What prop pitch?

[Fishingbud]

I have a 1989 70hp merc on a smoker craft Fazer 17.5ft. The boat came with a 21p but is this the best pitch for the boat. The RPM's seem to be a little low at WOT. It does jump out of the water good and my speed is around 33 to 35. Any suggestions.

 

[mpsyamaha]

Re: What prop pitch?

i would think a 17p would be about right, but without knowing your rpms now, its hard to say.

 

[Fishingbud]

Re: What prop pitch?

Thanks for the info. I think the RPM,s were about 46-4800

 

[trendsetter240]

Re: What prop pitch?

Thanks for the info. I think the RPM,s were about 46-4800

21p seems far to much to me. Without knowing your gear ratio, recommended rpm range, current prop material (alum. or stainless), number of blades on prop and average loaded boat weight it's only a wild guess.

If I were to take a wild guess I'd say a 17p would be much closer to what you want than a 21p.

My advice is to provide as much information as you can to get some feedback on here before buying a prop.


Cheers~!

 

[hwsiii]

Re: What prop pitch?

Bud, I show your motor to have a 2.3:1 gear ratio and a WOT range of 4,750 to 5,250 RPM, please verify this.

If this is the case, and you are doing 34 MPH at 4,700 RPM I show you to have a prop slip of 16%. If you will change to a 19" pitch prop in the same make and model you have right now your RPM will be right at maximum, where they should be.



H

 

[Fishingbud]

Re: What prop pitch?

Again thanks for the information. Going to a 19p is what i was thinking. The prop thats on now is a aluminum prop whis 3 blades which has been rebuilt. The part number for the prop is 48 77348 A40. What is a acceptable amount of prop slip??? Is 16% good?

 

[hwsiii]

Re: What prop pitch?

Bud, You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

My recommendations are that when you get to about 15% prop slip you should consider changing to a different prop design, but by the numbers you gave me they appeared to be best guess and not exact numbers with exact speed by GPS, since you appear to be going to buy a new prop it would be to your advantage, in my opinion, if you first took the boat out and got me some exact numbers, so we can do it right and see if maybe there is a Better prop design for your boat, as Trend stated.



H

 

[Fishingbud]

Re: What prop pitch?

When the Ice is off the lakes I will come back with more information. I am going to buy a 19p in the same modle and try it out. The prop that I have is in bad shape so I am not sure about the numbers. Thanks again!!!!!!!!!

 

[hwsiii]

Re: What prop pitch?

Just let me know when you are ready.



H