prop change question

[ewolf]

I have a Caravelle 188 with a 4.3 l mercruiser. It has a13.75 inch 21 pitch aluminum prop. It runs 44mph @4500 rpm. I want to change the prop to a SS 14.25 inch three blade. Should I go to a 20 pitch because of the diameter difference? A normal prop change in the same diameter will raise my rpm's 150 rpm which should be OK but what will the diameter change do to the formula? Will a 14.25 inch 20 pitch prop maintain the same speed as before but just help the hole shot?

 

[Chris1956]

Re: prop change question

I would stick with the 21P prop. Are you adding some rake? Consider a sternlifter type prop, as boats of your type will have a heavy stern.

 

[steelespike]

Re: prop change question

Is your prop old with plenty of wear? a Solas Amita or Rubex in the same size might improve performance.
They have a state of the art process.
There are others with better technical information.
But but to answer your question; speaking generally:: As you decrease pitch diameter goes up.
A 20 inch prop should behave like a 20" prop.A 1/2" diameter increase might might lower rpm 50.
But I'm thinking the ss prop will more than make up for any diameter change.
Prop calculator indicates about 20% slip.I believe the ss prop should improve slip.
Your looking for about 4800 max rpm I think the 20" might get it. Speed change may be a wash with a better hole shot
I did a little calculator check.
If, and it is a big if,A 20" ss prop can make 4800 with 10% slip you might see 50 mph.
Pure speculation.

 

[tpenfield]

Re: prop change question

Lots of variables to consider, so it is hard to say what the outcome will be

Changing material, which changes the blade thickness . . .
Changing pitch, which will change RPM & loading. . .
Changing diameter, which will also change RPM and loading

A 14.25 x 20 P prop does move a slight bit more water per revolution than a 13.75 x 21 P all else being equal . . . So, you might see a slight reduction in RPM with the bigger/lower pitched prop, but it should be real close.

To know for sure, you will have to try it.

 

[ewolf]

Re: prop change question

Thanks guys. All great points. This is much better than going to a salesman and have him sell what he only has on the shelf, if you know what I mean...

 

[ewolf]

Re: prop change question

With all of this discussion, should I consider a 14.25 inch 19 pitch instead of a 20p in order to get the rpm's up closer to the 4800rpm max?

 

[steelespike]

Re: prop change question

I think if you go down too far you could lose speed.

 

[Levinz11]

Re: prop change question

I would get a 4 blade 19" Solas prop. It really gets my boat out of the hole well, lower planing speed, better speed at a given RPM, etc. Just my .02. If top speed is your concern stick with a 3 blade. Good luck.