I've got a loud knocking sound coming from the prop shaft at various RPMS...Between 1500 and 2700 it sounds like it would if someone was hitting the hull with a wooden broom handle twice every second. There's no knocking when it's at a lower RPM or when it's at cruising speed of between 2900 to 3000 RPM. Here's what I know so far: 1. It's not the cutlass bearing...I just had that replaced & the noise is still the same. 2. The Zinc is new, too, and is clamped on. 3. I have about 1" of clearance between the hull & the tip of the propellor blades. 4. There's about 2" of space between the prop and the cutlass bearing. 5. I have a composite PYI "extendo" prop specifically built for my Yanmar 2GM20F that gives me about 5.6 knots at 3000RPM. 6. The hull extrusion that holds the cutlass bearing is solid..there's no play in it. So what can it be?? I replaced my stuffing box with a PSS Shaft Seal and it seems like that seal has a lot more "give" to it than the stock bronze one. I'm wondering if that's the problem. Maybe the shaft hits the hull at certain RPMs depending on the vibration in the motor. Does anyone have any experience with this type of knocking? Any advice on what you think it is? Thanks in advance! Allen Schweitzer s/v Drambuie C-30 hull # 632
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Todd Osborne
New zinc has come loose
New zincs have a tendancy to come loose on the shaft & bang around on your hull, especially when it is in reverse. Start there. A simple look under the hull would really help narrow down the options.
New zincs have a tendancy to come loose on the shaft & bang around on your hull, especially when it is in reverse. Start there. A simple look under the hull would really help narrow down the options.
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Allen
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Matt
Knocking on hull
Allen, I would suspect that your prop shaft is not perfectly true, and is hitting against the hull during certain RPM's. I had this same situation and found that the knocking went away when I payed special attention to the centering of the shaft in the hull tube when I checked the engine alignment. You can have perfect alignment of the engine and shaft, but still be too close to one side of the hull opening or the other. Hope this helps.
Allen, I would suspect that your prop shaft is not perfectly true, and is hitting against the hull during certain RPM's. I had this same situation and found that the knocking went away when I payed special attention to the centering of the shaft in the hull tube when I checked the engine alignment. You can have perfect alignment of the engine and shaft, but still be too close to one side of the hull opening or the other. Hope this helps.
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Mike
Knocking noise
I agree the zinc has probably come loose and is sliding free on the shaft. Same thing happened to me a year ago and I thought the engine was coming apart.
I agree the zinc has probably come loose and is sliding free on the shaft. Same thing happened to me a year ago and I thought the engine was coming apart.
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Allen
It's NOT the Zinc...Any other ideas? Anyone?
Just so everyone is crystal clear: It is not the zinc. I had the exact same problem last year. When I hauled for winter storage the zinc was locked on that prop shaft as solid as can be. Therefore, the source of the noise can not be the zinc. If anyone has another idea I'd love to hear it. Thank you! Allen
Just so everyone is crystal clear: It is not the zinc. I had the exact same problem last year. When I hauled for winter storage the zinc was locked on that prop shaft as solid as can be. Therefore, the source of the noise can not be the zinc. If anyone has another idea I'd love to hear it. Thank you! Allen
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John renfro
knock
hello Allen, how about a lose engine mount? an engine will shake at increased power loads until rpm's or load on the engine is reduced. It can sound like a thumping on the hull. john
hello Allen, how about a lose engine mount? an engine will shake at increased power loads until rpm's or load on the engine is reduced. It can sound like a thumping on the hull. john
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Allen
Thanks
I'll check on it this weekend. I have a suspicion it's the shaft alignment. Thanks for your replies! Allen
I'll check on it this weekend. I have a suspicion it's the shaft alignment. Thanks for your replies! Allen
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Brian Stevenson
Knocking sound
Allen---reading other posts, the shaft alignment and mounting bolts would be important to check. I wonder about the 2 knocks per second at 1500 to 2700 rpm. One might guess that this would be too slow for shaft noise, but I'm not sure. I read that your prop has one inch clearance from the hull. Is it possible that you are getting any prop cavitation at this speed. I ask because our hydro electic company had cavitation on the impellor blades at certain speeds and pitch (variable) and it sounded like pounding noise at a few beats per second as the air collapsed against the blade. Just a thought for what its worth, but if all else fails, perhaps you could borrow a different prop to see if this eliminates the noise? Hope this helps.
Allen---reading other posts, the shaft alignment and mounting bolts would be important to check. I wonder about the 2 knocks per second at 1500 to 2700 rpm. One might guess that this would be too slow for shaft noise, but I'm not sure. I read that your prop has one inch clearance from the hull. Is it possible that you are getting any prop cavitation at this speed. I ask because our hydro electic company had cavitation on the impellor blades at certain speeds and pitch (variable) and it sounded like pounding noise at a few beats per second as the air collapsed against the blade. Just a thought for what its worth, but if all else fails, perhaps you could borrow a different prop to see if this eliminates the noise? Hope this helps.
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