Mysterious tapping sound

[Ken Palmer]

We have noticed a tapping sound when motoring our 33.5. This sound happens while under way or not. The frequency of the tapping determines the intensity of the noise. If I go below and try to pinpoint where the noise is originating, it sounds like it is from the hull if I am in the aft berth. I have looked carefully under the hatch that exposes the motor coupling and stuffing box. Nothing to see there. In the cockpit, it sounds like the sound is from the locker under the starboard seat. Nothing found there.We had this same strange noise when we owned a 1981 Hunter 33 but never found the source.After motoring for 7 hours across Lake Ontario Tuesday, my head is going bang, bang, BANG, bang, bang, BANG. The caps indicate a slightly louder noise. It almost sounds like a cable is hitting a bulkhead at resonate frequencies while motoring. Anybody else experience this? What did you find?

 

[jtm]

tapping

I suspect it might be your cutlass bearing giving out too much play - its probably rotting out and there's play in the housing holding the prop shaft. It happens even when you are not motoring cause the props still have resistance and bonk around in the housing. Let us know what you find.

 

[walmsleyc]

Not the cutless

if it happens when you are NOT underway ..., or at least not likelyunless your motor is REALLY vibrating at idle, and thus jiggling theshaft around.mmmChris

 

[undathac]

Same sound on 37C

I have the same tapping sound whenever the engine is running at less than 1500 RPM. The noise seems to be coming from the area around the exhaust outlet. That's as far as I got. I will get down in there and see if I can isolate it further.

 

[Bill Roosa]

secondary fuel pump???

I don't know your setup but electric fuel pumps make that kind of sound.Does it very with engine speed?

 

[Ken Palmer]

Answer to Bill

Yes, the knocking will vary with speed. There is no electric fuel pump on this boat, so it is something else. I find that the knocking is minimal at 2100 rpm or so.Kris... I will take a close look at the cutlass bearing when Water Colors is hauled for the winter in October. I must correct my statement. We were under way, motor sailing, when I put the engine in neutral. The knocking persisted and changed rate depending on engine speed. True, the prop was rotating because of water passing, but that would have been a constant speed, not the varying speed I observed.What I really need to do is empty out the cockpit locker, crawl in, and carefully inspect with and without the engine running. Thank goodness this is a big locker. On my old 33 I always thought they would have to sell the boat with me stuck down there. It was truly confining.

 

[John]

Engine Alignment

Ken, this sounds like the same issue I had on our H31 when we bought it. The shaft was striking the stern tube making a thumping or knocking noise. I replaced the cuttlass, shaft seal, and had the shaft checked for straitness. The problem still persisted. The only thing left to do was check alignment. The rear of the shaft was too low, and striking the stern tube with every revolution, alignment solved the problem. Misalignment will also cause wear to the cutlass bearing. I hope this helps.JohnS/V Sanctuary II

 

[Bill Roosa]

Don't want to be jonny raincloud but

knocking that varies with engine (and not trans/prop) speed and is loud and less loud at certain RPMs is piston wrist pin or crankshaft bearings going. From what little I have to work on (text just does not cut it you really need a trained ear) I would strongly recommend that you get a mechanic to listen to it. If you catch these things soon you can get by with just replacing the "bearings" themselves and not do any damage to the actual crankshaft-connecting rods-or block. I've done it in an afternoon with out even pulling the engine. If the tapping gets louder it is surly a bearing going. This could leave you low and wet (high and dry) if you are out on the water as loosing a bearing can render the engine un-startable/runnable. In attempt to diagnose over the internet can you say how fast the tapping is? idle down the engine, note the RPM and count the taps for 6 seconds. Multiply the number you get by 10 to get taps per minute. If the taps match the RPM then you have a lower end (crankshaft-piston) problem. If the number is 1/2 the RPMs then the top end is making the noise which would be the valves-lifters or camshaft.Hope this helps and good luck. 8^)

 

[Ken Palmer]

Thanks John and Bill

Very good suggestions from both of you.Bill, this may be a clue: I started having to add oil to the engine after 8 to 10 hours of use. About 1/2 quart if I cruise at 2500 rpm. I used to cruise at 2700 rpm but noticed more oil loss. I do not see any signs of an oil leak. I also do not see smoke coming from the exhaust, but get the normal(?) black on the hull around the exhaust.I will try to get more answers this week if the weather holds up. The alignment question will have to wait until October after I am hauled.The engine is a Yanmar 2GM20F and the boat is a 1990 Hunter 33.5 bought used four years ago. Used on fresh water only.

 

[Neil Hodgson]

Don't wait Ken

Until you get the boat out of the water to check the engine allignment. It needs to be done with the boat in the water to get the true allignment picture, as when it is out of the water, the boat flexes differently as to when it is settled in it.Bill, great thinking, I will remember that for the future thank you.Neil

 

[Ken Palmer]

For all that responded to this thread...

I have contacted a professional Yanmar tech that is highly regarded in the Rochester area. So much so that he cannot look at the engine until the week of 9/24. I am scheduled to haul out on October 1st, so we have a small window to work with while still in the water. I'll post the findings when I know more what the problem is. Thanks for all the tips.