Prop vs. RPM

[Jer from CA]

I have a '88 OMC 4.3l with OMC outdrive. Recently replaced prop with OEM 15X17. Boat is slower then before. WOT rpm is 3200 @ 35 mph. Boat is 18' open bow. What should the WOT rpm be? Would pitch improve performance and speed?

 

[Wingnutt]

Re: Prop vs. RPM

Sounds like you are over propped. I'm not 100% sure about OMC, but on my 88 Regal, the 4.3 is supposed to run between 4600 & 4800 RPM at WOT. I'm not sure what your boat weighs (mine weighs 2300 lbs dry) and at WOT (which I rarely do) spining a 14.5x20 pitch 4 blade prop I'm turning 4850 RPM @ 40mph. Your stern drive must be geared way higher than a Merc because @ 3200 RPM (my cruse setting) I'm only running at 28 MPH. When I had a 17 pitch prop on I had a great hole shot but no top end due to over revving (32 mph or so @ 4800 RPM at just over 3/4 throttle)). She would only run 20-22 MPH @ 3200 RPM. See if you can borrow a lower pitched prop and give that a try.

 

[tylerin]

Re: Prop vs. RPM

I have 87 4.3 2barrel carb w/ same exact prop (15x17) and I get 4400-4500 rpms more if you have 4barrel. I have a 17 1/2 ft boat. something else appears to be wrong

 

[Jer from CA]

Re: Prop vs. RPM

Do you think that an adjustment to the throttle cable might be the answer.

 

[ufm82]

Re: Prop vs. RPM

No way that boat is overpropped. A 15X17 prop on a 4.3 should have no problem attaining normal WOT of 4500-4800 rpm. It shouldn't be that hard. if you are only getting 3200 rpm, you need to look elsewhere. <br /><br /> You said you just replaced the prop. What was the old prop? What were your numbers previously? If they were correct, why did you change props? Damage or wanting more out of the boat?<br /><br /> I would think that you have a engine concern more than a prop concern. Throttle linkage possibly but I rarely see a big problem there if the carb or linkage has not been removed at some time. It can't get that far out all by itself. You may want to look at ignition system to see if you are getting proper advance at WOT. I would look beyond the prop. Just my thoughts.<br /><br />UFM82

 

[Dhadley]

Re: Prop vs. RPM

Well, what prop were you running before?<br /><br />"Boat is slower than before" -- before what? Just the prop change or was something else changed?

 

[Jer from CA]

Re: Prop vs. RPM

I replaced the prop due to damage, but not enough to think anything would be wrong with gearcase. The old prop was OEM 15X17. The new prop is Michigan Wheel. I ran the boat most of the day. It was very smooth, just did not perform like it did with the other prop. Could the brand of prop make the difference. I really think the engine runs good.

 

[andrewgroup]

Re: Prop vs. RPM

I have a 1990 18' Mariah (looks like SeaRays) with 4.3L OMC V6, 2450 lbs, I had a 15 x 19 3 blade factory aluminum prop. AT 4500 rpm, I had 42.5 MPH on GPS. on smooth water and only me in the boat and the trim adjusted about as good as it could feel.<br /><br />I had the same PROP repitched to 18" to improve low end ski pulling and I lost 2 MPH on the top end at 4500 rpm.<br /><br />Is your tilt working properly and can go down to it's lowest position?<br /><br />Here goes the formula for all of this stuff to calculate slip and speed..<br /><br />Here goes the calculator. <br />· Pitch in Feet 17" pitch / 12" per foot = 1.42 <br />· x RPM at prop = 2738 (you calculate prop speed based on gear ratio)<br />· = Speed in feet per minute = 3888 <br />· x 60 (60 minutes per hour) x 60 233280 <br />· / 5280 feet (number of feet per mile) / 5280 <br />· = Theoretical Speed = 44.18 MPH <br />· x .82 (average slip = 18%) x .82 <br />· = Probable Speed = 36.22 MPH<br /><br />I have this all in an Excel Spreadsheet for anyone that cares.

 

[Dhadley]

Re: Prop vs. RPM

The Michigan prop will not perform like the OEM prop.