Spun Prop, Cavitation, Aeration or Engine Coupler

[ronboy109]

Hi

Boat is a 3.0 mercruiser, alpha 1 gen 2 leg.


Out on the boat in a new piece of water the other day and hit a rocky promontory going very slow, 3 or 4 knots max, no burning rubber smell, prop very bent for about 1/2 in backwards ath the edge of each blade.

Boat seemed okay in the low speed area, but later started to act like a spun prop, (not gaining speed under increased throttle) digging in very deep at the stern (4 people on Board)

I had a spare (not sure of the internal condition but badly pitted and lost some material from the edge) so replaced it by the water side, boat seemed fine after this, i.e. easily planed achieved 25 mph (2 people on board).

Left the boat in the water over night and had a bbq, to celebrate my manly exploits.

Next morning in the boat, acting like spun prop (this time on the replacement prop) not going above 7 knots, and digging in very deep at the stern(4 People on board).

Took the prop off for a look, retightened it went for a short test run... went up to 25 knots no problem and planed (2 people on Board).

shook my head puzzled and trailered the boat...

Got home, put the boat in gear, and hand turned the prop with the boat in gear (with a lot of effort), the coupler and shaft seem to move together with no deflection, replacement prop is bad (large chips (1 cm by 3 cm) and bad orange peel pitting other prop is wrecked.

no nasty noises coming form the top or bottom of the leg.

Anyone have any ideas??

 

[Rick Stephens]

Get a new prop. Pull the outdrive and inspect everything. Go boating.

 

[Bondo]

Got home, put the boat in gear, and hand turned the prop with the boat in gear (with a lot of effort),the coupler and shaft seem to move together with no deflection,

Ayuh,..... Welcome Aboard,...... You turned the motor over, by hand from the prop,..?? with the spark plugs in the motor,..??

 

[ronboy109]

Hi

yes, a lot of effort required, but yes...I am a big fella. Compression was tested recently (last week) and is fine (>120 in each cylinder)

 

[thumpar]

Hard to believe the engine turned over from the prop considering the engine turns 2x for every prop rotation with the ratio.

 

[Bondo]

Ayuh,.... I wouldn't think it possible,.....

Get a new prop, 'n try it,....

How weedy was the water you were in,..??
Weeds can load up the prop, 'n do that,....
A bust of reverse usually clears the weeds,....

 

[ronboy109]

Hard to believe the engine turned over from the prop considering the engine turns 2x for every prop rotation with the ratio.

I am a big fella, can bench a lot... I have no reason to lie... I am here looking for help.

 

[ronboy109]

Ayuh,.... I wouldn't think it possible,.....

Get a new prop, 'n try it,....

Weeds can load up the prop, 'n do that,....

Its a deep weed free loch (except where the outcrop was)

 

[thumpar]

I am a big fella, can bench a lot... I have no reason to lie... I am here looking for help.

I was not suggesting you were lying. I was wondering if maybe the coupler was spinning or hub spinning.

 

[TurtleTamer]

Each time you couldn't plane, you had four people on board but when you could plane, you only had two. I'm assuming that you were reaching higher RPMs (not lugging) since you suspect something is spun? Could very well be, considering you're a bigger person, that adding two more people put the boat at that threshold where you get more bow lift at the hole shot and it stresses the bad component too much, where only two people didn't.

Since it did this with both props it sounds more like the coupler.

I'd check for a bent prop shaft as well.

 

[ronboy109]

I marked the coupler when I turned the prop and there didn't seem to be movement, but the coupler and shaft rotated the expected amount.....

I have a new prop coming tomorrow, I'll try that... if it fails to rectify I guess the boat is destined for the great Loch in the sky.

 

[Rick Stephens]

I marked the coupler when I turned the prop and there didn't seem to be movement, but the coupler and shaft rotated the expected amount.....

I have a new prop coming tomorrow, I'll try that... if it fails to rectify I guess the boat is destined for the great Loch in the sky.

Sounds very dramatic anyway - "Found out I have a bad clutch in my car, off to the junk yard with it." What happened to figure out what's wrong and see if it's fixable at a bearable cost?

 

[TurtleTamer]

I would check the mark again after testing it out with the new prop. We could debate all day as to whether or not one would be able to get the engine to turn from turning the prop by hand, but I don't think anyone would argue that one could reproduce the stresses on the driveline created by a hole shot, by hand. Hopefully it works out either way.

 

[ronboy109]

Sounds very dramatic anyway - "Found out I have a bad clutch in my car, off to the junk yard with it." What happened to figure out what's wrong and see if it's fixable at a bearable cost?

Its almost impossible to get merc parts in the UK at a reasonable cost, the boat is old and not terribly great anyways.

 

[Maclin]

I may not understand exactly how you marked the engine coupler or even what you were watching for movement, but if the engine coupler has spun inside the rubber it might move with the shaft from the outdrive when you turn the prop, but will probably not turn the engine. And just thinking about the mechanical (un)leverage at play, moving the prop as means to turn the engine, seems like a good way to overstress the prop hub and even the coupler if either is about to let go.

It seems a new prop is worth a gamble just to see if that is what the boat wants Good luck!

 

[ronboy109]

the whole engine seemed to turn, I marked the crank pulley too and it moved the same amount as the coupler/shaft