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Old 08-06-2010, 19:48   #16
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Hauled out today after sea trial with boatyard mechanic. On the hard, he was able to shake the end of the shaft (full keel boat, no exposed shaft, just the end where the prop is) and hear the sound of shaft on tube. Keep fingers crossed that checking shaft for true, re-aligning, and new cutlass bearing will put an end to this.
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Old 08-06-2010, 21:00   #17
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Hauled out today after sea trial with boatyard mechanic. On the hard, he was able to shake the end of the shaft (full keel boat, no exposed shaft, just the end where the prop is) and hear the sound of shaft on tube. Keep fingers crossed that checking shaft for true, re-aligning, and new cutlass bearing will put an end to this.
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Originally Posted by delmerrey
On the old shafts, was there any wear marks indicating that the shaft may have been hitting the stern tube/shaft log.
That's what I suspected!
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Old 08-06-2010, 22:17   #18
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No. First shaft was badly pitted, second wasn't in that long and had to replace when we replaced the engine (new engine shorter, needed longer shaft). Current one went in with new engine.....only has about 60 hours on it.

So we may not have the answer.......but I think we've got a good game plan........
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Old 20-06-2010, 23:50   #19
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Update. Hauled. Shaft sent out and made true. Propeller sent out and balanced. Cutlass bearing replaced. Engine realigned. One coat bottom paint while we're at it. Back in the water,and much, much better. Some of the time, no noise at all. But below 900 rpm and over 1500 rpm, definite noise. Kind of a tapping in the shaft tube, frequency consistent with rpm. Hauling out again next week, will install another bearing on inboard end of tube and hope that does it. After aligning the engine again. Shaft is currently supported only at the coupling and the cutlass bearing, as dripless packing does not provide the support a packing gland did. Shaft is over 7 feet long, so another bearing should help (I think this is better than a pillow block as the bearing will be further aft (aft of the dripless packing) and we really don't have room for a pillow block.

Packing gland area is very hard to get to (hence the move to dripless). Must remove batteries. Ugh. But must be done.
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Old 21-06-2010, 10:45   #20
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Shaft is over 7 feet long, so another bearing should help (I think this is better than a pillow block as the bearing will be further aft (aft of the dripless packing) and we really don't have room for a pillow block.

Packing gland area is very hard to get to (hence the move to dripless). Must remove batteries. Ugh. But must be done.
If your shaft is 1" dia. and 7' long I would suspect a flexing especially under stress OR any misalignment of the coupling.
Are you using a soft coupler between the motor and shaft couplings? That helps to keep the whipping action out of the shaft if misaligned.

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Originally Posted by marshmat
A quick check against a few reference books suggests that shaft diameter is typically specified as approx. 1/14 of propeller diameter for a tobin bronze shaft, and 1/17 of propeller diameter for Monel (subtract 1 from the denominator for 4-bladed props, and always round up to the next standard size). As a rule of thumb, maximum bearing spacing without causing excessive vibration is usually around 40 times shaft diameter, and bearings should never be closer than 20 times shaft diameter.

Shaft vibration could be related to insufficient support (too narrow a shaft, or too much space between bearings); it could also be the result of too little tip clearance between the propeller and the hull (should be at least 10-15% of prop diameter, anything under 10% is virtually guaranteed to shake like crazy).
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Old 21-06-2010, 10:56   #21
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Be sure to recheck the engine alignment after the boat has been back in the water for a few days.
It can be quite different when the boat's on the hard.
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Old 10-08-2010, 04:45   #22
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Update:

Yard added a second bearing...just like the cutlass bearing....but at the inboard end of the shaft tube.

End of problem.

Just returned from a 250 mile trip up the coast and back. Lots of upwind bashing under power. No thumping.

Bliss.
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Old 10-08-2010, 08:05   #23
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prop to appeture clearance too small?
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Old 10-08-2010, 10:11   #24
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I think the clearance is OK. I'll post a photo if I can figure out how. I think the shaft was flexing inside the tube and the second bearing put a stop to that. I'm very happy with the results.

And yes, the alignment was done after the boat was put back in the water.
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Old 10-08-2010, 20:56   #25
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It is always nice to see a mystery solved.
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Old 11-08-2010, 00:44   #26
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Just reading through and I was thinking how many bearings? what length of bearing support to length of shaft ratio? and the yard solved it. We often fit longer bearings in this type of situation, rather than a 4" we will instal a 6" or two 6". You have to be careful that the gap from forward bearing to engine (with or without a flexible coupling) allows the engine to move through torque, thrust and weight transfer some I've seen have been so close it will cause the water film in the forward bearing to become marginal.
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Old 11-08-2010, 04:30   #27
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rattling noise

Damper Plate??? Bob
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