CD31 Vibration

[CruiseAlong]

CD31 Kaunis
Kaunis was refitted with a Universal M25xpb 3 cylinder, 2;1 Hurth transmission speed reduction.
Upper speed range is 2600 rpm with a 3 blade 13" dia, 13" pitch, 1 in shaft
There are definitely some speed ranges which have more vibration which I avoid
As stated, some of these do shake my diner table latches and sail latches, other speeds are very smooth
I will some times get a small vibration in the wheel recently under some power conditions but plan to investigate
Alignment should, as you know, be done only in the water
My shaft is short but I do not have a vibration dampener between my engine drive shaft flanges, so alignment checks are straight forward in the water. Just decoupling is needed....
If you are fitted with a dampener, could that be the problem????

Dana

[Maine Sail]

A few questions:

1- Was the prop balanced before re-installation? When I ask about "balance" I am not referring to a mom & pop shop that balances using a "bubble" I am talking about either Prop Scan or Hale MRI. These machines can make a HUGE difference. I have seen some extremely dramatic before and afters where the "bubble" showed the prop "perfect" and the Hale or Prop Scan shop actually made it perfect, which it was FAR from.

2- Do we know the specs on the shaft? Was it "true" if so can they provide you with how true? .001", .002", .003" over the length?

3- How was the coupling fitted and faced to the shaft? Can they give you specs on the "fit & face". The coupling, if not tapered, should be a light press fit over the shaft. Often times when split couplings are used they omit the fitting. Split couplings still need to be properly "fit" to the shaft depsite many shops taking the "easy way"...

4- Was the prop "lap fit" to the new shaft with valve grinding compound?

5- Are we sure the prop is not "key bound"? Who actually did the install of the prop? Was it done correctly or was this a "slap n go"?

6- Does the prop have the proper clearances? Eg: in of 15% of diameter to any portion of the hull?

7- Is this a two blade? Two blade props exhibit more vibration than a three blade no matter how well "balanced".

8- Are we sure the cutlass is in-line with the engine? Many boats are built without the cutlass lining up. Are we sure the packing gland is not side loading the shaft?

9- Are we sure that none of the engine mounts is "pre-loaded". Often times installers will pre-load a mount to make the alignment work. This changes the flexibility of the rubber mount, by pre-compressing it. This makes it less "flexible" than the others. This is not the proper way to install and engine but I come across it all the time and usually with high levels of vibration. When properly aligned the motor should sit as equally as possible on all four mounts as is humanly possible with straight studs and mounts sitting straight in relation to the stringer. Getting the engine to do this can be very time consuming and few take the time to get this right.

10- DriveSavers DO NOT stop or eliminate vibration and their installation can very often exacerbate or make vibrations worse. I remove more of them than I install and magically the "issues" go away once a proper alignment has been done. They are not and should be used as a Band-aid as many installers use them for. While they can protect a gear box I have yet to see one "self sacrifice" on a small aux engine. I have seen shafts, props, gears and even damper plates destroyed and the DriveSaver was still perfectly intact. Doh'... Simply over tightening one or two bolt slightly more than the others will throw some pre-load into the shaft causing "whip". It is very easy to compress these more than .005" during installation and ruin any alignment you had. A very accurate and sensitive torque wrench should always be used with DriveSavers. On a short shaft this can get pretty bad as the shaft has little length to absorb the "whip". At least yours was installed with the proper overhang, this is good! Many, no make that most, installers do not install DriveSavers correctly and the companies do themselves no favors by not explain this strongly enough in the installation instructions..

11- Unfortunately I think you will need to have a precision machined spacer made to conduct an in-water alignment. This spacer should be steel the same thickness as the DriveSaver with female tappings so that it can be brought up against one coupling or the other, but preferably the shaft coupling then the opposite face used for the feeler gauges. While the CD's move little in the water compared to some boats, with new mounts they DO settle after the first 50 or so hours so for that reason alone the alignment in-water really needs to be done. This spacer should be flat to .0001 at the very minimum. A good machine shop can make one for not a lot of money. This is the only way to align a boat that had a shaft properly sized for a drive saver if the prop hits the cutlass.

12- How high does the engine sit on the mounts? It should be very low on the mount bolts. The higher you go on the mount studs the harder it is to control vibration.


Until you've systematically ruled out all possible scenarios fixing the alignment may be dog chasing tail? Hopefully the machine shop and installer can accurately answer some of these questions.

