2003 johnson 115hp 4 stroke prop question

jojo1975

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I have a 1984 sea nymph 19.5 feet aluminum hull 1800pounds I have just put this engine on this past week and tried it over the week-end.

The engine came with a 14inch 17pitch prop I get out of the water like a bad out of hell. and du 37mph at 5800rpm. trottle is only 3/4 in I have lots of play on the trottle but I am already att the max safe RPM range.


I was wondering if I can go with a 14.5inch 21pitch to get the same speed and be att a lower rpm. or sould I go with something else?


any suggestions?
 

steelespike

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Calculator says with a 2.59 gear ratio you can't do 37 at 5800. Is your speed by gps?
 

jojo1975

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GPS was showing 35 dash gauge 37 mph...
I removed the prop and numbers on it shows 14-17
 

jojo1975

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RPM was showing 5800 I beleive that is the max recommended on these engine ?
 

steelespike

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Prop size,gear ratio,gps speed should be accurate the tach and slip numbers are harder to come by.
Most boats have slip in the low double digits. 5800 @ 35 mph calculates at 2%.Highly unlikely this seems to indicate the tach is off low.
If we guess 10% slip we get actual rpm about 6250.Keeping in mind 10% a guess.
The chart on the 2003 - 2005 Johnson 4 strokes indicates 5000-6000 is the rpm range.
I think a 19" pitch should reduce rpm 300 to 400.
A little slower hole shot, "perhaps" a little more speed.
 

jojo1975

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thanks for the info.
so I should only go 14-19 and not 14-21.


I am happy with a speed of 35mph not looking for more speed, but I am looking to reduce the rpm att that speed. And like I said, the take off right now is really fast , so, loosing a bit there is not a problem. if I want to do some wake or ski I can always used my 14-17 but when I go cruising I would prefer lower RPM.
 

steelespike

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thanks for the info.
so I should only go 14-19 and not 14-21.


I am happy with a speed of 35mph not looking for more speed, but I am looking to reduce the rpm att that speed. And like I said, the take off right now is really fast , so, loosing a bit there is not a problem. if I want to do some wake or ski I can always used my 14-17 but when I go cruising I would prefer lower RPM.
Marine applications are not like automotive applications.Your motor needs to be propped to rev freely within its rpm range.
Forcing the motor to a lower rpm with a prop actually may br harder for the motor.Like going up hill in overdrive towing a trailer.
Use the 19 and lower the rpm with the throttle.
 

jojo1975

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one more question I am not able to get WOT right now I am reaching my max RPM at 2/3 3/4 in there is still lots of travel before I get to the wide open throttle WOT.

is that ok because all my previous 2stroke engine I was able to go WOT and not excide the RPM max.... are 4 stroke different in that sense?
 

steelespike

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one more question I am not able to get WOT right now I am reaching my max RPM at 2/3 3/4 in there is still lots of travel before I get to the wide open throttle WOT.

is that ok because all my previous 2stroke engine I was able to go WOT and not excide the RPM max.... are 4 stroke different in that sense…
This was very important info and I think the 21" pitch made sense.Apparently I lost contact with the partial open throttle
I'm a little surprised the 21 gives good performance on a almost 20 ft boat.It must be the 2.59 gear ratio.
The typical Johnson/Evinrude 115 2 stroke uses a 2.00 gear ratio. and usually run a 17 or 19" prop
2.59 makes it easier to spin the prop so obviously the 21 suits it well.
 
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