Vibraration and Binding Prop Shaft - Where do I start?

[Vincentc]

On Thursday's trip from Pascagoula to Biloxi I noticed a vibration in the aft cabin above the port prop shaft.

I figured it was barnacles and while we were out Friday I went overboard with a scraper. There were very few barnacles on the props and as I tried to spin the port prop to get to the inside blades I noticed that it was much more difficut to rotate the port prop than the stbd prop.

I have not hit anything with the prop, lately, and am guessing that there might be an alignment/engine mount issue. 43 DC 671TI 1.75" shaft.

Any suggestions on likely causes and advice on how to best diagnose and fix the problem will be appreciated.

Regards,

 

[eze2bme]

Just a thought. When did you replace the cutlass bearings last? Those can get tight when old.

 

[Vincentc]

Thanks,

I wondered about the cutlass bearing, but would that cause a significant vibration? The bearings have not been replaced in the 3 years I've had the boat. They were checked about 2 years ago when we pulled the shaft and the bearings looked good then. I was going to replace them anyway, but the strut "nose cone" complicated the task enough that I decided to wait.

I received a suggestion to check the mounts and put a dial gauge on the shaft. Believe I will try that.
Regards,

 

[MikeP]

Although there could be an alignment issue, it seems unlikely that it just suddenly occurred though I suppose engine mount bolts could be loose and some shifting occurred.

I would not be concerned about one prop being harder to turn by hand - the tightness of the packing in the stuffing box can easily account for that.

Barnacles will certainly cause vibration - did the scraping reduce the vibration?

Also one surprise/learning experience I got one time re prop vibration is that water depth/particular engine RPM can cause it. To make it short, I found that every time I went over a certain area on the Chester river, the boat would vibrate. The first time it happened I thought I must have picked up a line or something so we turned around to head back. But as soon as we got out of that area, it went away. Did another 180 and the vibration started again but we kept on going and the vibration again quit in deeper water.

Good luck!

 

[Vincentc]

I cannot say if the scraping made a difference because it occurred just south of the entrance to the channel by the harbor. I will get a chance to study things when we take the boat back to Biloxi tomorrow.

I do not think this is a shallow water issue since, most of our running is in 6 to 12 feet of water and this is something new. The vibration becomes most noticable above my usual running speed of 8.5 kts at 1150 rpm. However there was a vibration at at about 900 rpm. In addition to feeling excessive vibration, I watched the 24" ss rod guide for the windshield opening support shake pretty severely at that speed.

Thanks

 

[rsmith]

Thanks,

I wondered about the cutlass bearing, but would that cause a significant vibration? The bearings have not been replaced in the 3 years I've had the boat. They were checked about 2 years ago when we pulled the shaft and the bearings looked good then. I was going to replace them anyway, but the strut "nose cone" complicated the task enough that I decided to wait.

I received a suggestion to check the mounts and put a dial gauge on the shaft. Believe I will try that.
Regards,

I've got the nose cones too. the only way I've been able to get the bearings out is with a sawzall and a 15' blade.

 

[Vincentc]

We brought the boat back this afternoon and the vibrations was much less. I am having trouble figuring this out. On the way over I felt a strong vibration when I stood in the aft cabin above the port shaft. Today at the same 8.5 kt speed and similar water depth no vibration was felt through the aft cabin floor.

I did not take off more than 10 barnacles from the port prop Friday. However I also noticed that the aft zinc on the port shaft was missing. I am wondering if it was partially off on the trip over causing an imbalance and vibration. I haven't checked anything else yet.

I'm still puzzled, but pleased most of the vibrations is gone.

Regards,

 

[MikeP]

It was the barnacles - it takes very few. This happens so often for me that it has become almost normal.

 

[rsmith]

barnacles up in the shaft log tube can make the shaft hard to turn.

 

[MikeP]

Another barnacle example - a few weeks ago, top speed 12 k and max RPM 1800/considerable vibration. Cleaned shafts/props of barnacles - by no means more than what I'd call a "light scattering," Didn't run the boat again until yesterday. Ran down and back to St Michaels and spent 40 minutes at WOT - 2300 RPM/17.5 knots/no abnormal vibration.

 

[SeaEric]

St Michaels? I hear there's a damaged Day Marker down that way.

 

[MikeP]

LOL! And probably a pair of P-O'd ospreys who are now without a nest...