Prop Noise

[mwhitten]

Gents

I changed prop pitch from a 15" to 11" so I could reach the RPM required. It's a 115 Merc on a 24' pontoon. After the prop change I noticed a noise near the prop area at the lowest RPM (about 600) just in gear, forward or backward. Once the engine RPM rises the noise is gone. My dealership claims this is to be expected with a low pitch prop, not to worry. Do I have need for concern?

Michael

 

[walleyehed]

Re: Prop Noise

There "is" some truth to that....I see no need for worry. Some are worse than others.

 

[Texasmark]

Re: Prop Noise

Ok guys, can of worms opened.

Ken, what do you mean by that comment "is"?

Sounds like the place where clutch dog rattle occurs, but mwhitten is talking about a 11" prop not a 24 and it possibly could be alum not the heavy SS.

Thanks,

Mark

 

[WillyBWright]

Re: Prop Noise

The 11s have thick large blades and are rather heavy for aluminums. Live with it. Nothing you can do about it except don't idle it around. A 13 wouldn't be any better.

Edit: If that has the plastic hub, you might tame it with a different hub. Either FloTorq 3 or 4, don't recall which one. But I'll bet it's a rubber hub and then you're stuck.

 

[Texasmark]

Re: Prop Noise

Ok Willy, is it "clutch dog rattle"?

Mark

 

[WillyBWright]

Re: Prop Noise

Yup. It goes away as soon as throttle is applied, probably around 700-800 rpm and it's gone. The blades on his are huge and round, not swept back and tapered. Stump puller.

 

[Texasmark]

Re: Prop Noise

Thanks. Funny, I have been boating all my life and I am just now being exposed to the phenomenon.

Does OMC have the same type of gear/shifter setup in the lower unit like Merc, and do they suffer from it too?

Thanks,

Mark

 

[mwhitten]

Re: Prop Noise

Guy's

Thank you for all of the advice. I agree it is a stump puller. Most of the time there is three - four GROWN adults plus gear. I have considered going to a "13". Currently WOT it will go all the way to 6000 RPM. "13 would put me around 5600 RPM. WOT now is about 23MPH. The prop is a Quick Silver, the hub is what ever they reccommended. I do not understand what "clutch dog rattle" is. The noise did increase from a "15" to the "11"

Michael

 

[Texasmark]

Re: Prop Noise

Archives should have lots of info. Basically it is the shift mechanism in the lower unit gearbox bouncing into and out of gear while at very low rpm's. Makes an undesirable noise, but no one has admitted that it hurts anything and my nearest Merc dealer's engine mechanic says he has a factory bulletin covering the subject.

Mark

 

[WillyBWright]

Re: Prop Noise

Thanks to this topic, I finally started collecting a few previously posted explanations. This is the first. I copied and pasted this and made a few minor revisions...

Clutch Rattle Explained

Your powerhead doesn't run smooth at all, especially at idle. The crankshaft is constantly slowing down with compression and speeding-up with combustion. A 2-stroke 3-cylinder slows and speeds 3 times each revolution. If you graphed crankshaft rotation velocity, you would have something closer to a sine wave than a straight power line.

What's happening down in the gearcase is that the mass of the propeller keeps it turning constantly...the old object in motion tends to stay in motion thing. Graphed-out, it would be a straight line. So every time the crankshaft slows down, the mating surfaces of the cogs of the sliding shifter (clutch dog) and gear form a tiny gap as the prop continues it's constant spin. When the sparkplug fires and speeds-up the crankshaft, the cogs slam together. That's the noise you hear. At idle, those speed-slow cycles are much more pronounced than at higher RPMs.

It sounds awful, but it's not generally destructive since the power at idle is so low. It is much more noticable with stainless props than aluminums because the added weight (mass) keeps the stainless prop spinning smoother when the gap forms and then resists speed-up when the cogs slam together again.

I wouldn't advise trying this at home, but imagine pushing a car with an SUV. Slow down the SUV and you get a gap between the vehicles. Accelerate to resume pushing and there's a bump. Now imagine that the pushed vehicle is a big truck. The bump is more like a jolt.

I haven't heard the clutch rattle complaint with makes other than Mercury, but I work mostly on Mercurys. As a marine mechanic, it seems that Mercury products in partcular give me a certain degree of job security.

 

[Texasmark]

Re: Prop Noise

Ok Willy, I had to laugh at your comment about Mercs and a living.

Back in the '50-60's I was an OMC guru. We had a standing saying: The Merc owner goes to the (repair) shop on Friday afternoon and picks up his boat for the weekend's fun. Monday morning, he drops it off at the shop so that he can use it the next weekend.

I think what moved me away from OMC was the gearcase and the shearpin. Merc had a smooth front that didn't collect weeds and hang up on floating logs....where I liked to fish. The prop had a slip clutch not a shear pin so when you were fishing on the windward side of riprap (liked to fish that too) with a 15mph wind blowing and hit something you could get out and not wind up on the rocks. I know OMC had a slip hub too, but seems the shear pin was always breaking anyway.

I've been criticized on here for disagreeing with some of the senior level posters. One of those disagreements was just what you said. They said the flywheel didn't allow that surge to happen and I said it did (as you) as the OB flywheel was lightweight (to keep the engine's weight down)....made the comparison to the flywheel on a John Deere 2 cylinder "popper" (where weight helps to keep the rear tires digging and not slipping). I still stand by my guns.

