High rpm, low speed, checked hub

[leakytarp]

Hi all, I just joined the forum and am relatively new to boats, just bought a used 2006 Tahoe Q4 with a Mercruiser MCM 3.0 I/O. I know it?s a cheapo boat but I like it and I don?t want to get into debt. That said, the RPM?s seem to be way too high for relatively slow speed, to reach a decent speed approx. 25 MPH it revs up to 6000 rpm so I checked the specifications and on the engine it says WOT rpm range should be 4400 ? 4800. I Hope I didn?t ruin anything by running it on such high rpm?s. At low speeds RPM it?s not noticable but sometimes the needle jumps. Based on research on this forum I checked for a spun hub on the prop but it looked ok, no damage is visible on the inside where the aft drive adapter goes but I am not sure about the outside of the hub as I did not manage to get the hub out of the prop. I tried to spin the prop by hand when it?s in gear with engine off to check for a spun hub but it doesn?t move, indicating that the hub is fine.
Prop also does not seem bent or damaged and appears to be 21 pitch.
Any ideas what else could cause the high RPM?s?

 

[alldodge]

Hi all, I just joined the forum and am relatively new to boats, just bought a used 2006 Tahoe Q4 with a Mercruiser MCM 3.0 I/O. I know it’s a cheapo boat but I like it and I don’t want to get into debt. That said, the RPM’s seem to be way too high for relatively slow speed, to reach a decent speed approx. 25 MPH it revs up to 6000 rpm so I checked the specifications and on the engine it says WOT rpm range should be 4400 – 4800. I Hope I didn’t ruin anything by running it on such high rpm’s. At low speeds RPM it’s not noticable but sometimes the needle jumps. Based on research on this forum I checked for a spun hub on the prop but it looked ok, no damage is visible on the inside where the aft drive adapter goes but I am not sure about the outside of the hub as I did not manage to get the hub out of the prop. I tried to spin the prop by hand when it’s in gear with engine off to check for a spun hub but it doesn’t move, indicating that the hub is fine.
Prop also does not seem bent or damaged and appears to be 21 pitch.
Any ideas what else could cause the high RPM’s?

Howdy

If the boat starts speeding up and goes a bit faster as the throttle moves forward, you probably don't have a spun prop. At 6000rpm your engine would be coming unglued. I think your tach is bad

 

[steelespike]

Agreed, you need to verify your tach.Borrow a shop tach or get an induction tach.
If a hub slips its usually a total failure it will fail at a certain power and gradually get worse.
It is possible the motor coupler is slipping Usually it smells like burning rubber.
You may want to confirm the prop size.

 

[leakytarp]

Ok thanks - I don't smell burned rubber, I guess it could be the tach but I feel like it used to go faster the first few days I had it. Now it doesn't go faster than around 25-30 mph with the rpm's and sound of the engine being very high. I am still wondering if maybe after all it is the hub because I remember recently wondering if I hit a sandbank or not, the prop still looked fine so it is hard to tell but the paint was off at the tips - could also just be from it sitting in the water for weeks now. Maybe it's just minor damage to the hub only allowing it to slip at high RPM's? The problem is that I haven't had the boat long enough to be able to tell what it should feel like when everything is right. I think I will just change the hub (if I somehow manage to get it out of the prop) and see if that makes a difference to narrow it down.

 

[steelespike]

Some hubs are fitted to the prop intended to be removed by hand.Others are pressed into the prop.
I suppose if the removable hub was damaged it might still work enough to turn the prop.
If the boat has been in for weeks perhaps marine growth has fouled the prop or the hull.Maybe you could borrow a prop from
someone.If the the 3.0 is healthy a 21" prop could produce speeds around 40 with a sluggish hole shot
and poor performance with a load.

 

[wahlejim]

I agree with all of the above. Your tach is way off. It sounds like you are overloaded for your prop as well. How many people are on board? How much gear? Also keep in mind that just because the boat came with that prop, it doesn't mean that it is right for you. Possibly consider a new prop. Try this prop selctor:

http://legacy.qsprops.com/prop-selector/#

Quicksilver and mercury props are essentially the same thing. Plug in your boat info, your typical usage, and see what is available in blade numbers, size, and pitch. If you know anyone running a mercruiser hub on a prob that you can borrow, even better! Try some things out and pick what works for you.

 

[leakytarp]

Thanks for all the suggestions, I will look into it this weekend.

I agree with all of the above. Your tach is way off. It sounds like you are overloaded for your prop as well. How many people are on board? How much gear? Also keep in mind that just because the boat came with that prop, it doesn't mean that it is right for you. Possibly consider a new prop. Try this prop selctor:

http://legacy.qsprops.com/prop-selector/#

Quicksilver and mercury props are essentially the same thing. Plug in your boat info, your typical usage, and see what is available in blade numbers, size, and pitch. If you know anyone running a mercruiser hub on a prob that you can borrow, even better! Try some things out and pick what works for you.

Don't think we are overloaded, it's just my wife and I and the required safety gear. Maybe it's waterlogged but I don't think so, it's more of a feeling that any additional power is not delivered fully to the prop after reaching a certain speed and RPM

 

[leakytarp]

Ok it is definately a faulty tach. Managed to hammer the hub out of the prop and it still looked perfectly fine, replaced it anyway. And the speed issue is resolved after checking with gps on flat water everything is fine, it was probably just too choppy last time with headwinds where i felt i wasnt moving.
The tach is going crazy up and down sometimes, will see if i can calibrate it or if i need to replace it. Thanks again for the responses.