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GILow · 20-10-2016, 03:34

Quote:

Originally Posted by a64pilot

My problem I believe is with driveshaft overhang, I have 2.5" sticking out and should ideally have no more than 1.5" or so. I have a 1.25" thick shaft.
I attached three shaft collar zincs to the driveshaft to the inside of the boat and removed the one outside and moved my resonance freq from 2200 to 2700, and in my mind validated that I simply have too much weight hanging off of a too long of unsupported shaft.

Totally agree with this. We have a 1.5" drive shaft, less than 4 feet long and very little protrusion beyond the cutlass. But I accidentally set the engine about half an inch too far aft when I replaced the engine mounts and immediately got odd little vibrations at various revs. Brought the engine forward again to the CORRECT set of measurements and the vibrations went away.

ramblinrod · 20-10-2016, 08:56

Quote:

Originally Posted by Hoohaa

https://youtu.be/fUkl3v5gD4k
I posted a short clip on YouTube. Hopefully this link will work.
Lokking at all the advice so far. And as i had this same problem before the work was done. I am leaning towards the prop being bent or similar.
Or the shaft is still missaligned.

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

Thanks for the video clip.

The most common culprit for prop shaft vibration is a worn cutlass bearing.

About 50% of boats over 20 years old, have this issue. I replace several every year.

People avoid replacing worn cutlass bearings because it can be expensive (hired out) or a very time consuming DIY, and a DIY can run into all kinds of trouble in the process. (If you hire it out, make sure the contractor has a professional cutlass bearing extractor. They are an expensive tool but it turns a ~$1300 job into a ~$300 job.)

A worn cutlass bearing can loosen the strut/hull join and take out the transmission and engine bearings.

I heard an engine knock in the audio at the same frequency as the shaft RPM (though X frames per second can throw off the visual RPM).

Here is the sequence of operations I would take in this case.

1. Check for play in cutlass bearing. There should be < 0.005" play as measured at the end of the supported shaft. If it you can see ANY play with the naked eye, replace it.

2. Verify the strut is securely mounted to the hull. (No movement whatsoever.)

3. Inspect the motor mounts and ensure they are correct.

4. Disconnect coupling and check the tranny bearing. < 0.002" play at end of unsupported output shaft.

Do all of the above regardless. If the problem is not solved, consider the following one at a time.

5. Have the prop checked and balanced.

6. Perform a compression test and ensure all cylinders are equal. (If one cylinder is low, the vibration can transmit through the shaft, amplified over it's length.)

7. If that fails, but there is definitely an RPM synced knock, it may be engine internal, could be a crank or conrod bearing, excessive valve gap (you would likely notice lost performance / smoke if it had a stuck valve), wrist pin, or piston slap.

ramblinrod · 20-10-2016, 09:11

Quote:

Originally Posted by GILow

With all deference to the many wonderful professionals on this forum, having the job done professionally, in my experience, is no guarantee it was done right.

That's the fastest thing I learned from having a few boat things done by "marine professionals".

One has to be careful how they define "marine professional".

If you hire someone qualified and experienced IN THAT FIELD, and pay them an appropriate amount for the value of the job, the job will almost always be done right the first time. (A true "marine professional" can still make a mistake, or have something completely out of their control go wrong, but they will make it right.)

If you hire someone who will accept pay for performing work that they are not qualified to do and have no experience with, you did not hire a "marine professional".

If you hired a marina service shop who sent out a summer student, you did not hire a "marine professional".

In reality, no "marine professional" after a 15 minute assessment, can know a particular boat better, than a knowledgeable and experienced owner of 25 years. It just isn't possible. What the "marine professional" brings to the job, is general, or specific system / component knowledge, education, and experience, that may be greater than the owner. Sometimes, even a second pair of equally qualified and experienced eyes is a benefit.

ramblinrod · 20-10-2016, 09:14

Quote:

Originally Posted by GILow

Totally agree with this. We have a 1.5" drive shaft, less than 4 feet long and very little protrusion beyond the cutlass. But I accidentally set the engine about half an inch too far aft when I replaced the engine mounts and immediately got odd little vibrations at various revs. Brought the engine forward again to the CORRECT set of measurements and the vibrations went away.

Likely another issue, masked by the change.

Many boats (both sail and power) have 12" to 24" of shaft extending beyond the strut, with zero vibration.

Cheechako · 20-10-2016, 09:15

I think you said you replaced the shaft so that is very unlikely to be your problem. You also say it's not a vibration but a wobble. Do you mean it's just a perceived problem because the coupling/shaft appears to wobble when viewed inside the boat? Another thought is how close to the hull is the prop blade? Does it have 15% of diameter clearance?
I have had a couple boats with enough shaft overhang to get a bullet zinc on in front of the prop, so that's probably 2" or so. (1.25 and 1.5 shaft) Wasn't an issue on those boats....
I see nothing unusual in your video.

**a64pilot** · 20-10-2016, 09:19

Quote:

Originally Posted by ramblinrod

Likely another issue, masked by the change.

Many boats (both sail and power) have 12" to 24" of shaft extending beyond the strut, with zero vibration.

Really? I've not seen one, But then I guess I haven't seen many

Cheechako · 20-10-2016, 09:22

Quote:

Originally Posted by ramblinrod

Likely another issue, masked by the change.

Many boats (both sail and power) have 12" to 24" of shaft extending beyond the strut, with zero vibration.

You're kidding right? Never seen one...

ramblinrod · 20-10-2016, 11:17

Quote:

Originally Posted by Cheechako

You're kidding right? Never seen one...

OK, if I've been told once, I've been told a MILLION times, "Stop Exaggerating"!

Yer right, 12" to 24" is excessive and not likely.

But I quickly checked some service photos and of the first ten, two had prop shaft extension at least 6" (no exaggeration) between the aft of the strut and the start of the prop (Mirage 33 and Beneteau 46.)

But all that aside, an inch or fraction of an inch either way, (as long as min clearances are maintained) won't make any difference if the prop shaft and alignment is true and the bearings are all good.

Three Sisters · 20-10-2016, 11:20

Quote:

Originally Posted by Cheechako

You're kidding right? Never seen one...

That is something I have never seen also, save for the arneson surface drives.

delmarrey · 20-10-2016, 11:22

Even 6" could be catastrophic if one were to hit a log or rock.

The clearance is to be 1-1/2 times shaft diameter + or - a bit. As to let cooling water pass thru the bearing.

**sailorchic34** · 20-10-2016, 11:23

One note for those with three cylinder diesels. Three cylinder diesels are not balanced very well and certain RPM's can induce vibration. Yanmar in their operation manual recommend avoiding those rpms when operating.

It's a feature.

Three Sisters · 20-10-2016, 11:25

Checking "play" at the cutlass bearing could be free. Then maybe a consultation with Prop Scan if no other possibilities are apparent. Good luck to the OP

delmarrey · 20-10-2016, 11:25

Quote:

Originally Posted by sailorchic34

One note for those with three cylinder diesels. Three cylinder diesels are not balanced very well and certain RPM's can induce vibration. Yanmar in their operation manual recommend avoiding those rpms when operating.

It's a feature.

Cruising speed for my 3JH is between 2000 and 2600.