Droning vibration at 2700 rpm

mytime

mytime

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Aug 26, 2014
86
Hunter 2007 Marlow Hunter 33 Middle River, MD
I have a 1994 Catalina 30 with a Universal 25xp.
Over the winter I had:
  • A new prop shaft spun and balanced (Marina outsourced machine shop work, installed by marina)
  • 2 blade prop machined and balanced (Marina outsourced machine shop work, installed by marina)
  • New cutlass (installed by marina)
  • New dripless with gortex packing (installed by marina)
  • New motor mounts (done by a reputable mechanic)
  • Shaft to motor alignment (done in water by a reputable mechanic)
I took the boat out this weekend and it was smooth and sounded good up to 2500 rpm's, once you come above 2550 rpm's you start to here a droning type of vibration which gets significantly worse as rpm's increase. The noise was not there last year. I typically cruise at 2700 rpm's. There is no grinding or metal to metal type of noises. I ran the motor momentarily to 2700 rpm's and sounded smooth. I put my ear to the dripless and there was no noise there and it was cool to the touch. I put my head back in aft birth and it seems like it is originating in the area where the prop is.
Does anyone know what it could be and how to get rid of it? Is it ok to run the boat with the droning?
My first guess is an unbalanced prop or some sort of cavitation?
 
B

Benny17441

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May 24, 2004
7,213
CC 30 South Florida
Motor mounts? It is hard to harmonically balance a 3 cylinder engine across its entire RPM range. What is mostly done is to dampen vibrations in certain RPM ranges. The motor mounts play a role in dampening vibrations and the change to a new stiffer set may influence how engine vibrations are transmitted. We each interpret sounds differently and to me the sound of a droning engine does not raise any level of concern but I'm not there to hear your engine.
 
Last edited:
mytime

mytime

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Aug 26, 2014
86
Hunter 2007 Marlow Hunter 33 Middle River, MD
They were just installed last week by a reputable mechanic
 
kloudie1

kloudie1

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Nov 6, 2006
10,212
Hunter 34 Mandeville Louisiana
Could be something like the exhaust hose vibrating against a hull part or attachment ??
 
Ralph Johnstone

Ralph Johnstone

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Jan 4, 2006
7,641
Hunter 310 West Vancouver, B.C.
Maybe time to get sophisticated. Find a screwdriver that has a handle which fits comfortably into your outer ear. Then touch the tip of the screwdriver to the various points which may be emitting the noise. Crude, but often pinpoints the source of the noise.
 
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C

Capt jgw

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Feb 8, 2014
1,300
Columbia 36 Muskegon
Screwdriver works but a mechanic stethoscope is better. Harbor Freight has them pretty cheap. The long flexible hose is much safer. I don't like getting my head that close to moving machinery.
 
jibes138

jibes138

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Jan 27, 2008
3,092
ODay 35 Beaufort, NC
Engines have certain natural frequencies where they will vibrate more than normal, I have a M25 engine and it will vibrate at certain RPM's and then it goes away with a small change in throttle. Replacing all the stuff you did likely changed the natural frequency of the engine so it is resonating at 2700. Try going to 2800 or 2600 and see if the noise changes. As the bearing wears a bit and the mounts get older you may find it changing a little and will go back to the 2700 cruising speed..
 
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mytime

mytime

.
Aug 26, 2014
86
Hunter 2007 Marlow Hunter 33 Middle River, MD
Based on some of the forums..Many owners are happy @ 2500 and below....Am I just too aggressive for a 25 xp?
 
Ukeluthier

Ukeluthier

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Jan 7, 2015
77
Menger 19 Catboat Annapolis, MD
Jibes138 is on the right track. The new motor mounts will likely either be stiffer or slacker than the old, worn out ones, which will affect the way the engine's natural resonances are transmitted to the hull.
 
B

Benny17441

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May 24, 2004
7,213
CC 30 South Florida
mytime said:
Based on some of the forums..Many owners are happy @ 2500 and below....Am I just too aggressive for a 25 xp?
Click to expand
The engine is rated for 3200 RPM so I do not think you are being overly aggressive as long as your tachometer is accurate. If 2700 RPM is causing a vibration then choose a lower RPM range where the engine runs smoother.
 
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jibes138

jibes138

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Jan 27, 2008
3,092
ODay 35 Beaufort, NC
You don't necessarily have to go slower, faster will also move the system out of the natural frequency RPM and reduce the vibration, try both and see what works best
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
23,342
Catalina 34 224 Maple Bay, BC, Canada
Benny17441 said:
The engine is rated for 3200 RPM
Click to expand
Actually it's 3000 rpm.

Running at a constant rpm is not good for diesels anyway, so keep changing it until you find a few sweet spots. They all do that.

You might be interested in this:

Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too :)

http://www.c34.org/wiki/index.php?title=Diesel_Engine
 
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