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Old 02-06-2010, 08:58   #1
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Rattling Noise

I have a 1979 CT 41 ketch......classic, full keel. Have owned it for five years. Love it.

From the very beginning, there has always been a rattling/tapping/vibration sound coming from underneath when the engine is running.......best guess is sound is emanating from the shaft tube.

For unrelated reasons, we have been through two engines, three shafts, and three propellors on this boat. With each change the pitch/inensity of the noise changes, but it never goes away.

The noise only occurs under way under power. Not sail. More importantly, (I think) it does NOT occur if we run the engine in the slip. In gear.

I don't think it's the rudder, as my husband has crawled inside and held the rudder post while this is going on. No vibration of any kind there.

Cutlass bearing has a clean bill of health from the shipyard.

Any ideas?

I'm thinking we pull the shaft next haulout and take a look-see up the tube. Take the shaft to the shop to check for straightness (even though we had this problem with previous shafts that were known to be straight).

New engine is quiet (Yanmar 4JH3-TE). Hate ruining it with this cockpit noise.
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Old 02-06-2010, 09:26   #2
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Rudder was my first guess.

I can't find a design doc on the internet of the underbody so I don't know what cavities etc to make an accurate guess. Engine mounts? Loose bolts to something or other when under load and moving through water...
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Old 02-06-2010, 09:44   #3
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Not loose engine mounts. Two engines, different mounts. Last ones were loose, this one, brand new, all tight.

Considering reinstalling the red donut between engine and shaft....forget what it's called.....had it before.....don't now.......I think it's supposed to compensate for slight misalingment.......had the noise with it in the old days anyway......
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Old 02-06-2010, 09:58   #4
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Not normal. I'd pull her out and check her all around. Jiggle the rudder. Open her up. Maybe there is a cavity someplace that is holding a loose bolt. Doesnt hurt to have her out every year anyway.
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Old 02-06-2010, 10:10   #5
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Could be a harmonic vibration causing something perhaps a little loose below decks to manifest itself. Assuming your shaft alignment checks out, engine mounts and transmission are sound, and all bearings are good, the problem is most likely vibration from the prop in the water and interaction of the hull moving through through the water..
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Old 02-06-2010, 10:18   #6
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At just the right rpm ALL, and I do mean ALL, my standing rigging vibrates(a bit noisily) to the point I think it looks like the boat is excited to be going out..........m
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Old 02-06-2010, 16:26   #7
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engine vibrations

Who did the alignment? If the mounts are bottomed out to achieve proper alignment the engine will be in contact with the base of the mount and will transmit vibration throughout the boat.This may be a shot in the dark but worth a look since I experienced this on my brothers boat.
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Old 02-06-2010, 16:55   #8
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I agree with Perchance. Is there a piece of relatively hard rubber between the engine and the boat at the mounting points. According to Yanmar's website it's supposed to come with them.
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Old 02-06-2010, 21:37   #9
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Engine mounts are standard Yanmar,and brand new (like the engine). Husband went to great lengths (new stringers) to make sure engine mounts were mid-range to allow maximum adjustment ability.

I had this problem with the old engine, too. Two engines, three shafts, and three propellors......still the same, just slight differences in pitch.

I suspect Astrid may have the answer. Looking into vibration dampeners (though I had that donut in the past and still had the noise).
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Old 02-06-2010, 21:54   #10
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What was the reason for replacing the shafts and props?

How much distance between the coupler and the packing gland?

It the packing gland soft mounted (with a hose)?
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Old 02-06-2010, 21:55   #11
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Did you also change the transmission when you changed the engine? How about the pulse dampener plate on the engine flywheel?
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Old 03-06-2010, 08:29   #12
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What was the reason for replacing the shafts and props?

Replaced shaft when we replaced packing gland with PYI dripless, as old shaft was pitted. Replaced propellor because we wanted more bite -went from two to three blade. Replaced again when we replaced engine to get prop compatible with new engine.

How much distance between the coupler and the packing gland? I'll have to measure. I'm guessing three feet.

It the packing gland soft mounted (with a hose)? I guess. It's one of these:
PSS Shaft Seal: The world’s leading dripless seal.

Did you also change the transmission when you changed the engine? How about the pulse dampener plate on the engine flywheel?

Both engine and transmission were replaced.

Old engine was Volvo Penta MD21A. Could Not Get Parts.

Thanks all. Hope this helps.
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Old 03-06-2010, 12:52   #13
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On the old shafts, was there any wear marks indicating that the shaft may have been hitting the stern tube/shaft log.

On my boat I just replaced the strut, my old strut was out enough (crooked) that no matter how the motor was aligned it would still hit when going from forward to reverse or vise-versa. And, it was a real b$#*ch trying to get the new one lined up with only 1/8" gap on both sides/all around. Just the shake of the motor was enough for the shaft to hit the log.
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Old 03-06-2010, 16:56   #14
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G'Day SP,

A couple of diagnostic queries:

Does the frequency of the vibration/noise change with RPM? And did the frequency vs rpm change when you went from 2 to 3 blades? And finally, how much tip clearance is there between the prop and the nearest bit of hull?

Two possible sources of noise that have not been mentioned so far are cavitation and turbulence from inadequate tip clearance.

A vexing problem for sure, and you have already done most of the things that one would ordinarily suggest. Interesting...

Cheers and good luck

Jim and Ann s/v Insatiable II lying Salamander Bay NSW Oz, northbound at last
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Old 03-06-2010, 18:33   #15
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Delmarry....thanks for the photos. Very clear.

Mine is a full keel boat.....shaft tube ends with the cutlass bearing, so I think I'm OK on that front.

Jim....yes to all.

Does the frequency of the vibration/noise change with RPM? Yes. And did the frequency vs rpm change when you went from 2 to 3 blades? Maybe.And finally, how much tip clearance is there between the prop and the nearest bit of hull? Enough, they all say, but will be checking.......
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