Some vibrations left...

[cmedsailor]

In the past I had lots of vibrations especially in high rpm. During the period the boat was out of the water I did the following:
- Changed the vetus front engine mounts with the Volvo ones (the vetus were relatively new and in great condition but since I had two brand new Volvo ones I bought in the past by mistake it cost nothing to replace them).
- Changed the cutlass bearing (the typical rubber style you find in lots of Beneteau) which was in a very bad condition and definitely causing vibrations
- Removed the shaft and checked that was perfectly straight
- Aligned the shaft – engine.
- Replaced the 2-blade prop with a 3-blade (2 blades behind the skeg could also be a reason for vibrations)

The boat is back in the water and things are much better. However I do get some vibrations at around 1100-1300rpm, less vibrations at idle (800-900rpm) and no vibrations above 1500rpm (even at 3000rpm). The engine is a volvo penta MD2030D.
Do you think these vibrations are now a matter of some fine tuning of the alignment, or does the current situation sounds normal? I know that many engines do have some specific rpm that “shake a little bit”. I cannot decide if it’s worth messing with the alignment again.
Thanks

 

[vyv_cox]

I have done just about everything possible to eliminate vibration at around engine 2000 rpm. Aquadrive, new shaft, new prop, new P-bracket, new cutless bearing, without total success. I haven't calculated it but I suspect that these revs correspond with the natural frequency of the shaft. Now I just avoid 2000 rpm.

 

[Neil_Y]

There will always be resonant frequencies when you have a structure that can flex or is on flexible mounts. Fitting hard as opposed to rubber shaft bearings has been known to reduce vibration/noise, however there are still some designers that will tell you rubber bearings help damp vibration.

If the boat has a P bracket make sure this is aligned as this is often something that is missed.

But sounds like you've done quite a thorough job.

 

[essexboy]

You could recheck the alignment now that the boat is back in the water, they alway alter a little.

 

[theoldsalt]

No engine mounts can isolate the boat from all frequencies produced by the engine - it's a compromise. Anything in the boat that has a natural frequency near those frequencies not isolated by the engine mounts will vibrate.

Most people seem to choose a cruising speed that gives a minimum of vibration.

I know that my cooker seems to love vibrating at a certain engine RPM so I cruise slightly above or below that speed.

 

[cmedsailor]

Continuing my old thread I can now verify 100% that any vibrations left at low rpm (from idle until around 1300) are from the engine. I mean even if I don't engage the gear they are there, so nothing to do with shaft/engine alignment.
Any ideas what I could try to reduce them?

 

[macnorton]

3 pot lumps will vibrate and you have to rate the engine mountings to tune the vibration to where you want it to be (at revs you rarely use), as yours are 1300 and below that should be ok, I have fitted a motorcycle steering damper to the head in the past which helped a little but is no cure.
Perkins have released an uprated injection system which helps (your engine is Perkins based?), I fitted one on a Perama (same engine?) and it has reduced vibration and fuel consumption.
You will need to re-check the shaft alignment now the boat is afloat as boats change shape when ashore.

 

[rob2]

Actually 3 pot lumps vibrate a hell of a lot less than a single or twin! Just watch a Yanmar 1GM rev up!

My mate's boat had a nasty resonance just above crusing revs, which luckily died away if you went to full throttle. It sounded like the shaft was coming to join you in the cockpit - yet we never found a "cause" other than the plywood cockpit sole seemed to like it.

Rob.

 

[sarabande]

as a raggie with an old Stuart Turner 5hp bolted to wooden bearers, can I ask if proper mobo engines have bearers or mountings designed to absorb a wide range of frequencies. It seems that any flexibility in engine mountings will allow flexure of the prop shaft - unless that unit is also mounted flexibly. And then that means the deadwood also needs tomove a bit.

Logically then, the whole engine, prop shaft, and prop should be in one organic unit quietly vibrating away on its own with no transmission to the hull or internal panels.

 

[Bosun Higgs]

If you still get the vibes when drive is not engaged, then it isnt alignment as you have said. Its clearly a resonance of some sort or it would be present throughout the rev range. So you need to check both the engine and surrounding bodywork to see if anything is vibr5ating badly at the key revs or is loose.

One issue people often forget when changing engine mounts is to get the engine sat equally on all 4. You can end up with three taking most of the load and one lightly loaded and that will give vibes.

 

[Tranona]

as a raggie with an old Stuart Turner 5hp bolted to wooden bearers, can I ask if proper mobo engines have bearers or mountings designed to absorb a wide range of frequencies. It seems that any flexibility in engine mountings will allow flexure of the prop shaft - unless that unit is also mounted flexibly. And then that means the deadwood also needs tomove a bit.

Logically then, the whole engine, prop shaft, and prop should be in one organic unit quietly vibrating away on its own with no transmission to the hull or internal panels.

It is - if you have a saildrive. One of the claimed advantages is that you can isolate the whole propulsion package from the hull structure through the mounts. Largely successful, although you still get some vibration transferred through connections such as cables, piping and the exhaust which are all attached to both the engine and the hull.

There are various strategies for reducing transmission of vibration on conventional drives from fully floating as you described, to more commonly flexibly mounted engine and flexible coupling to the shaft so that the engine can move relatively independent of the shaft. In both cases, as in the saildrive the thrust is through the engine mounts, so it is impossible to eliminate all vibrations. An alternative is to use something like an Aquadrive which takes the prop thrust fixed to the boat and the engine is attached to it through a flexible articulated joint. As vyv cox will tell you, having fitted one there are still some vibrations.