Looking for ideal WOT RPM on 1984. 2.5 L OMC

peteyjarns

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Sep 4, 2017
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First time boat owner, not much experience but am learning fast. So we recently bought a used boat...Starcraft cruiser with OMC sterndrive. So far things have run pretty smooth, haven't had to replace much at all, and fortunately was able to handle it all myself.

Being a first time boat owner, I dinged the prop up pretty bad on about the 4th or 5th time we had it out. Long story short I got flustered by the dock for a moment and the prop hit the pvc guide next to the dock. I know I know, dumb. So now I need a new prop.

The prop that the guy had on the boat is a 13.5" 15p three blade solas. I looked up the number stamped on it and found out its this one here: http://boatpropellers.iboats.com/Solas-Amita-Propellers/2411-135-15/

Now, I don't know anything about boats much, but I've read online that prop matters in terms of lots of things...speed, plane on, hole shoot, etc. So, i have no idea if this is the "right" prop for the boat. I do know the guy who sold me the boat seemed like he knew a thing or two, but not everything, as already some of the stuff he told me I have since found out is inaccurate. But this guy told me "I bought the boat off some kid that didn't know anything about boats, it wasn't running right, I did all this stuff to fix it up, still wasn't running right, and later found out he had the wrong prop on it. I put that prop on it and it runs a lot better know."

So, no clue if that guy made the right call in switching the props. No clue what made him decide that the old prop was wrong or that the new Solas prop was right. But that's what he said.

What I do know is:
  • the boat usually runs at about 3800-3900 RPM when the throttle lever is pushed all the way forward.
  • the boat was struggling to get past 3000 RPM after I buzzed it on the PVC pipe in the lake
  • engine model is OMC 262STHRCRA
What I would like to know is what is the RPM that this engine should run at WOT while in the water? I've searched and searched, and I haven't found anything. A guy at West Marine in town said I should "really figure out that math" and encouraged me to determine the WOT RPM that I'm shooting for before buying another propeller. Right now, I'm considering having the Solas 13.5" 15p reconditioned. After I determine the ideal RPM, I will then either get another new propeller with the same specifications, or get a different prop with the "right" dimensions and use the refurbed Solas as a back up.

I have some pics of the engine attached

Any tips or information would be greatly appreciated.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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4600RPM @ WOT
if your motor cant spin the 15p prop, your motor is sick and needs to be corrected

check compression
check fuel to make sure you dont have crud in the fuel
check to make sure that the throttle plates are opening fully (OMC bell crank linkage at carb/manifold often needs new bushings)
check dwell and timing
check anti-siphon valve

PS, you need 3 posts under your belt before you can attach photos, so a few more replies and attach away
 

southkogs

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Is THIS the same boat we're talking about here?

Before you ground up the propeller, how did the boat do? What kind of cruise speed could you get out of it? What kind of RPM?
 

peteyjarns

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Is THIS the same boat we're talking about here?

YES, it is...my wife posted while we were at the store and didn't tell me about it until I already had made this posting...

Before you ground up the propeller, how did the boat do? What kind of cruise speed could you get out of it? What kind of RPM?

I would usually get around 3800 RPM...usually top speed was 29 mph (measured via two different phone GPS programs, both matched)
 

Bondo

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Ayuh,.... Welcome Aboard,..... Like Scott says,... If ya prop for 4600 rpms at Wot, with a light load, you'll be Just Right,....
 

peteyjarns

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Ayuh,.... Welcome Aboard,..... Like Scott says,... If ya prop for 4600 rpms at Wot, with a light load, you'll be Just Right,....

thanks... OK so when you say "prop for 4600 rpms at WOT" ... what exactly do you mean. I'm taking it to mean, you put on a propeller that gets the engine on YOUR boat moving at 4600 rpms...

Scott above says to check a few things. I think I can figure out the fuel and the throttle plate...No idea how to check dwell/timing, no idea how to check compression, no idea how to check anti-siphon valve...(are you noticing a theme?)

i will look into all those checks, and I will also try and figure out how to check the plugs and make sure all are in good shape.

thank you guys for the tips, i've been enjoying learning more about engines as a result of owning the boat
 

southkogs

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I would have expected that drive to have a 16p prop (14" diameter) - not that my expectation would mean that much, rather you've got a prop you should be able to get the high RPMs out of. You should be getting 4600 RPM with that prop. Something isn't right from the engine side right now.

If you can have the current prop reconditioned, I would use that prop to figure out if the engine is running right.
 

Rinker85

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Aug 29, 2010
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I have the same engine on a 1985 Rinker v170. I have the same prop as you. 13.5 x 15. When I first got the boat in 1992 I tried about 3 different sizes until I found the right one. I do remember calling the Rinker boat company and they recommended the 13.5 x 15. My top speed is 32 mph. I can't tell you what my RPM's are. My tachometer does not work right now. Getting new gauges soon.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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. I think I can figure out the fuel and the throttle plate...No idea how to check dwell/timing, no idea how to check compression, no idea how to check anti-siphon valve...(are you noticing a theme?)

either learn to do engine maintenance and troubleshooting, or pay someone to do engine maintenance and troubleshooting.

you have a points style ignition. you will need to have access to a dwell meter and a timing light. if you dont own one, you can borrow one from the auto parts store

same goes with a compression gauge

the answers you seek are in the stickies

http://forums.iboats.com/forum/engi...ion-for-diyers-an-work-safe-search-version-of

http://forums.iboats.com/forum/engi...6-list-of-possible-causes-of-low-wot?t=295070
 

Silvertip

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Sep 22, 2003
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28,758
Do not spend money on props until you verify the engine is "tuned" properly. That means fuel pump pressure is correct, carburetor is clean and properly adjusted, ignition timing and dwell (point gap) is properly set, and that the engine itself is "healthy". Healthy means not worn out. An engine that has everything set properly cannot perform unless the internals are in good shape. A compression test proves it is or isn't. You can rent a compression gauge, timing light/dwell meter, and other special tools from some auto stores. Some are even free. A little on-line study will teach you how to do it. As for timing and dwell, you set dwell first, then timing. All the other stuff can be done in any order but prove one system is operating before messing with another. Use a "logical approach" to troubleshooting.
 
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