Slipping 1978 115 hp Johnson

[cjhols]

Hi all,
I have a 1978 115 hp Johnson that seems to slip under load (towing - I can't think of the "C" word for the process.) and you have to drop off the revs to pick up speed. Anyway the engine seems to run fine but at WOT it is doing roughly 80 km (50mph) at 6500 rpm. This is to high rpm isn't it. The prop is 17'. Should I put a higher prop on it to bring down the WOT rpm?? and also increase the potential pulling power without it (That C word again). What should the WOT RPM be on the old girl???
Regards
Peter

 

[SkiDad]

The prob hub might be spun. You take the prop off. Mark the back with a stait line through the hub and rubber part. Then look at it after you return from a short run. If mark is not lined up that is your problem. I think 5500 is about what you want. If the prop comes back good maybe your engine is mounted too high ? Or just the wrong pitch for your boat.

 

[cjhols]

Ok Thanks. Will have a look at it. Thank You

 

[steelespike]

The C word your looking for is "cavitation" In the modern vernacular Cavitation refers to the erosion on the back of the blades in the low pressure area.
The low pressure lets the water boil eventually scaring the prop.You don't feel cavitation.
What ypou may have is the spun hub referred to or "ventilation".With ventilation the prop gets fouled with air losing its grip on the water.
Could ba a number of problems;poor prop condition or design,a poor setup motor too high or even possibly too low.excessive trimming.

 

[steelespike]

Your numbers don't add up. What year, brand,size and dry weight is your boat?
My feeling is your tach and speedometer are both off.Your speed needs to be by a gps only accurate speed,even then you need to be aware of any currents
wind or tides.To make 50 mph you would need over 7000 rpm.Your hp is developed at 5000. If your motor could make 7000 it wouldn't be for long.
A 17 or 19" pitch is pretty typical for a 115.
We need your lightly loaded wot rpm and gps speed,boat size and weight,remind us of your prop size.

 

[Chrisravosa36]

Your numbers don't add up. What year, brand,size and dry weight is your boat?
To make 50 mph you would need over 7000 rpm.Your hp is developed at 5000. If your motor could make 7000 it wouldn't be for long.

Did not know this... Is this just a guideline? is it always true? I heard of people running 4500revs on a tinny blastin 60+mph?

 

[steelespike]

Did not know this... Is this just a guideline? is it always true? I heard of people running 4500revs on a tinny blastin 60+mph?

I think that first of all when talking all out speed you need to be suspicious unless you run it yourself with correct rpm and gps speed.
If you read the stickys at the top of the prop page it may help you understand prop selection better.
Marine motors are working very hard .Picture a car pulling a trailer up a never ending hill. Prop selection is like trying to pick one gear to do everything.
Your motor needs to operate within its "wide open throttle"rpm range Will say 4500-5500. The right prop will keep it within that wot range.
Pitch on a prop is the" theoretical "distance it moves in one revolution a 17" (higher gear)would go farther than a 12"(lower gear). But
the 12 has more power.For the health of the motor and best performance motor needs to operate within its wot rpm range.The 12" would
over rev badly on a boat that works with say a 17" prop.
Once established you can operate at what ever throttle setting is appropriate for the conditions.Higher pitch (higher numbers) will be harder
for the motor to turn but will be faster if it can reach wot rpm.Higher pitch has less power like a high gear in a car.
Lower pitch will have more power but less speed because more like low gear in the car.
The 4500 you mentioned is close to what an I/O might run at 4800. A 18 ft 4.3 I/O with about a 23" prop could make "about" 50 mph
4.3 usually have about 190 to 220 hp.4400-4800 wot rpm. The results depend on the condition, load,prop etc.
A healthy 115 could make 50 on the right boat right setup.
There is a special modified 20 hp motor the runs over 40 on a utube video.