Clanking/rattling sound after prop change

[Joshto]

Hi Folks,

I searched the forum briefly but couldn't find anything similar.

17' Cadorette (Thundecraft) BR Nuova 170 .
1998 Mercruiser 3.0 Alpha one Gen I (Replaced head gasket last year - ~8 hours of use)

A few days ago, I switched props from a 3-blade SS 14.25 x 19P to a 4-blade Aluminum 14.25 x 19P. I took the boat to the water as I wanted to measure the new WOT. Excellent handling and better hole shot. I was expecting the top-end would decrease but not a clanking/rattling sound from the engine.

Three people in the boat. The boat runs smoothly, goes on plane quickly (around 2800 rpm) and accelerates quickly too. Until it reaches about 3900 RPMs (~30 mph) when the engine starts making a clanking/rattling sound as if two metal pieces were banging into each other rapidly. With the 3-blade prop, I could do 36 mph at 4300 rpms without it making any abnormal sound.

If I slow down immediately when the noise begins, it disappears below 3800 rpms. Performance doesn't seem compromised, but it literally sounds like a rattling can of beans about to break apart. Although all gauges are normal, I didn't want to keep it at that speed. Nothing is visibly broken on the outside, nor are any fluids in the bilge. I can cruise at ~ 3800 without any noise.

Is this because of the prop change? I understand one needs to drop 1" of pitch when going from 3 to 4 blades, but shouldn't that only affect the revs (wot) and general performance (top speed)?

I am thinking of putting back the 3-blade prop and testing it again before taking apart the engine (head cover to start with).

Appreciate your input. Thanks.
Josh.

 

[flashback]

You might be hearing detonation, I agree, put the old prop back on

 

[Rick Stephens]

Pull your plugs and post pictures. If you are in detonation you are either running lean and/or timing is off. You will injure your motor with the SS prop as well, just won't hear as much.

 

[dingbat]

Could be prop chatter....normally happens going from aluminum to SS
What hub are you using with the prop?
Hub and prop nut torqued to specification?

 

[Joshto]

You might be hearing detonation, I agree, put the old prop back on

Thanks. Is it normal for that to happen after a prop change?

Pull your plugs and post pictures. If you are in detonation you are either running lean and/or timing is off. You will injure your motor with the SS prop as well, just won't hear as much.

Will take pictures next time I'm close to the storage. Timing has been unchanged since I bought the engine a few years ago. It has always performed well...at least, as you said, I never heard that sound before.

Could be prop chatter....normally happens going from aluminum to SS
What hub are you using with the prop?
Hub and prop nut torqued to specification?

I doubt it, the noise is definitely coming from the engine (it's crazy loud). The rubber hub (flow torq) is new. The SS has a pressed-in hub. Also, I have a 3-blade aluminum (same specs as the 4-blade); it runs great too without noises. The prop is installed and torqued to the specs.

 

[flashback]

I only mention detonation because the 4 blade at the same pitch as your 3 blade would put additional load on your motor and make it more obvious but not cause it.

 

[QBhoy]

I’d check the prop assembly again. Make sure the thrust washer is there

 

[dingbat]

I doubt it, the noise is definitely coming from the engine (it's crazy loud).

Prop chatter is generated in the lower unit. Resonates up the drive shaft to the motor

The rubber hub (flow torq) is new.

So was mine.
Ended up swapping it out for the Rubex to eliminate the noise

The SS has a pressed-in hub.

So was mine

Also, I have a 3-blade aluminum (same specs as the 4-blade); it runs great too without noises.

Same number of blades?
Same weight/inertia in prop shaft?
Same newly installed hub?

 

[Joshto]

I’d check the prop assembly again. Make sure the thrust washer is there

Thanks. It is there. I've replaced at least 10 propellers in the last few years. Thrust washer > hub > prop > adapter> tab washer > nut. Nut is tightened to the specs and tabs bent. The prop runs smoothly.

 

[Scott Danforth]

Two red flags.

Going from aluminum to stainless, you normally need to drop 1" in pitch. You kept it the same

Going from 3 blades to 4 you normally need to drop 1" in pitch. You kept it the same

To go from a 3 blade aluminum to a 4 blade stainless, you should be at 17p. Not the 19p you bought.

You are lugging your motor. That rattling is the motor detonating and eating itself

 

[Joshto]

Two red flags.

Going from aluminum to stainless, you normally need to drop 1" in pitch. You kept it the same

Going from 3 blades to 4 you normally need to drop 1" in pitch. You kept it the same

To go from a 3 blade aluminum to a 4 blade stainless, you should be at 17p. Not the 19p you bought.

You are lugging your motor. That rattling is the motor detonating and eating itself

Thanks. To clarify.

I went from SS 3 blade to Aluminum 4 blade. But I agree with you. I think the 4blade is lugging the engine. Never thought it could have such a big impact.

 

[Joshto]

Prop chatter is generated in the lower unit. Resonates up the drive shaft to the motor

So was mine.
Ended up swapping it out for the Rubex to eliminate the noise

So was mine

Same number of blades?
Same weight/inertia in prop shaft?
Same newly installed hub?

As much I would like that to be the problem, I pretty much sure the noise is from the engine, almost certain from starboard side. It's a sharp metallic sound. Like a rod or valve.

The hub is quicksilver, brand new. I have never had issues with those.

I'll switch the hub to my 3 blade aluminum to rule out the hub.

 

[Scott Danforth]

Thanks. To clarify.

I went from SS 3 blade to Aluminum 4 blade. But I agree with you. I think the 4blade is lugging the engine. Never thought it could have such a big impact.

Then it should even out. Check your timing. Check the contents of your fuel filter

 

[Joshto]

Pull your plugs and post pictures. If you are in detonation you are either running lean and/or timing is off. You will injure your motor with the SS prop as well, just won't hear as much.

Here. That's the first cylinder

 

[Joshto]

Pull your plugs and post pictures. If you are in detonation you are either running lean and/or timing is off. You will injure your motor with the SS prop as well, just won't hear as much.

This the the fourth cylinder

 

[ScottinAZ]

from what I see, either those plugs have nearly no run time on them, or you are running lean. Center electrode porcelain should be more of a tannish color than the bright white you have there.

 

[Joshto]

The manual says 20 hrs. Wonder if I should retorq the head to specs as a precaution? It has about 8 hrs of use until now.

 

[Joshto]

from what I see, either those plugs have nearly no run time on them, or you are running lean. Center electrode porcelain should be more of a tannish color than the bright white you have there.

I installed them when I replaced the head gasket. So they literally have approx 8 hours of use.

 

[ScottinAZ]

I installed them when I replaced the head gasket. So they literally have approx 8 hours of use.

then I would say its a little early to get a really good plug read on them to determine lean. I dont see it in the pics, but are there any shiny metallic looking specs on the plugs? this is also a sign of detonation/preignition. You may still be running lean, but at this point I would call the plug read "inconclusive"

 

[Joshto]

then I would say its a little early to get a really good plug read on them to determine lean. I dont see it in the pics, but are there any shiny metallic looking specs on the plugs? this is also a sign of detonation/preignition. You may still be running lean, but at this point I would call the plug read "inconclusive"

Thanks. They look quite clean. Nothing metallic on them