Question about changing prop diameter

[Mike Robinson]

Hello All

I have a 19 foot welded aluminum boat with a '94 johnson 150 and am currently using a 3 bladed aluminum 15 by 15 prop which runs WOT at 5200 rpm at 32 knots.

Where I live is fairly remote and I would like to buy a stainless prop through the internet. Are there guidelines I can use to tell my what pitch I would need in, for example, a 14 inch stainless prop that would give me 5600 rpms? Is there a formula one can use for predicting how a change in diameter will affect WOT RPM?

Thanks in advance for your feed back!

 

[Ron G]

Re: Question about changing prop diameter

i i would really think you would be turning alot more rpms with that 15 then you are!are you sure your tacs right and it is a 15p prop?

 

[Mike Robinson]

Re: Question about changing prop diameter

Thanks for your response Ron

I've been told that before. A mechanic said I should be able to "spin the paint off" a 15" prop but I'm pretty sure my tach is accurate and it's definitely a 15 by 15 prop. I did a prop slip calculation a while ago and it seemed OK. (I don't remember what the slippage was)

A couple of things. I did do a compression check a while ago and only got around 65 to 70 psi on all 6 cylinders. I did a decarb and it didn't change anything significantly. Also when accelerating the engine jumps from about 1500 to 2500 without much movement of the throttle.

Thanks!

 

[Plainsman]

Re: Question about changing prop diameter

I'm no expert, but recall reading that compressoin should be above 100 and within 10% of each other. Might want to post the compression in your motors repair forum to get the absolute correct info.

Eric

 

[walleyehed]

Re: Question about changing prop diameter

65-70lbs on all 6 isn't good, but we don't know if the gauge is accurate..
As for your speed, 32kts (36.8mph) figures out to just over 9% slip...I would say that's extremely good for aluminum, if that's what you have. SS, it would be about where it should be if set-up is correct.
I do question tach accuracy though, and yet if the compression is really what you came up with, I'd say the engine is about done...

 

[Crownie2]

Re: Question about changing prop diameter

Mike - a decrease in diameter, which is probably what you will be doing, will probably increase RPM's but as far as I know there is no way to calculate this.

Bob