Question about smaller diameter prop

[svkb]

Hi all,

I currently have a 15.25 x 15 pitch prop on my 3.0 (it has the wrong gears) and I can't reach proper rpm. By my calculation I need to go down to a 13 pitch. Problem is it seems I can't find a 13 pitch prop without dropping to a 14" diameter prop. Will the decrease in diameter cause me a lot of grief?

I should add that my current prop has been repaired and it doesn't measure 15.25 diameter anymore, it's probably more like 15.5 or 14.75 diameter.

I understand the situation is far from desireable given the wrong gear ratio, but it's what I've got to work with at this point.

My local "parts guy" said that the decrease in diameter would cause cavitation, but I was under the impression that the faster the prop turns, the smaller the diameter should be (within reason of course).

Will a 1" decrease really cause that much problem?

 

[rpatton]

Re: Question about smaller diameter prop

I have the same issue as you with the gear ratio being wrong. I put a 800 series on my 3.0 so my ratio is also off. I started with a 19 pitch but only had 2200 rpms then switched to a 15 1/2 x 13 and it is better but am only at about 3200. I am considering repitching to a 11 pitch but am afraid of breakaga of a blade plus am lossin way to much speed so mighr hust leave mine alone.

 

[svkb]

Re: Question about smaller diameter prop

With the 15 pitch prop I turn 3900 - 4000 rpm (confirmed with a timing light) and I'm getting 34 mph (gps confirmed). I figured a 13 pitch prop would bring me right into the optimal range. Problem is I can't find a 13 pitch without going down a bit to a 14 diameter. Hoping someone here can give me some idea of what to expect.

 

[dazk14]

Re: Question about smaller diameter prop

What boat is this running on and what is the rpm range.

The standard formula is 1" decrease in diameter will raise rpms by 400.

What 13" prop are you looking at? If the boat is in the 18' or less category, 14" diameter should work fine, especially on the top end.

 

[svkb]

Re: Question about smaller diameter prop

What boat is this running on and what is the rpm range.

The standard formula is 1" decrease in pitch will raise rpms by 400.

What 13" prop are you looking at? If the boat is in the 18' or less category, 14" diameter should work fine, especially on the top end.

My apologies for the lack of info, the boat is a 1991 searay 170 bow rider with a 3.0 litre mercruiser. WOT is 4,200 - 4,600. I haven't looked at any prop specifically, I just figured that given my rpms I would need to drop at least 2 inches to get with spec.

The hull is just shy of 18'.

 

[jestor68]

Re: Question about smaller diameter prop

If you check the prop section at iboats, you will find a Solas Amita(aluminum) 15.5 X 13 for $95.36. That will increase your rpm's by 300-400 and give better all around performance.

They are other brands on the page of like size, but the Solas is the best bang for the buck.

The rule of thumb is 150-200 rpm per inch of pitch; not 400.

 

[dazk14]

Re: Question about smaller diameter prop

If you check the prop section at iboats, you will find a Solas Amita(aluminum) 15.5 X 13 for $95.36. That will increase your rpm's by 300-400 and give better all around performance.

They are other brands on the page of like size, but the Solas is the best bang for the buck.

The rule of thumb is 150-200 rpm per inch of pitch; not 400.

Who said 400rpm per inch of pitch?

Oops! I guess I did. It's 400rpm per inch of diameter. I fixed my above post.

Good catch jester!