Prop pitch

[Dbuesing]

Hi guys I have a 95 omc cobra 3.0 gs with cobra outdrive. I'm wondering what pitch prop I should be running? My mechanic says that the 21 pitch that I'm running now is way to big for my motor to be turning. He recommended a 17 pitch but the prop shop I talked to said 17 would be too much rpm and they recommended a 19 pitch? Any feedback would be appreciated

 

[lynnboy021]

Re: Prop pitch

Lol I am in the same boat as you lol

 

[bruceb58]

Re: Prop pitch

Depends on the weight/size of your boat which you haven't mentioned yet.

What RPM are you spinning at WOT right now?

Welcome to iBoats.

 

[Dbuesing]

Re: Prop pitch

17.5 ft four winns freedom. Weight is around 2070 lbs! Not sure on rpm

 

[Bt Doctur]

Re: Prop pitch

Your Wide Open Throttle rpm reading is what determines your prop pitch.

 

[Howard Sterndrive]

Re: Prop pitch

Not sure on rpm

go for a run and see. If between 4200 and 4600 the prop you have is perfect.

 

[Starcraft5834]

Re: Prop pitch

if the prop you have takes you forever to get out of the hole.. (meaning get up on plane) within a reasonable amount of time...ie- when you punch it with a normal load.. say two people and it takes more than 5 seconds to get up on plane, you might want to consider less pitch..than 21. then again, that might be just right for you... heavy boats benefit by use of 4 blade compared to 3..more thrust power..

 

[jestor68]

Re: Prop pitch

It probably came new with a 21", but 19 years later I doubt the motor can handle that any more.

My guess is for a 19", but the only way to determine what pitch is correct is a test run with a light load.

The WOT rpm should be near the top of the recommended rpm range in order to handle the extra load of a few more friends along.

 

[Dbuesing]

Re: Prop pitch

Thanks guys! Big help. I'm taking it to get de- winterized this weekend and I will run it and check rpm's! Thanks again

 

[fishrdan]

Re: Prop pitch

I will run it and check rpm's! Thanks again

Also, get the boat's speed at WOT in case you need prop recommendations. Use a GPS cell phone app to get the speed as it's more accurate than a boat speedo, I'm using ulyssespeedometer on my android.

 

[Lager]

Re: Prop pitch

An accurate tachometer is necessary, My original was not.
You can also use your cars GPS if you have one and just run it on battery power when you do your top speed runs.
Hope this helps.

 

[steelespike]

Re: Prop pitch

Just to refine this we need your prop size, lightly loaded wot rpm and gps speed to be able to evaluate your setup.

 

[Dbuesing]

Re: Prop pitch

Okay guys so I took it for a run yesterday and wot rpm's were around 3800 at a speed of 36-37mph. That was with me and another adult and a 6 year old in the boat

 

[SkiDad]

Re: Prop pitch

Okay guys so I took it for a run yesterday and wot rpm's were around 3800 at a speed of 36-37mph. That was with me and another adult and a 6 year old in the boat

you definitely want to go down to a 17 pitch for sure - that should get you to the recommended 4600 rpm

 

[Howard Sterndrive]

Re: Prop pitch

yep, or you might like an 18P 4 Blade.

 

[Maclin]

Re: Prop pitch

How accurate was the speed measurement. I show a 4% slip assuming a 2.0 drive ratio with your RPM's and MPH, that is near perfect no slip.

Just saying, if the slip is more like 12-15% then a 19p gets you to 4300-4400 rpm. Then Howard's recommendation for an 18p 4 blade is in play as well.

 

[jestor68]

Re: Prop pitch

How accurate is your tachometer?

A 2.0 ratio and 21 pitch works out to 37 actual mph using a realistic 10% slip; which works out to 4123 rpm, and you're showing 3800.