15 hp RMPs

A lil back ground on the motor. Its a Craigs list find, An Evinrude OB appears to be mid 80's model judging by the paint . no identifying marks , tag has been removed.
it is either a 15 or a 9.9 with a 15 carb. The carb is defiantly a 15hp carb . Prop is a 9.5 X9 as stamped on the hub. Long story short ,I cleaned it up as it had sat for a looong time. Re-surfaced the head , cleaned up the head and installed new gaskets , fuel lines, impeller, plugs ect ect.
It will according to my GPS it push the 14 ' flat bottom 22 to 23 mph. that's with me 230lbs batt, trolling motor , gear.

Got around to getting a tiny tach I set it at one pulse per revolution . It shows 820 rpms at idle and 6,680 rpms at WOT. Idle Sounds good by ear but at WOT that sucker sounds like she's gonna blow at any second.

Motor is clamped directly to transom. no jack plates , spacers or anything. anti -vent plate is about 1.75 in below keel of hull. Prop is in good shape , cleaned it up with a file a bit. but otherwise it ok.

Why is this motor spinning so many RPM's ?

That prop is way too small for your boat you should go up 1 or 2 in pitch this is what I had to do on my boat it's a 14ft jon boat v bow my original prop was 9x9 I went up to a 9.25x11 it made quite the difference my motor does sound like it ready to come apart any more. My motor is a 1973 Johnson 25HP.

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Rednecknproud, after some looking around I found from https://www.rubexprops.com/boat-propellers/rpm-range/?make=Evinrude

that the 15hp max rpm range from 1981 to 1993 is 5500 to 7000 I never thought they would turn that much stock.

I'm agreeing with ya and I have a new solars 9.25x10 that I will be putting on it . I had gotten that for my 1978 15 but it was too much and bogged it down.

I owned a 74 Johnson 15 and was told to run WOT @5500 and a max of 6K. Never heard of a 2S doing 7 K rpm on an outboard, but I may be wrong.

Pappy said:

7000RPM easily for these engines. (9.9-15)
The 9 pitch prop is holding you back. Go to at least the stock 10" pitch for the engine.

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Done Pappy , put on the 9.25 X 10 yesterday. I'll run it on Saturday morn before the storm front comes through . Just out of curiosity though what's the stock prop for a 1978 15 HP ?? that motor has a 9.5 X 8 Seems to run ok, not screaming like the other one. I wonder if the old coot that I got that one from turned down the WOT on it ? I'll have to check that out. compression is 120 & 121.

well , the 9.25 X 10 dropped the revs to 6,420 at WOT. Against the wind it was running 23 to 24 mph , with the wind 25mph. A bit scary in the turns on Mattaponi . Had to slow down a bit for most of them , pucker factor was up there..........

Charger25 said:

well , the 9.25 X 10 dropped the revs to 6,420 at WOT. Against the wind it was running 23 to 24 mph , with the wind 25mph. A bit scary in the turns on Mattaponi . Had to slow down a bit for most of them , pucker factor was up there..........

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I ran those figures past the prop calculator. 10% slip, 12/29 gear, 10p at 25 gives 7099 rpm.

I know mine at 7400 with 4b10p blade, 1232 jon will do 23.8 according to my neighbor's satellite speed controlled ski boat.

Thats .17 slip as my 1232 digs a big hole with my 210lbs fat a$$.

https://www.kirschbaum.at/ki_prop/pdf/2015/RPM%20Chart.pdf

I looked over this link quickly and only spotted one O/B that had a max RPM of 7K recommended was 5.5-7K 15 hp.

Be careful your not over revving your engines.

Crazyboat said:

https://www.kirschbaum.at/ki_prop/pdf/2015/RPM%20Chart.pdf

I looked over this link quickly and only spotted one O/B that had a max RPM of 7K recommended was 5.5-7K 15 hp.

Be careful your not over revving your engines.

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Yep , in my first post I stated that judging by the paint , it was a mid eighties Evinrude ,which according to your link is max at 7,000 RPM.

Sinkingfast said:

Charger25 said:

well , the 9.25 X 10 dropped the revs to 6,420 at WOT. Against the wind it was running 23 to 24 mph , with the wind 25mph. A bit scary in the turns on Mattaponi . Had to slow down a bit for most of them , pucker factor was up there..........

Click to expand...

I ran those figures past the prop calculator. 10% slip, 12/29 gear, 10p at 25 gives 7099 rpm.

I know mine at 7400 with 4b10p blade, 1232 jon will do 23.8 according to my neighbor's satellite speed controlled ski boat.

Thats .17 slip as my 1232 digs a big hole with my 210lbs fat a$$.

Click to expand...

Well , the tach is a tiny tach knock off set at one pulse per revolution.

the GPS is a 7yr. old TomTom.

The boat is a 1448 (or 42 can't remember) Fisher purchased at Walmart in 1997. Don't know the gage of the aluminum hull ,but its not the heaviest.

The prop is a new Solars 9.25 X10 ( ran once on the other 15hp rude)

The numbers I posted were indeed the ones I saw on the tach and the GPS.

The motor was trimmed out to the 2nd hole, boat was riding flat on top of the water. Very little wake and only a little spray off the sides.

The boat is still loaded as it was when it was in the water, I'll post pics of it , the motor and prop in the morning.

Here is the calculator I use for diagnostics.

https://www.rbbi.com/folders/prop/propcalc.htm

My 1992 9.9hp Johnson with a 15 hp carb runs perfect at 7000 rpms. I too was skeptical when I first saw the WOT range in the manual but I made some pretty long river runs and checked the powerhead temp with a digital IR thermometer and it always stayed within 130-140 degrees. The intake ports got a little hotter, but they also don't have a water jacket.

I run mine WOT most of the time because I have it on a relatively big boat (1648 flatbottom) so I just add a hair more oil and some Seafoam. I probably mix it at about 40:1.