Engine Performance
 

I have 1 2012 Mercury 115 Four Stroke. I noticed over the summer that my engine was losing MPH. I was running 6000 RPM and only going about 19 MPH. Took it to shop and they got it up to about 24 MPH. I know that in previous seasons it would run much more efficiently. Could this be from a fouled spark plug? I am confident that running the engine at 6000 can not be good. Any advice would be appreciated. Thanks!

Re: Engine Performance
 

Something is not making sense here.. You said before you took it to have it looked at you were making 6000 rpms and going 19 mph and when you got it back you were making 6000 rpm and making 24 mph... If your rpms are reading correctly, and you have it on the same boat, the only way that would be possible is if they did something with the props since 6000 rpms with the same prop/same boat should give you the same mph. Did they change the prop or perhaps re hub it?

And, it looks like the Wide Open Throttle rating for this particular engine is 5800- 6400 rpms, since you are in that range, that would indicate that the engine is performing as it should. If there was something wrong with the engine it you wouldn't be able to achieve those RPMS..

Need more info here. something is not adding up....

Re: Engine Performance
 

Yes something is amiss in the information.

If it still puts out 6000rpms but moves through the water slower it's NOT the motor per se. It may be a slipping hub on the lower unit. It may be trim. It also may be bottom growth (commonly called a beard) on the hull creating drag.

Re: Engine Performance
 

Agreed Frank... If however he had bottom growth, I doubt the engine would turn 6000 RPMS.... The motor would likely just lug and not get up to the rated RPM. I've seen boats with growth that wouldn't even get up on plane...

Re: Engine Performance
 

True if its bogged down in displacement plowing water it would not make top WOT. But if it is doing almost 20 knots so it's on plane.

We are also assuming 6000rpm was WOT. If it were 6400-6500 and now dropped 500rpms on WOT. Maybe not.

I think we agree that something changed in prop efficiency or the hydro-dynamic load/drag is different? The power head is fine.

And most people power wash the boat when hauled. That could be his speed difference.

Re: Engine Performance
 

Thanks for getting back to me. We did check the bottom and it was as clean as a whistle. The mechanic did suggest a new prop but I couldn't see any damage to it. One year ago we had the prop looked at. I don't know, maybe I'm just being over-cautious because I have had problems with the engine. Power head was replaced two years ago and had to have a water separator installed. It's been a problem of one kind or another every year. Just trying to avoid another catastrophe.

Re: Engine Performance
 

I have to agree with the others. If you are still getting 6000 rpm it has to be in the prop. slippage most likely. When my Mercs slow down it is either fuel line restriction thru the external filter or bottom crap. But with both of these the rpms drop. Good luck. It sounds like something simple. Give Jack at Almars in DE a call. They are the best around in Mercury. That were I get all my work done. 2hr drive but its done right every time

Re: Engine Performance
 

Thanks for your help

Re: Engine Performance
 

As suggested above, the first thing I would check for is a spun hub. If you're not familiar what a prop hub is, it's a insert in the prop. When there's an impact the hub breaks free (or spins) within in the prop so the sudden impact of something stopping the prop doesn't ruin the prop shaft and or your gears...

The symptoms of a spun hub is an engine that races but does not get up to speed... The hub breaks free and spins within the prop instead of spinning the prop. It's kind of like spinning your car tires on ice... The engine will race, but you won't go anywhere...

This video will help to explain https://www.youtube.com/watch?v=d7Lz7cI-srQ

I were you, I would take the prop off the engine and take it to a reputable prop shop and have them see if the hub is spun... If it is they can re hub it for you and it will likely be around $100 or less to re hub it.

If that's not it, you have something else going on.

Re: Engine Performance
 

Gerry,

I'd vote elsewhere given his symptoms. Hub speed this consistent?

The more times it spins free of engine-to-prop the less friction the hub has left for the next time it happens. And progressively slippage gets worse.

And during a hub slippage it emulates cavitation where prop thrust speed would drop off to near 0. He's maintains consistently 3 knots slower?

Could it be as simple as his GPS was knots v mph on the read out? Perhaps.

I think we both agree it's not the powerhead.