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Power Loss at High RPM's

J

John Haggerty

Contributing Member
1989 70hp Johnson

Other than this problem, the motor runs fine. On rough water at about 3000 -3400 rpm the boat looses thrust power and the rpms increase to about 4000. When I reduce the rpms I feel a click in the controls at about 3000-2800 rpm. After the click I can throttle back up with power until around 3200 rpm when again it will rev back up to 4000 rpm with a reduction in thrust. What's happening?

Thanks,
John
 
Sounds like the sheer-pin (aka key) is gone. Sometimes there is enough friction to keep the prop spinning to a certain RPM and then after that it spins on the shaft.

Assuming it is a prop. If it is a jet unit, then maybe the jet foot is coming out of the water?
 
To check the propeller hub for a slipping problem.... remove the prop-nut and spacer momentarily, mark the hub and the propeller in a straight line with a magic marker.

Run the rig in the water to encounter that problem. After the problem occurs, once again remove the prop-nut and spacer to observe that line you drew. If the line on the propeller has moved away from the same line on the hub, then obviously the hub is slipping and the propeller will require re-hubbing.

If the line is unbroken, the propeller is okay... and since this problem only occurs in rough water, I'd suspect that you have the engine trimmed too high... OR... the engine is mounted too high on the transom.
 
I marked the center of the bushing to prop with a line. I had the problem occur again yesterday. I just removed the nut and spacer and the lines still align. Motor is running fine and strong with the exception on the problem. Yesterday the water was calm the the trim was where I've been running it for the last 26 years. I don't mess with the trim/tilt much with the exception of shallow water. The boat has a "trim sweet spot" that I normally run in with no problems. Typical symptoms; on plane at approximately 3400 rpm, lightly give it more throttle and it revs to 4000 with no power increase.

Any more ideas?

Thanks,
John
 
Since you've been running the same rig combination for 26 years... and the propeller has checked out fine (no slipping), and this takes place on both choppy and calm waters... That would lead me into the boat/hull area.

Excess water trapped in the false bottom of the hull?

Something new added to the bottom of the hull that's interfering with the water flow into the propeller?

Something old, attached to the hull, changing in some manner to interfere with the water flow into the propeller?

A trailer roller, even if adjusted properly, over the years finally causing a hook in the hull?... Raises the stern as speed increases!
 
The prop isn't dinged up a bunch to where it causes cavitation?


You can kind of see where the suggestions are leading. If the prop or the shaft is not slipping, it points to some sort of cavitation or keel issue that is creating air pockets around the prop.

Did you recently change props? Is the pitch the same as your old ones? To wrong pitch on the wrong boat can cause issues like cavitation.
 
I asked my brother who is a lot more knowledgeable about this. Here's his theory:

Well, its actually more serious then he makes it out to be. I believe he has the shift control cable not fully engaging the clutch dog in the gearcase. That click is when it reengages but when more thrust is put on it there is a sort of a neutral that happens and though the prop may still be spinning it has no go to it.
If it runs to long like this then the gears will need replacing because they will get rounded off where they match the clutch dog.
If he can take the hood off and force it to stay in forward by grabbing the shift mechanism and holding it forward, in gear, he will get his answer.
As long as the thrust washer is on the prop shaft prior to the prop and everything else looks good, which it sounds like he has checked it then inward you go to the gears. It may be the shift mechanism/cable adjustment is the root of the problem as well. If it isn't "held" into gear firmly it will eventually start to slip.
Think of the clutch dog like a sliding bushing on a shaft that when it moves forward you go forward by engaging that gear. Both the forward gear and the reverse gear spin via a pinion gear all the time but the connection to the prop shaft is through the clutch dog that slides either forward or backward on the prop shaft. If the shifter cable is not adjusted to travel to the full forward position, eventually it will just slip under high power and when you back down, it reengages.
 
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