Periodic Engine Vibrations

[klubko]

Hello,
we've got a new propeller (fixed 3-blade), which is supposed to be a copy of our old one. The engine is now vibrating every 2-3 seconds. All RPMs. Vibrations are worse in bigger waves. The prop doesn't seem to be key-bound. What other reasons could there be?
Many thanks for any insight!

[Rapanui]

Assuming you have a conventional prop shaft, it sounds like it could be a shaft vibration triggered by some out of balance in the propeller. The out of balance which could be difference in pitch between the blades, static mass imbalance or dynamic imbalance. Or you just could have picked up damage or something round the prop. Whatever the cause, that can trigger a resonance in the shaft/prop system.
If this never ever happened even momentarily with the old prop then my guess is that the propeller has a significantly different mass and/or moment of inertia which has shifted the resonance into the running range.
My suggestion would be to take the prop off and get the pitch checked on each blade at several diameters. Also, weigh the prop compared to old one and try and get the prop static and dynamic balance checked maybe at a tyre shop.
Although I used to do this sort of trouble shooting for patrol boats, ferries and frigates, that usually followed exhaustive measurements and analysis. My suggestions here are based on very sketchy info and my experience. It could be that other things have changed in parallel to make a nonsense of my castle built on quicksand

[ccgarnaal]

Before you take the boat out again check the engine - prop shaft alignment. That's the number one cause of vibrations.

[Lajos]

It the alignment is off that means the engine shifted, if the engine shifted it could be engine supports?...

[ccgarnaal]

I'm unfamiliar with this case, did you have any problems before or did they start after the new prop. Did they only replace the prop or also the shaft or stuffing box?

If they only replaced the prop and didn't have to force anything on the shaft and you didn't have any problems before then its the prop.

And yes sometimes the engine shifts, all it needs is 0.2mm and its too much.
Usually because the vibration dampers are getting old and the engine sacs in lower. On a new installation its even recommended setting the engine 0.1-.02 mm to high to anticipate the aging
Sometimes because the boat deformed over time (wooden boats that get whet inside have this problem). Or the boat was deformed during the alignment. This happens when the alignment is done on the yard with the boat on the dry.
Because its standing on its keel- support its slightly bend compared to when it is in the water. Alignment should always be rechecked once in the water.

[klubko]

Thank you all.
The vibrations started right after we switched the prop. The boat was in the water the whole time. Nothing else was changed. That's why we suspect that it must the propeller.
In fact, the guy that made the prop for us, didn't get the taper right the first time. In fact I was able to convince him that the tapper is of by using calipers. When he adjusted the taper, I was able to slide to prop on to the shaft without problems. Not sure if this tells something about that guy. Or is it generally difficult to make a copy of an old prop?
Since the boat was in the water, he didn't have access to our shaft.
Naturally he claims the propeller is balanced and all that. So I guess that now we should take the propeller to someone else and have it checked. Then we will realign the engine.

[klubko]

Btw. he has impressed the pitch and diameter on the body of the propeller between two blades. I suppose that's normal and cannot be the cause of our problem, right?

[Rapanui]

It is normal to stamp the pitch and diameter.
The additional information regarding the taper is interesting. It seems to me that getting a perfect fit on the taper presumably from underwater measurements is asking a lot so the chances are its not a perfect fit. If the prop can move even slightly then I can see it exhibiting the out of balance characteristics of the prop wobbling and hence exciting the drive train in the manner I previously suggested. That would be worth a look to see if there is movement even very small. Otherwise, get the prop off and test it for pitch, static and dynamic imbalance.
Good luck

[nhschneider]

My guess is that the new prop could be causing the vibration even though there's nothing wrong with it. I've been on plenty of ships, motor yachts, even aircraft carriers that would have a cyclical vibration. Engine realignment might take care of it but it might not. Even changing to a different prop might not solve the problem, but, then again, it might.

Fair winds and calm seas.

[GordMay]

Originally Posted by Rapanui

... It seems to me that getting a perfect fit on the taper presumably from underwater measurements is asking a lot so the chances are its not a perfect fit. If the prop can move even slightly then I can see it exhibiting the out of balance characteristics of the prop wobbling and hence exciting the drive train in the manner I previously suggested. That would be worth a look to see if there is movement even very small. Otherwise, get the prop off and test it for pitch, static and dynamic imbalance.
Good luck

It “shouldn’t” be that difficult, unless you have a non-standard shaft taper.

ABYC Standard P-6 Shafting (Appendix B) specifies that tapers should be as specified in SAE J755 Marine Propeller-shaft Ends And Hubs.

SAE J755 specifies 1 in 16 up to and including 5 1/2" diameter. A 1 in 16 taper is 3/4" inch to the foot.

[Ziggy]

Originally Posted by klubko

Hello,
we've got a new propeller (fixed 3-blade), which is supposed to be a copy of our old one. The engine is now vibrating every 2-3 seconds. All RPMs. Vibrations are worse in bigger waves. The prop doesn't seem to be key-bound. What other reasons could there be?
Many thanks for any insight!

What I find very strange is that the vibration comes and goes every 2-3 seconds. Does the frequency of vibration depend on the rpm?

[klubko]

We were finally able to take the prop off and took it to another prop manufacturer. One blade was way longer than the other too. Little grinding did the trick, so it seems. We've tested in calm water and most of the vibrations are gone. Naturally we are now fairly paranoid and any vibration seems wrong... We will have an experienced mechanic have a look just to be sure, but already the world seems a better place
Thank you all for the input

[Ziggy]

Glad it worked out for you!

[GordMay]

Originally Posted by Rapanui

It is normal to stamp the pitch and diameter.
The additional information regarding the taper is interesting. It seems to me that getting a perfect fit on the taper presumably from underwater measurements is asking a lot so the chances are its not a perfect fit. If the prop can move even slightly then I can see it exhibiting the out of balance characteristics of the prop wobbling and hence exciting the drive train in the manner I previously suggested. That would be worth a look to see if there is movement even very small. Otherwise, get the prop off and test it for pitch, static and dynamic imbalance.
Good luck

Originally Posted by GordMay

It “shouldn’t” be that difficult, unless you have a non-standard shaft taper.

ABYC Standard P-6 Shafting (Appendix B) specifies that tapers should be as specified in SAE J755 Marine Propeller-shaft Ends And Hubs.

SAE J755 specifies 1 in 16 up to and including 5 1/2" diameter. A 1 in 16 taper is 3/4" inch to the foot.

Received this PM from NCboatrx:

“Careful Gord. Regarding the post by Klubko about engine vibration. The shaft taper that is used on American boats is only used on boats built here. In Europe there are completely different tapers and these are used world wide but not here. The boat in question is a Halberg Rassy so if the shaft is still the original one, the taper will be 1 in 10 or 1 in 12 depending on the age of the vessel.
Stanley”

Thanks for the correction Stanley (NCboatrx).

European Propeller Hub Tapers
1:10 ~ ISO 4566
1:12 ~ ISO 8845

➥ http://pdf.nauticexpo.com/pdf/clemen...30-401-_3.html