I get a knocking noise that sounds like it is coming from somewhere aft when under power with my 2GM in a narrow band around 2800 rpm. When listening close to the engine it does not seem like that's the source as the sound still seems aft. Other than that the engine runs well with no problems. I have replaced the cutlass bearing, which was somewhat worn. Has anyone experienced this? Any suggestions as to what it might be? Thanks.
Knocking Noise
- Thread starter Ca Va Encore Bien
- Start date
check your motor mounts, they can transmit the vibration to the hull and it can sound like it is coming from somewhere else
Thanks for your reply. It's not the motor mounts. I've already checked them. Also, the frequency of the knocking does not change with the rpm of the engine. It does get louder and softer with a change in rpm. A sister boat next to me has the dames noise but much less severe.
Frequency does not change with rpm......just gets louder? That's a tough one. Does it make the noise in neutral without the shaft turning? Even so you would think the frequency of the noise would change. Only thing right now I can think of is the muffler. I know mine just sits on the floor under the battery shelf and is only attached to the exhaust hoses. Since you said it starts around 2800 rpm it could be banging around. Just a thought.
Knocking noise may be transmission...
My 34' Pearson used to make a knocking noise between 1800 and 2800 RPM. One time I was trying to avoid threatening weather and ran it wide open, the knocking stopped until I slowed back down. I later found out it was the cone drive in the transmission. If the linkage is a little off it will allow the gear to slip and wear nearly smooth, that is the knocking you hear. At low RPM, idle, there is not enough resistance to cause it to slip and knock. At a high RPM, wide open, there is enough force from trying to push the boat through the water it forces the gear to take a bite. Try it and see if it goes away.
Butch
My 34' Pearson used to make a knocking noise between 1800 and 2800 RPM. One time I was trying to avoid threatening weather and ran it wide open, the knocking stopped until I slowed back down. I later found out it was the cone drive in the transmission. If the linkage is a little off it will allow the gear to slip and wear nearly smooth, that is the knocking you hear. At low RPM, idle, there is not enough resistance to cause it to slip and knock. At a high RPM, wide open, there is enough force from trying to push the boat through the water it forces the gear to take a bite. Try it and see if it goes away.
Butch
Thank you very much. This sounds promising and I'll check it out. I've never worked on a transmission. Does this necessarily mean a rebuild or is it possible to solve with an adjustment?
Transmission cone drive
An adjustment may take care of it temporarily, but if it has been going on for a while the cone could be shot. The rebuild is not that tough. A good book, a clean bench and access to the parts is all you need. I would keep a good mechanic on call just in case. I also remember it did not knock in reverse. We had this problem for over 5 years. I thought the engine was going to blow up. I was talking to the parts man at our Yanmar distributor, I needed an oil filter. He told me what the problem was. I had talked to factory reps at boat shows they had no idea. Once I replaced the cone gear it worked like a Yanmar usually does. Good luck.
Butch
An adjustment may take care of it temporarily, but if it has been going on for a while the cone could be shot. The rebuild is not that tough. A good book, a clean bench and access to the parts is all you need. I would keep a good mechanic on call just in case. I also remember it did not knock in reverse. We had this problem for over 5 years. I thought the engine was going to blow up. I was talking to the parts man at our Yanmar distributor, I needed an oil filter. He told me what the problem was. I had talked to factory reps at boat shows they had no idea. Once I replaced the cone gear it worked like a Yanmar usually does. Good luck.
Butch
I had the same problem at low rpm. It turned out to be the muffler knocking against the hull under the aft berth. The problem was resolved by inserting a piece of foam under the muffler. Good luck!
Thanks everyone. Sorry for the delays in my responses. My internet access at the moment is somewhat unreliable. Thanks Butch23065. I have a Yanmar manual. I'll see what I can do. This has been a concern but I may take it easy until I haul out. In MN that's not too far off.
Wouldn't the knocking noise increase in frequency if it is indeed the transmission?. Like Sailaway said...it was his muffler banging. Just saying, worth a look at anyway before you haul out and tear into your transmission. Take it out and get it knocking and have someone move around the exhaust hose off the elbow and see if it stops. If very well may be your tranny but I would check for "easy" fixes first.
Exhaust noise???
If it were an exhaust noise, wouldn't it make noise either in or out of gear? The noise I'm referring to shakes the entire drive train and makes you think your engine is going to blow but goes away when wide open or at idle. I can't guarantee that is what is causing it and I would definitely rule out all other possibilities before I dropped the transmission. I would also only attempt that during a haul out.
Butch
If it were an exhaust noise, wouldn't it make noise either in or out of gear? The noise I'm referring to shakes the entire drive train and makes you think your engine is going to blow but goes away when wide open or at idle. I can't guarantee that is what is causing it and I would definitely rule out all other possibilities before I dropped the transmission. I would also only attempt that during a haul out.
Butch
The OP mentioned a knocking noise and not a severe vibration problem too. He mentions the knocking noise remains at the same frequency. Maybe he can chime in and tell us approximately how often it happens per minute.
CaVa-
It could be that u have an alignment problem and the prop shaft is knocking against the shaft log.
Bob
It could be that u have an alignment problem and the prop shaft is knocking against the shaft log.
Bob