Do I have a spun prop?

[Mordekai]

Boat: Tahoe Q4 w/ Merc Alpha 3.0

Ok basically I've just lost about 10mph on my boat @ WOT for no apparently and I can't figure it out for the life of me.

Last outting everything was all good and right now as I'm typing this, the boat will only do 30mph @ WOT instead of 40 like it's supposed to.

The boat is reaching the same RPM at WOT too, around 4800.

Everything else is the same with weight and fuel, 2 ppl on board.

If it's a matter of a wet foam, I don't think one outting could make the foam absorb that much water? The boat is trailered not slipped.
I've checked the engine, all plugs are in and connected.
The only thing I can think of at this point is that I'm starting to develope a spun hub?
At the beginning of this season, I changed the impeller and everything slid back in perfectly.

Help please!

 

[ericp501]

Re: Do I have a spun prop?

Does it feel slower or is that just what the gauge says?

 

[halfmoa]

Re: Do I have a spun prop?

Make a mark on your prop and a corresponding mark on the hub. Take the boat out. If they're out of alingment after you're back on the trailer you've got a spun prop.

 

[Mordekai]

Re: Do I have a spun prop?

Does it feel slower or is that just what the gauge says?

I'm going off the RPM gauge on the boat and the GPS on my phone also.

 

[Mordekai]

Re: Do I have a spun prop?

Make a mark on your prop and a corresponding mark on the hub. Take the boat out. If they're out of alingment after you're back on the trailer you've got a spun prop.

Good thinking,
I also got a spare prop that I can bring. Never tried switching props in the middle of the lake before.

 

[halfmoa]

Re: Do I have a spun prop?

Remember if the prop is a different pitch it'll skew your results.

Keep a tight hold on those parts if you're gonna do it on the water! Better yet, pull it on to the trailer and change it in the parking lot!

 

[Chris1956]

Re: Do I have a spun prop?

A spun prop will usually keep the boat from planing. Did you hit anything and damage the prop? That will cause the scenario you are seeing

 

[Mordekai]

Re: Do I have a spun prop?

I did hit something but that was last year and even then, everything was fine until now.
The boat is planing and is reaching speed 30mph but will not seems to go pass that.

Almost as if it's "slipping"
Is it possible to have a hub on the prop that's holding ok until a certain rpm?

 

[Chris1956]

Re: Do I have a spun prop?

It is doubtful the prop is slipping at 30 MPH, and not below that. A prop hub is a piece of hard rubber. Once it slips, it starts to slip worse and worse. if it is a bad prop hub, you will likely get stuck somewhere, real soon.

Maybe you are not trimming the motor like you did last year? Maybe a piece of prop fell off or bent more?

 

[jpo1953]

Re: Do I have a spun prop?

Did you have the prop re-worked? I had a similar problem and found that the shop I took my prop to shaved it down. I was losing top-end and the fuel mileage was very bad. Took the prop to a different shop and they showed me with a pemplate that the blades were 3/8" shorter.

 

[TyeeMan]

Re: Do I have a spun prop?

So if I'm reading you correctly, last year you could get about 40 MPH at WOT, , meaning about 4600-4800 rpm.
Now this year you can only get about 30 MPH at the same 4600-4800 rpm?
If that's the case I suppose your hub could be slipping. Could also be possibel that your coupler could be slipping, , not sure how to check that one.
And yeah as previously mentioned, if the guy didn't re-work your prop correctly that will throw you off too.

 

[WIMUSKY]

Re: Do I have a spun prop?

Make a mark on your prop and a corresponding mark on the hub. Take the boat out. If they're out of alingment after you're back on the trailer you've got a spun prop.

I think this is a great idea. Start with the easy stuff first and see where you're at...... If nothing else, it's one area you can eliminate if it's not slipping...