props basically as pitch goes up diameter goes down. There are exceptions probanly props designed for a specific application ie: pontoon boats.

My 'theory' on that is that the propeller manufacturer is trying to stay within a certain range for the volume of water the prop will try to move per revolution. (say that 3 times fast) They *probably* do this to stay within a certain intended power range, i.e. application.

 

[emilsr]

Re: "Prop-er" etiquette?

If it were only that simple.....

Blade shape, blade geometry, rake angle, cup....there are a myriad of other variables in the props themselves. Pitch should be considered a primary variable and diameter is more secondary in nature.

Change "material" to "design AND material" and you've got the pitch rule correct. Just bear in mind it's a general rule of thumb and can vary somewhat.

You're backwards on the aluminum/stainless formula; stainless blades are thinner and therefore have less drag and will turn more rpm.....assuming the design is EXACTLY the same.

Different prop designs have different diameters, so you can't really assume that a diameter change isn't going to change some other parameter. Your boat and the intended use will determine which prop designs (and therefore diameters) are appropriate. After that it's pretty easy to determine which pitch to run. I've never found a "prop selector" that was worth using; there's better ways to pick wheels.

This is already hard. Don't make it harder.