How will this change affect RPM's

[CJY]

I currently have an aluminum 13 3/8 x 17 prop on my boat. If I switch to a 12 3/4 x 21 stainless, do you have any thoughts on how this will affect the RPM's at WOT?

Any thoughts in general on such a change? Is this a drastic change?

Thanks,

CJY

 

[Bondo]

Re: How will this change affect RPM's

Is this a drastic change?

Ayuh,........ Sure Is.......

The rest of your question is another,...Depends,......
On the present WOT,..... What the motor,+ drive are,..... Etc.....

 

[CJY]

Re: How will this change affect RPM's

Thanks Bond-o,

It's a 1989 Johnson 70 HP. The current RPM at WOT is around 5700 with the 17 pitch prop.

I am due to have the prop reworked or buy another. If I buy another, I will most likely go with stainless and I want to have a good understanding of how pitch and diameter affects RPM's @ WOT and engine performance in general.

I think I read somewhere that pitch affects RPM's by 400 for each degree? I would have to look it up again though. I am absolutely clueless regarding blade dia. and how it affects rpm's.

Thanks

 

[Bondo]

Re: How will this change affect RPM's

Diameter is almost a moot point,.....

Pitch,+ Blade Design are What Counts.........

Usually an inch of pitch is 200rpms at wot,......
But,..... Available Horsepower comes into play with that # too.....

I imagine that the SSteel prop you're looking at would Lug your motor down Way Too Low...........

 

[Silvertip]

Re: How will this change affect RPM's

Pitch is stated in "inches" not "degrees".

 

[kp913]

Re: How will this change affect RPM's

For each 1" of pitch you increase, you will lose approx. 200 RPM, in turn for each 1" of pitch you decrease, you will gain approx. 200 RPM. Diameter is a bit different, I think only about 100 RPM gain or loss for each 1" of diameter.

 

[kp913]

Re: How will this change affect RPM's

inches, degree's.....who's counting?

It is referred to as inches, but it is actually "degrees" of pitch.

 

[CJY]

Re: How will this change affect RPM's

Based on the info so far, can I pick a prop for a faster top speed? Hole shot? etc? How would I go about this?

Or, is this something done by trial and error?

If I stick to a 13 3/8 X 17, but go to stainless rather alum, are there considerations I should make before doing so? Will it change performance?

I apologize for what may be ignorant Q's, but I am attempting to become an informed consumer and I appreciate all of your help. Thanks

 

[Bondo]

Re: How will this change affect RPM's

inches, degree's.....who's counting?

It is referred to as inches, but it is actually "degrees" of pitch.

Ahhhhhhh................ No it Ain't..............

It's Inches of Travel, without Slippage, Per Revolution.............

 

[kenmyfam]

Re: How will this change affect RPM's

inches, degree's.....who's counting?

It is referred to as inches, but it is actually "degrees" of pitch.

Ahhhhhhh................ No it Ain't..............

It's Inches of Travel, without Slippage, Per Revolution.............

And I will second that statement.

 

[walleyehed]

Re: How will this change affect RPM's

Yes, it IS in inches...theoretical fwd movement....in inches....
As for the 70HP at 5700, I wouldn't change a thing.
If you go with SS, you may have to raise the engine a hole...BUT, if the prop has better over-all lift, it may provide less wetted area, giving more speed and RPM.