Engine Performance - NJFishing.com Your Best Online Source for Fishing Information in New Jersey

Down The Hatch · #1 11-24-2018, 08:23 AM

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!

Gerry Zagorski · #2 11-26-2018, 07:55 AM

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....

Capt. Debbie · #3 11-26-2018, 10:35 AM

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.

Quote:

Originally Posted by Gerry Zagorski

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....

Gerry Zagorski · #4 11-26-2018, 11:54 AM

Quote:

Originally Posted by Capt. Frank

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.

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...

Capt. Debbie · #5 11-27-2018, 10:18 AM

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.

Quote:

Originally Posted by Gerry Zagorski

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...

Down The Hatch · #6 11-27-2018, 05:25 PM

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.