Propellor seems to be slipping?

[cocobolo]

2007 Suzuki 25hp o/b. Until a short while ago it was great. Now it seems to lack drive when I use the throttle.
What happens is this. I turn the throttle to about 3/4, the motor revs up fine, but seems like it is cavitating. If I leave the engine speed up - maybe even a little over 3/4 throttle, the overspeed protection kicks in and I have to go back to idle again.
Previously, 3/4 throttle would drive the 15 1/2 foot f/g skiff at 19 mph on the gps. It barely makes 1/2 that now. The engine seems like it still has the power, but something seems to be slipping.
I have spoken with the Suzi mechanic in town, and I asked him if there is some sort of clutch mechanism that would allow mechanical slippage - that's what it sounds like to me - but he says no. He figured a prop problem.
I have just put a brand new prop on and no improvement at all.
Then I go on the forum here and I find posts about a clutch, which leads me to believe there might be such a beast.
Any help anyone can offer will be so hugely appreciated you can't even imagine it!
I have not bought a manual yet, but looks like I should do that PDQ.
Thank you.

 

[hawkf250]

Re: Propellor seems to be slipping?

I dont think it would be the prop if you replaced it with a new one and are seeing the same symptoms. Typically if it were a clutch dog in the lower unit you would feel it start to power up then feel like the motor lost power suddenly the feel a thud like you hit something and feel the boat start to go againg, this would repeat rapidly. if you want to eliminate the prop, use a grease pen and put a line from the hub to the prop, take it out for a ride, then check the prop to see if the line moved

 

[GoogieMan]

Re: Propellor seems to be slipping?

quick question? is the leg low enough in the water? IE the cavition plate level to the keel?

 

[sturdavj]

Re: Propellor seems to be slipping?

You say a new prop, was the problem there before the new prop was installed, was the replacement the same pitch as the old one ?

Do you have a manual, if so what pitch does your motor call for and are you in that range. You may also want to put a mini tack on your motor and check your RPM's.

Check with fellows on the prop forum,

 

[cocobolo]

Re: Propellor seems to be slipping?

I dont think it would be the prop if you replaced it with a new one and are seeing the same symptoms. Typically if it were a clutch dog in the lower unit you would feel it start to power up then feel like the motor lost power suddenly the feel a thud like you hit something and feel the boat start to go againg, this would repeat rapidly. if you want to eliminate the prop, use a grease pen and put a line from the hub to the prop, take it out for a ride, then check the prop to see if the line moved

I agree with you...I don't believe it is the prop now either.

And it doesn't feel like a thud. The motor will speed up normally, as it has always done, and the boat will accelerate slowly to about half speed. Which is about 9 - 10 mph on the gps. It feels like the prop isn't grabbing the water very well. Either that or some sort of mechanical slippage.

I will have another check today and see if I can feel that thud you speak of, and I will try the grease pencil trick.

Many thanks for this.

 

[cocobolo]

Re: Propellor seems to be slipping?

quick question? is the leg low enough in the water? IE the cavition plate level to the keel?

This is one thing we have thought of. There has been no change at all in the engine mounting, and no alteration of the adjustment pin - it is set in the lowest hole.

It worked perfectly for about 2 1/2 years, but the problem has developed over the past several months. The boat wasn't used much this spring due to the inclement weather. We use it to make a 4 mile trip across to Vancouver Island from where we live. That's about all the use it gets.

Thank you.

 

[cocobolo]

Re: Propellor seems to be slipping?

You say a new prop, was the problem there before the new prop was installed, was the replacement the same pitch as the old one ?

Do you have a manual, if so what pitch does your motor call for and are you in that range. You may also want to put a mini tack on your motor and check your RPM's.

Check with fellows on the prop forum,

With the original prop, it was a 10 5/8 x 11, this was the factory installation. I cannot see any blade damage on this prop, but it is going in to the shop to be checked next time we go over to town.

There are two factory props listed, and the new one I have is 10 3/8 x 12. Either one is recommended by the manufacturer. They are both Suzuki props.

The problem started 4 or 5 months ago, although I never noticed it as much as I do now. We often use the boat to move building materials over here, and the boat is only driven slowly then. So, I wouldn't notice the problem I don't think.

Your idea of a tach is definitely a good one and I will try that. It almost sounds to me as though the engine isn't reaching its' proper rpm level at higher throttle. But then, I have always thought the engine sounded as though it ran slowly. Yet it always had plenty of power to get the boat up on plane quickly, and maintain around 19 mph at 3/4 throttle or so.

We have tried to use a process of elimination to figure the problem out, and about all that we are left with is some sort of internal mechanical problem.