[gca]

Thanks Phil, it is good to know that it is not a systemic problem with the CD31. What do you have for an engine, gear reduction and prop pitch? What rpm range do you run? With my old M25 I had the same as Dana, a 2:1 gear reduction with a 13 X 13 prop and had vibration with that as well.
Thanks for your suggestions Maine Sail. I will copy and take to my installer.
Evergreen was hauled yesterday and I think I have a prop cavitation issue which is what the yard that did the installation has been thinking might be the problem. The zinc is badly pitted, there is pitting on the pressure side of the prop tip and pretty equally spaced wear marks in the paint on both sides of the rudder. The paint is totally worn off on the leading edge starboard side. The wear is worse on the starboard side, which may be why my vibration was worse turning to starboard. I am going to try and attach some pictures.
The rudder is pretty tight but does have enough movement that the cavitation could be causing it to vibrate. The cutlass seems to be about the same as it was when launched. I will measure it as soon as I get a chance.
From what I read the solution to cavitation is increased blade area to reduce the blade loading unless there is something else causing it. Am I crazy to worry about increased drag with more blade area? Does anyone know if it could be cause by the closeness of the prop to the keel or the size of the nose (not sure what you call that thing) that houses the cutlass? It was built up some when the new cutlass was installed to give more beef to hold the setscrews.

[sharkbait]

I can see the prop is not centered in the aperture and that zinc has totally unbalanced everything.

[Maine Sail]

Thanks Phil, it is good to know that it is not a systemic problem with the CD31. What do you have for an engine, gear reduction and prop pitch? What rpm range do you run? With my old M25 I had the same as Dana, a 2:1 gear reduction with a 13 X 13 prop and had vibration with that as well.
Thanks for your suggestions Maine Sail. I will copy and take to my installer.
Evergreen was hauled yesterday and I think I have a prop cavitation issue which is what the yard that did the installation has been thinking might be the problem. The zinc is badly pitted, there is pitting on the pressure side of the prop tip and pretty equally spaced wear marks in the paint on both sides of the rudder. The paint is totally worn off on the leading edge starboard side. The wear is worse on the starboard side, which may be why my vibration was worse turning to starboard. I am going to try and attach some pictures.
The rudder is pretty tight but does have enough movement that the cavitation could be causing it to vibrate. The cutlass seems to be about the same as it was when launched. I will measure it as soon as I get a chance.
From what I read the solution to cavitation is increased blade area to reduce the blade loading unless there is something else causing it. Am I crazy to worry about increased drag with more blade area? Does anyone know if it could be cause by the closeness of the prop to the keel or the size of the nose (not sure what you call that thing) that houses the cutlass? It was built up some when the new cutlass was installed to give more beef to hold the setscrews.

That zinc is a big part of the problem. That thing is GINORMOUS and acts like an tire weight, only to unbalance everything, not balance it. Try using a "thin" zinc in front of the prop. They are available and will still allow enough water flow to the cutlass. I have used the BD-25's before, designed for Beneteau's, and they work well snugged up next to the prop. These are a 25mm zinc so they fit snug over the shaft but you can reem or sand them them slightly bigger. I'm not a huge fan of prop nut zincs and try to avoid the rocket or bullet style whenever possible.

BD-25 Beneteau shaft zinc: http://www.boatzincs.com/beneteau-donut ... specs.html

[Dean Abramson]

We also have no vibration issues on our CD31. I virtually always cruise at 2400 RPM with the M25. 3-blade prop. This boat has less vibration (and noise) than our old 25D w/ the Yanmar 1GM. I think motoring in our 31 is quite smooth.

I hope you can sort it out. I don't have a clue, but I'd lean towards a prop issue. Wish I could help.

Dean

[Dean Abramson]

Only just now, I saw the pix of the zinc. I agree that could be an issue. We use a doughnut 1" zinc in front of the prop. Now, looking at your photo, I am not sure it would fit.

Had a Perry Nut on the old 25D. It came loose once while motoring, but with the boat going straight, it could not escape. It made a hellacious racket, some vibration, and did some damage to the rudder. (I figured all this out later.) My wife steered while I looked things over, but then at some point we turned, and then the racket ended. I concluded later that when we turned, the zinc could finally be on its way to Davy Jones's Locker. There was no zinc at the end of the season, and the rudder was a bit chewed up just abaft the prop.

Perry Nuts: They're Expensive, but They're Crummy!

I would ditch that zinc before spending any more $ chasing a fix, and try again. Maybe that thing, besides being oversized and off-balance, is loose. In reverse, the water flow pins it to the prop, but in forward, the thing is trying to escape, but can't. Your zinc, when I look at how off-center it is at the end, it LOOKS loose. ??

Best of luck. Maybe this is a cheap fix.

Dean

[JWSutcliffe]

Looking at your picture I have to agree - the zinc has to be a large part of the problem. I use the same zinc arrangement, but with the zinc "grouper" I also use I never have that degree of electrolytic erosion on the prop nut zinc. In the marina we are in I cannot rely on a smaller zinc mounted forward of the prop - it would never last through the season. But with the additional zinc mass hanging overboard and well connected to the rest of the underwater metal (I have a good bonded wire from the after stay mount) the prop zinc doesn't take much of the electrolytic effect.

[moctrams]

I would advise you to buy the “Propeller Handbook”. After talking to a friend of mine who is considered a real prop Guru, regarding the vibration he referred me to Fig. 7.1 that shows the critical measurement for the prop distance to the hull and he came to the conclusion the prop does not meet the 3% distance from the hull. In other words, the vibration is caused by this 3% factor.