how much is too much

[raevin51]

Hi all I amworking the kinks out of my 65 Cobia 20DB. Currently running a 15p prop on a Johnson 115 but wot is way out of whack, the rpms will climb well in excess of 8500 and I need to get it down to between 5500 and 6500generally what pitch prop do you recommend. I was thinking a 21-23p in either 3 or 4 blade. Any suggestions would really help.

 

[GrindKore]

Re: how much is too much

I would start with a cheap 19 pitch 3 blade aluminum and see the RPM at WOT, if you are still over-revving the motor go up in pitch.

 

[sprouticus]

Re: how much is too much

I'm sure you know this, but don't run your rpms that high anymore.

I had a 19p ss on a 90horse. I'm thinking a 21p 13.25" 3 blade ss prop is right for you.

 

[Irv964]

Re: how much is too much

I would verify your tach first, with those numbers I am surprised it didn't grenade on you already?

 

[steelespike]

Re: how much is too much

Something "is" screwy.I doubt you could find a prop to get the proper rpm.
Something like 9"would only get you to about 7,500.so your venting or have unbeliebable slip or an inaccurate tach.Is the prop in good shape?
Max rpm and gps speed would be very helpful.

 

[sprouticus]

Re: how much is too much

Something "is" screwy.I doubt you could find a prop to get the proper rpm.
Something like 9"would only get you to about 7,500.so your venting or have unbeliebable slip or an inaccurate tach.Is the prop in good shape?
Max rpm and gps speed would be very helpful.

I was a little surprised his engine didn't blow up at those rpm's, and I'm not an expert at the math involved with the slip, but I know a 15p will allow much higher rpm's than what he should be running. I'm temporarily running a 15p on my 90, but as soon as it's out of the water, I drop down to half throttle.

I'm smiling right now because my GF just got back from the post office with the new/used prop I got off of ebay. too bad the weekend is over.

 

[raevin51]

Re: how much is too much

I verified the tach, it's accurate, the gps speed 24. The rpms spiked out pretty fast and I throttled back faster than is humanly possible lol. fortunatly for a geezzer my reflexes are still pretty good. I pulled the prop this morning and even though I will not be useing it again I am having it checked just to satisfy my own curiosity, I think the hub may have spun on it, the rubber bushing looks kind of distorted.