Mark

 

[mwhitten]

Re: Prop Noise

Does anyone see the need to change fron the 11" to a 13" ?

Michael

 

[WillyBWright]

Re: Prop Noise

6000 for an inline-6 115 doesn't concern me. I assume that it isn't an inline-4 115 or it wouldn't make 6000. And a 4-stroke 115 wouldn't have clutch rattle. You'd be complainig about difficulty getting it out of gear instead. A 13 might use a bit less gas for cruising cuz you wouldn't have to wind it so tight to get the same speed.

 

[mwhitten]

Re: Prop Noise

Mr. Wright

It is a 115 - 4 stroke

 

[WillyBWright]

Re: Prop Noise

I have never heard of clutch rattle on any 4-stroke. Is this a squeaky noise we're talking about? The prop would have nothing to do with that. But make sure the propnut is good and tight. The squeak is caused by the driveshaft in a plastic guide bushing that serves no other purpose. There isn't anything you can do about that either. The shaft can be greased there, but it doesn't last more than a few hours. It either spins off or washes off. Like clutch rattle, it is more annoyance than destructive. The only reason you hear it at all is because the motor idles so darned quietly. Heck the pee hitting the water is louder than the motor. If it weren't a 4-stroke, the other motor noises would drown it out. 6000 is a good RPM for that motor. The rev limiter would kick in at about 6200.

 

[Texasmark]

Re: Prop Noise

Willy, no, not a squeak. A metallic banging and clanging that is irregular and can be changed by water conditions, boat velocity, or changes in throttle. Only occurs at very low rpm's where the impulses you mentioned are the farthest apart thus causing the most disruption to the power flow from the engine to the prop shaft.

Like mentioned, the ratcheting mechanism of forward gear is the culprit. Like mentioned, the dog and F gear get partway separated [due to the prop screwing thru (holding in) the water] and along comes another bang from a cylinder firing and "klunk" the gears lock back and then the interim cycle where they separate slightly and along comes another firing and klunk.

At any rpm (1000?? ) above idle, the impulses are so close together, the dog doesn't have time to back off F gear and there is no noise.

Mark

 

[WillyBWright]

Re: Prop Noise

Oops, forgot about 115 Verados. Kinda shakey. Big pistons. Big bangs.

Or is it a Yamaha powerhead? Does the powerhead idle rough? They usually don't. Otherwise I'd make sure there wasn't something amiss with the prop. Haven't heard of one of those doing that.

6000 is good for both those motors too. Even 115 Optis. Only the inline-4 2-stroke 115s should run fewer.

 

[mwhitten]

Re: Prop Noise

TexasMark

You are correct. It is a "Klunk" only at it's lowest RPM, in rhythm. Forward or reverse. As soon as a load is applied the noise is gone. Do you still it is clutch rattle?

Michael

 

[WillyBWright]

Re: Prop Noise

We still kinda need to know exactly which 115 Merc you have. They made inline-6 2-strokes and inline-4 2-strokes. Until recently they made 4-cylinder EFI 4-strokes with Yamaha powerheads. Now they make direct injected 3-cylinder 2-stroke Optimaxes and 4-cylinder EFI 4-strokes with Mercury powerheads which are Verados without the blower. They even had a V-6 Optimax 115 for a few years. That's six very different Mercury 115s. Four are 2-strokes and two are 4-strokes. Model year alone would narrow it down. (Odds are it's the Yamaha powerhead cuz the newest are available starting this year. But the Yamaha powered ones aren't known for what you describe unless something is wrong. ) Exactly which Mercury 115 do you have?

 

[Texasmark]

Re: Prop Noise

Possibly.

I have the normal klunk when you engage a gear. That is what you want, a good solid engagement of the gears; not a slow movement of the shifting lever...by you, causing a ting ting ting as the gears find each other.....wears the tips off.

Then at idle, either making my way out of the no wake zone, or when aligning the boat to "drive on" the trailer I get this very obvious metal to metal clanging noise that is not synchronous....very intermittent, as stated, depends on several things as to whether or not it's present and if so how much and how long.

This is the first "big bore" 3 cyl OB I have owned. This is the first 2.3:1 gear ratio OB I have owned. This is the first 13 1/2 x 24P HD SS prop (new out of the box from iboats) I have put on an engine. This is the first time I have heard such a thing.

I can replace the prop with a lesser pitch (21 SS) and it is almost gone, as with a 23P aluminum, and if I go back to the 17P SS prop that was originally on the engine it is gone.

Mine does not rattle in reverse. The shift dog does not ratchet in R like it does in F. There is a slot and once in it, you are in it. You can determine this by hand at home on the trailer. Put it in F and rotate the prop both directions. One it locks, the other it ratchets with a clicking noise.

Then go thru N to R and do it slowly (don't force it to try to shift) while you slowly rotate the prop with your hand to get the R dogs lined up. Then it will go easily into R. Then the prop should be locked to the powerhead in both directions. If it doesn't and ratchets in one direction (like in F) on your engine, then you can get the same klunking in R that you get in F because your dog has sawteeth (slipclutch/overrun clutch...whatever) on both ends.

HTH,

Mark