"Tap tap" sound from engine/alternator belt?

[Tenedos]

We have taken the ownership of our new boat. Needless to say we are pretty excited. But each boat comes with new things to learn and challenges.... After our maiden trip to bring it to our home marina, when we fired up the engine, we noticed a new sound that wasn't there before.

It is hard to describe, but something like "tap tap". I couldn't diagnose the source though it feels like it is coming from the front side of the engine. I captured it on a video, available here: http://saillogger.com/static/IMG_4106.MOV

Any ideas on what this might be or how to troubleshoot?

[Sailmonkey]

What was the load on the alternator at the time? That almost sounds like a small amount of belt slip.

[Tenedos]

No, there wasn’t really any load. Battery is full and we are on shore power.

I am suspecting the belt but it is a new belt and alternator and I cannot see how/where it would make the sound.

[Cheechako]

Can't open the link to hear it. Either by cliking or cut and pasting.

[Cheechako]

OK, finally got it open. The engine sounds good. Do you mean that metallic "clik" now and then... which isn't timed with the engine rpm? Not sure what that is, but doesn't sound like an "in engine" thing. maybe a Morse cable vibrating or...?

I would start grabbing things that might vibrate as it's running, like a cable, or dipstik or wire loom or.... . Maybe even press a hammer handle on the alternator or things like that.

[Narfi]

Originally Posted by Cheechako

Can't open the link to hear it. Either by cliking or cut and pasting.

Right click -> Save As

[Orion Jim]

Originally Posted by Cheechako

OK, finally got it open. The engine sounds good. Do you mean that metallic "clik" now and then... which isn't timed with the engine rpm? Not sure what that is, but doesn't sound like an "in engine" thing. maybe a Morse cable vibrating or...?

I would start grabbing things that might vibrate as it's running, like a cable, or dipstik or wire loom or.... . Maybe even press a hammer handle on the alternator or things like that.

I fully agree with Cheechako but I’d warn the new owner to keep his hands away from any moving parts unless he feels qualified. For someone who doesn’t consider himself a reasonably proficient mechanic it might make sense to pay a qualified mechanic to diagnose the issue. Might cost an hour of the going rate but you will retain all of your digits.
I suspect the aftermarket add-ons.

[Thumbs Up]

Check your freshwater pump. Try to wiggle the pulley. Sounds like a water pump bushing gone bad.

[Tenedos]

Ok, after reading some troubleshooting guides and watching videos, I tried something that was suggested.

I sprayed a tiny amount of water on the rib side of the serpentine belt while the engine is running. "Tapping sound" went away for a while and came back maybe after a minute or so. So wondering if it is the belt. It is a new belt though and don't see any issues with it with a visual inspection, doesn't seem to be misaligned either...

[GreenWave]

How much deflection is there on the belt at rest when you press on the middle of the belt? If more than 1/4" then probably just needs to be tightened.

[Scubaseas]

Originally Posted by Tenedos

Ok, after reading some troubleshooting guides and watching videos, I tried something that was suggested.

I sprayed a tiny amount of water on the rib side of the serpentine belt while the engine is running. "Tapping sound" went away for a while and came back maybe after a minute or so. So wondering if it is the belt. It is a new belt though and don't see any issues with it with a visual inspection, doesn't seem to be misaligned either...

New belts will sometimes do this. Rare but possible. If the belt is tensionable make sure it's properly tightened. Or carry a spare. Pretty sure the noise comes from something wrong with the splice inside the belt, like a high spot. Sometimes it's like a harmonic that goes away after it's "worn in". Some go away, some the belt breaks but it usually takes a while for it to happen. What belt manufacturer is it? Cheap Chinesium belts will do this more often than say Dayco XL or Continentals. My comments are from automotive experience but they apply to boats too.

[beirutsailor]

Originally Posted by GreenWave

How much deflection is there on the belt at rest when you press on the middle of the belt? If more than 1/4" then probably just needs to be tightened.

Check visually, whether the pulleys for the water pump belt are properly aligned. After a decade of belt trouble, I found my Yanmar 3YM20 misaligned by the original installation. Fixed it by adding an extra nut inside two retaining screws.

[boatman61]

Originally Posted by Thumbs Up

Check your freshwater pump. Try to wiggle the pulley. Sounds like a water pump bushing gone bad.

^^^This^^^

[stormalong]

Sounds like a bearing failure. Could be alternator, water pump or idler pulley.

Relatively easy fix. Pull it apart and listen/feel for roughness - sometimes hard to find when turning by hand. Might just be a good time to replace all the bearings for a fresh start.

[Mike Banks]

That sounds like something to do with the flywheel. That is where I would begin--see if something is stuck in the belt groove on the flywheel. If it was the alternator it would not be in synch with the engine RPM--it would be more frequent
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