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Old 03-10-2004, 00:51   #1
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Shaft Bearing

Although I have engineering experiance, I have had none when it comes to the water lubricated shaft bearings. So I was wondering just how much play in the Shaft/Bearing should there be. For some info, the shaft is at least 2" dia. It is the main bearing that the shaft exits the stern through. I could get a small amount of movement by hand. I didn't think it was excessive, but I wouldn't want it any more either. Should there be any movement at all is my question.


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Old 03-10-2004, 05:25   #2
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I don't think there should be any perceptible movement of the shaft through the stern tube (shaft log) bearing. Beware of “ovaling”, particularly on the lower half of the bearing. Does the boat suffer from vibration, underway?

The most common cause of vibration are: motor, shaft, or stern-bushing misalignment, followed by motor mount, and coupler problems.

The most common reason why drive systems go out of line is that over time, the motor mounts or bushings wear. Progressively the motor or shaft settles into an off line position until it is eventually bending the shaft. This can also be cause if the hull changes shape.

Improper alignment, weak engine beds, or loose couplings can cause incredible Cutless Bearing wear. A properly installed Cutless Bearing should last many years and should be checked yearly upon haul out by grabbing the propeller and attempting to shake it in order to judge the Cutless wear. When you grab the shaft or propeller and shake it you should feel virtually no play or motion. If you remove the shaft, the bearings interior should be smooth, without cracks, cross wear, low spots, tears or indents. Cross wear on the bearing indicates a major alignment problem. The shaft log or shaft tube is just that, a hollow round tube which penetrates the hull through which the propeller shaft goes with no bearing surface, hopefully. Ideally the propeller shaft should be perfectly centered.

The general rule is, the longer the shaft and the larger the diameter, the more critical proper alignment is required. That's because larger shafts tend less toward self-centering than small ones, because they're more rigid.

ABYC standard P- refers to the alignment tolerance between the parallel flange of the coupling with the connection the bolts loose. That tolerance is defined as 4 thousandths of an inch. The alignment is specified by the ABYC to be done with "the boat floating". This allows the boat to obtain its normal flex so that an accurate alignment can be done. The measurement should be made by feeler gauge with the bolts loose, four times, 90 degrees out each time.

From ABYC P-6 table III, for shafts over 15/16" to 8" in diameter, and with a length of over 4' to less than 8'; you are allowed three thousandths of an inch tolerance. Table II calls out that for shafts supported at 42" where the shaft diameter is over 15/16' and under 1 15/16" the permissible variation is .006". Shafts 1 15/16 to 2 1/2" the variation is .007".

Some good resources:

Cruising World published (Sept. & Oct. 1966) a couple of good articles by Steve Antonio:

Heavy Metal Part 1 - Careful attention now to below-the-waterline running gear can prevent problems later on - By Steve D’Antonio

Heavy Metal (Part 2)
In the second of a two-part series, we look at the tools and techniques needed to pull the shaft, replace the cutless bearing, repack the stuffing box and put things back together - By Steve D’Antonio

SHAFT LOGS AND STERN GEAR (from Reading College):


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Old 03-10-2004, 05:28   #3
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Yes, some movement is okay, forgot the exact number.
I changed mine 4 years since the boat was on the hard. The bearing cost only $75 and will last several more years.

Also carry a spare one on the boat.

Problem was on the last haul-out: The guy that painted the hull had also painted over the cutless bearing and it go no lubrication when I motored from the yard. (I should have inspected better, but did not)

Heard a horrible noise from down there, and han an hunch that the noise would come from the bearing turning.

Got the boat to a dock and dove down with a flashlight and sure enough, all vent holes were clogged. Scrapped the paint off and no problems since, but the yard manager got a call from me...

So, if yer boat is on the hard, and ya have some movement, why not just change the bearing and get it over with.?
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Old 03-10-2004, 12:22   #4
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Thanks Gord and CSY. Here's more to the story. We had the boat out last Dec. It was an anti-foul and anode replacement and a pre-purchase check over. As I said, I didn't think it was tooooo bad, although a small amount of movement could be had when shaking the prop/shaft by hand. IT was only after putting the boat back in the water, that I noticed when reverse was applied, a vibration could be heard. Now, this also could be cavitation or reverse thrust on components, etc, but I did suspect maybe the shaft coiuld have had too much play after all. I also get water driping from the main front bearing/seal, although a couple of pumps of grease after we have docked, puts keep to that. She has done 430hrs, so it isn't alot. However, I am wondering if we should lift her earlier than I was thinking and check all that out, or if I let it be till after the season.
I tend to like lifting anually and giving a coat of fouling and mai reason is to check the anode. But were we are berthed, has a high amount of fresh water flusing through the marina and we are getting very little growth. So I can check the anode in the water, which means if I don't have to lift her out, it will be a big saving. But if you guy's think maybe I should, then I will. I will read up on those links Gord and maybe I can get more info from them as well.

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