A4 Vibration in drive

[Peter S]

I have a 1973 C&C30 sailboat with an Atomic engine. The problem is the engine will run in forward drive for about 20 minutes & a grinding sound / vibration occurs briefly for a few seconds. After another 10 minutes of motoring it returns again for a few seconds, & after another 5 minutes it comes back. If I continue motoring it will return a couple of minutes late & eventually the grinding noise will stay permanently, its so bad I have to stop motoring. If I change into neutral it goes away immediately, if I shut the engine down for 10 or 15 minutes, & restart everything is reset and its back & it will run for 20 minutes before the problem returns. It can be lessened by motor sailing, if there is enough wind it goes away. Presumably driveline thrust into the gearbox/ reversing gear, is reduced when the sails make a contribution, implying it might be a thrust bearing.
I don’t generally motor great distances, so is generally not a problem. I have tried various other fixes, non of which have worked. I changed the prop, it made no difference. There was some cutlass bearing play and the shaft was worn, so I replaced the cutlass bearing and drive shaft, it made no difference. The engine & drive line were re-aligned (its solidly mounted), it did not fix it either.
Has anyone experienced a similar problem?

[Don Moyer]

Peter,

The lack of response to your post is no doubt due to the fact that rear thrust bearing failures are quite rare in the Atomic 4 fleet. That being said, you do make a good case for such a failure.

If your thrust bearing is bad enough to cause your symptoms, I would think you could grab the prop shaft close to the output coupling and tug on it vigorously and feel lateral or vertical play. Otherwise, you'll have to remove the bearing for inspection.

Unless access is unusually bad on your boat, it's not all that difficult to separate the shaft coupling from the output coupling, and then pull the output coupling from the tail shaft of the reversing gear. You can then remove the two rear flanges and separate them to expose the thrust bearing.

Don

[Peter S]

A4 Vibration in Drive

Don,

Thanks for your reply, I will try grabbing the shaft and moving in laterally and vertically as you suggest.

Peter

[Peter S]

A4 Vibration

Don,

Just a short update, I tried moving the shaft up and down and horizontally and found no excess play. I have decided to try using a lubrication additive, on the theory that perhaps the oil thins out as the engine warms up, and a worn bearing is aggravated. If I add a lubricant additive perhaps the oil remain on the bearing longer. If this doesnot help I'll replace the bearing at the end of the season.

Regards,

Peter

[hanleyclifford]

May I suggest that you inspect your motor mounts including the sleepers - then make sure that your drive line is within the .003" limit.

[sastanley]

Hi HC,

Slight hijack if Peter doesn't mind..

I attempted to do this the other day, but could not get the coupler apart, but I got the bolts off....I also had some loose mount nuts so the engine may have moved...any tricks to separate the coupler? It is so tight I can't get my smallest feeler gauge (0.005") anywhere in it, bit I see the stuffing box move as the shaft rotates, so I am pretty sure it's out of round/alignment.

I was reading Calder's book about alignment but it seems to assume you are just installing the shaft so the coupler halves would be apart, and clean & fresh for coupling..not stuck together for 30 years.

[hanleyclifford]

If your dock is strong enough, tie her off real tight, get her going in gear and hold a metal brush against the seam while she turns. Once the metal is clean start with the P blaster. This is another one of those love/hate procedures.

[sastanley]

Hi HC, thanks....corrosion in the joint...Roger!

[FelicityRebuild]

What's a P blaster? I have 3 boys under the age of 9 so to me P blaster means something that's probably different.

[hanleyclifford]

P blaster is one of those chemical spray cans, like WD 40 only better.

[FelicityRebuild]

I used a sawz-all. My bolts were rusted so bad that I could barely find them. Ordering new plates in about a month once the engine is working. I have very little patience for rust.