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Old 09-23-2018, 11:31 AM   #1
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two prop-shaft bearings or one

While on the hard for bottom paint this spring, I installed a dripless shaft seal. It appeared to work great at 1500 rpm but at 1800 rpm the shaft wobble caused it to fling water.

Before hauling for repairs, I set out to either eliminate it or shift the 0.004" coupling misalignment that was the best I could achieve years ago. After improving the alignment I can now run 1800 RPM without flinging water.

Does anyone know a 1979 Marine Trader 34 should have a bearing in the stuffing box (at both ends) or a single cutlass bearing at the prop shaft end?

A friend with a similar vintage MT says that he has a bearing in the stuffing box. A friend with a newer Grand Banks says that it does not have the stuffing box bearing.

Based on the shaft free play in the stuffing box, if there was a bearing, it is long gone. Over the past 9 years, I have experienced a series of related problems so I don't know what to expect and I don't think it was ever right. The stuffing box water flow has always increased from a few drops at idle/in gear to an excessive amount of flow on long trips. I always need to tighten it after a trip.


Thank you.
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Old 09-23-2018, 12:44 PM   #2
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The 34 Marine Trader I had only had one Cutlass bearing where the chef exit the boat none in the stern tube
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Old 09-23-2018, 07:44 PM   #3
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Several factors can be causing your problem. First a traditional stuffing box may have cut down the wobble so that you didn't notice it. The wobble that appears above a certain RPM may be caused by several different things.

First, you realized engine alignment was a partial cause.

The propeller can also cause wobble if one or more blades is slightly out of pitch. Growth (barnacles) on the blades can cause an imbalance.

You could have a worn cutlass bearing that doesn't cause the shaft to jump until you reach the magic RPM.

You could have a slightly bent shaft.

The objective is to figure out what is causing the problem through analysis, not monetary injections. When next you haul out, I would pull the prop and have it scanned. This will tell you if it's true or needs to be reconditioned. Most prop shops will do this for a small fee or less. With the prop off, I would check the cutlass bearing for play by trying to move the shaft up and down, and side to side. If there is play and you don't know if it's too much, seek a professional's opinion. I would also then consider pulling the shaft and coupling, and take them to the prop shop for analysis. Is the shaft bent? With the coupling locked on the shaft, does the coupling face spin true to the shaft? Hopefully, without having spent much money, you can determine if any of these things are causing or contributing to the problem. While you're at it, check your motor and transmission mounts. A broken or worn out mount can be the problem.

Ted
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Old 09-24-2018, 11:43 PM   #4
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I would add one more thing to check and that is , is the prop riding the key. THat can also tip the prop off kilter.
It will not seat properly on the shaft taper.
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Old 09-25-2018, 08:37 PM   #5
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Good point! Last year, the diver said that the prop nut was loose and tightened it after letting me know.
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Old 09-25-2018, 09:08 PM   #6
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If you have a bearing near the shaft seal, you need some exhaust water piped to lube the bearing, or some other water flow. A bearing far up a shaft tube can boil away the water w/o a forced flow. A shaft seal that doesn't drip can make an air pocket. Some boats have a small scoop facing forward and bonded to the hull with a path to the front of the bearing.
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Old 09-26-2018, 01:46 PM   #7
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Quote:
Originally Posted by O C Diver View Post
Several factors can be causing your problem. First a traditional stuffing box may have cut down the wobble so that you didn't notice it. The wobble that appears above a certain RPM may be caused by several different things.

First, you realized engine alignment was a partial cause.

The propeller can also cause wobble if one or more blades is slightly out of pitch. Growth (barnacles) on the blades can cause an imbalance.

You could have a worn cutlass bearing that doesn't cause the shaft to jump until you reach the magic RPM.

You could have a slightly bent shaft.

The objective is to figure out what is causing the problem through analysis, not monetary injections. When next you haul out, I would pull the prop and have it scanned. This will tell you if it's true or needs to be reconditioned. Most prop shops will do this for a small fee or less. With the prop off, I would check the cutlass bearing for play by trying to move the shaft up and down, and side to side. If there is play and you don't know if it's too much, seek a professional's opinion. I would also then consider pulling the shaft and coupling, and take them to the prop shop for analysis. Is the shaft bent? With the coupling locked on the shaft, does the coupling face spin true to the shaft? Hopefully, without having spent much money, you can determine if any of these things are causing or contributing to the problem. While you're at it, check your motor and transmission mounts. A broken or worn out mount can be the problem.

Ted

If you take the shaft to a shop, be sure to take the propeller, key, and nuts, so they can check it as a system, rather than an individual part.
Vibration can occur, even on a perfect prop if the taper and key do not fit precisely.
Also note that props installed underwater often do not receive the same degree of scrutiny as props installed on the hard!
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