Over revving...spun prop?

[dmanmtl]

Took my 2005 Bayliner 175 out for a spin today. Engine ran great however I found that the boat was not as fast as it used to be - couldn;t seem to break 40mph and the tach was reading above 5k rpms...I know that these tachs aren't terribly reliable but last year at about 4800 RPM I could easily get the speed up over 40.

A guy back at the dock suggested that maybe my prop was slipping - I think this is called a spun prop...

Aany suggestions on what I should check. Same prop as last year, 3.0l mercruiser. New leg oil, engine oil, spark plugs, clean gas (super).

D

 

[alldodge]

Start by removing the prop and look at the center rubber section. If its slipping you should find a crack in a circular pattern. If that's not it, there could be the engine coupler.

Are you smelling anything like burnt rubber?

 

[dmanmtl]

Start by removing the prop and look at the center rubber section. If its slipping you should find a crack in a circular pattern. If that's not it, there could be the engine coupler.

Are you smelling anything like burnt rubber?

Thanks for the reply - no smell of burnt rubber at all. As for the coupler...I didn't thing it could slip...is it like a clutch? I thought it was bolted direct but made of rubber just to protect from "shocks" between the engine and drive..

 

[Bondo]

no smell of burnt rubber at all. As for the coupler...I didn't thing it could slip...is it like a clutch? I thought it was bolted direct but made of rubber just to protect from "shocks" between the engine and drive..

Ayuh,.... It's built just like the ole style prop hubs, rubber insulated break-away,....

But,.... If either one of 'em Slips, it's Beat,....

A prop hub slippin' won't lower yer speed abit, nor raise the rpms abit,...
It'll be Radical, ya loose all speed above idle, 'n the motor redlines goin' 4 mph, not 35 mph,....

If yer lookin' at the boat's Tach, 'n the boat's Speedometer, I wouldn't believe either one of 'em,....

Cross-check the rpms with a known good shop tach, 'n check the speed via Gps,....

 

[dmanmtl]

Thanks for your replies. I had a close look at the prop last night (boat is in the driveway) and it seems that I may not have tightened the prop bolt enough. There is some play in the prop - about a quarter inch back and forth (I call is sloppy), obviously I can fix this but am now wondering if this could have cause my problem. My top speed was only off by 4 or 5MPH, could this cause the prop to somehow be out of balance and over spinning inefficiently?

D

 

[Bondo]

There is some play in the prop - about a quarter inch back and forth

Ayuh,.... That's normal,......

If top end is only off 5 mph, I'd be lookin' at motor tune, rather than prop/ drive slippage,....

If a prop, or drive, or coupler goes out, ya pretty much loose all thrust beyond idle speeds,....

 

[dmanmtl]

Ok, so a slipping drive coupler would probably not let me run 38MPH? It's more the high revs that worry me, but the tach may be funky, no access to a shop tach though at this point...

Thanks again for your response!

D

 

[thumpar]

It is probably the tach reading wrong. With a coupler or prop spun I highly doubt you would get above 10mph and your RPMs would be hitting the rev limiter. Also don't trust the dream-o-meter on the dash for speed.

 

[dmanmtl]

Thanks guys - My speed is based on iPhone GPS apps (Navionics - amazing app) and the speedo is pretty close to that too.

Appreciate your replies - pretty glad I don;t have an engine coupler issue, from the sounds of it.

D