Vibration/ harmonics

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DaveJRH

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Hi all,
Had my Princess 33 Lily for a year in September and she has always had the following problem.
The 80hp Mermaids when both running create a vibration/harmonics at tick over and up to 1100 rpm, either in or out of drive. If I run just the port or starboard engine, either in or out of drive, either are as smooth as could be expected until I start the remaining engine. No problem at all over 1100rpm.
All the engine mounts are secure and intact, no misfire or excessive mechanical noise.
The vibration/harmonics is quite intrusive, indeed my wife finds being in the galley most uncomfortable.
Is this just normal for these engines ? which I have read as being somewhat "agricultural " or am I missing something obvious?
Many thanks in anticipation
Dave
 
Are the engine mounts the originals? If so they may have lost their flexibility and are transmitting more vibration through to the hull. Probably worth replacing them with something more modern and see if that improves things.
 
kashurst said:
Are the engine mounts the originals? If so they may have lost their flexibility and are transmitting more vibration through to the hull. Probably worth replacing them with something more modern and see if that improves things.
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Good point Kashurst, I shall take a couple off and compare their flexibility to a new one.
Thanks for the reply.
Dave
 
If you are going down the mounts route anyway - you could try changing just one set with a harder or softer set than the originals. A mis-match may help to get the harmonics out of synch.
 
gordmac said:
Does it happen only when both are at the same rpm? One at tickover, other1100rpm?
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It happens when both engines are between tickover and 1100rpm, but if I stop either engine in that rpm range the vibration ceases.
No direct mechanical connection between either engine....
 
DavidJ said:
I’m thinking the beats are a sound thing so can you improve/renew your exhausts
being a KAD person we don’t have exhaust pipes as such so I’m shooting blind.
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I believe the exhaust system is relatively new, the exhaust note appears to be similar from each engine.
I reckon engine mounts may well be the problem, especially if they are the originals which are almost 41 years old...
 
rafiki_ said:
Sounds like heterodyne to me. Heterodyne - Wikipedia
Just run the engines at slightly different revs and this should reduce.
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Yes, that's precisely what I do to remove the Heterodyne effect now
Just wondered if I could remove the effect and have both engines running smoothly below 1100 rpm.
I shall investigate the engine mounts further and imagine that a new set will most probably cure the problem.
Thanks for your reply
Dave
 
Do the engines vibrate excessively when in harmonic or is it just the noise with panels etc shaking? Mine will at low revs create a harmonic that will make the admiral feel ill but the engines aren't vibrating. As such I did not believe it was the mounts and tuned the idle revs to match, ensured the same volvo alternator pulley and put in a analogue synchroniser. Job done. When the engines are synchronised the harmonics go and everyone is happy.
 
BruceK said:
Do the engines vibrate excessively when in harmonic or is it just the noise with panels etc shaking? Mine will at low revs create a harmonic that will make the admiral feel ill but the engines aren't vibrating. As such I did not believe it was the mounts and tuned the idle revs to match, ensured the same volvo alternator pulley and put in a analogue synchroniser. Job done. When the engines are synchronised the harmonics go and everyone is happy.
Click to expand...
It appears to be engine harmonics, because I can improve matters by increasing or decreasing rpm on individual engines. When navigating passed moored craft on a narrow river I cut one engine and this is pure bliss.
My Admiral has suggested we continue to do this rather than change eight engine mounts.
We shall see....
 
DaveJRH said:
It appears to be engine harmonics, because I can improve matters by increasing or decreasing rpm on individual engines. When navigating passed moored craft on a narrow river I cut one engine and this is pure bliss.
My Admiral has suggested we continue to do this rather than change eight engine mounts.
We shall see....
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I honestly recommend you try what I did to resolve mine, especially setting idle revs and control throws exact. Once done it's a pleasure to cruise at low speeds. In addition as hard as it is to believe the engines sound quieter. I removed the old speedo and spitot tube which were redundant for the sync (below)

GQHi9tN.jpg
 
BruceK said:
I honestly recommend you try what I did to resolve mine, especially setting idle revs and control throws exact. Once done it's a pleasure to cruise at low speeds. In addition as hard as it is to believe the engines sound quieter. I removed the old speedo and spitot tube which were redundant for the sync (below)

GQHi9tN.jpg
Click to expand...
Hi Bruce,
I shall certainly try your solution the next time I visit.
I shall report back accordingly
Thanks
Dave
 
BruceK said:
Do the engines vibrate excessively when in harmonic or is it just the noise with panels etc shaking? Mine will at low revs create a harmonic that will make the admiral feel ill but the engines aren't vibrating. As such I did not believe it was the mounts and tuned the idle revs to match, ensured the same volvo alternator pulley and put in a analogue synchroniser. Job done. When the engines are synchronised the harmonics go and everyone is happy.
Click to expand...

I am interested in your analogue synchroniser Bruce. Can you share details please?
.
 
There are two types of analogue sync gauges. Magnetic pickup and W terminal pulse from alternator. As my tacho's are driven by the alternator the first thing to do is ensure that the alternator pulley's are an exact match. Volvo supply the pulley with the correct reduction for your tacko's so use that. While doing that calibrate your tacho's with a hand held rev counter although this is not essential for the sync it helps you co-ordinate the throttles to sync. Also important is to set throttle throws to be uniform and that both engines idle at the same speed. Again this is just to assist you. I personally find that when both throttles are exactly matched it does not fit my hand as I increase revs and prefer the stbd lever to lead slightly. Visually it also looks straighter then. A bit of a faff but worth the effort if you are going to be in sea states that require frequent throttle changes.
As for the W terminal tacho installation is super simple. Instrument lights is obvious, so is positive to ignition and negative. Then there are two W terminals port and starboard that piggy back onto each tacho W feed with a split connector. Very simple.
ASAP supplies do sell some gauges but they change periodically. I special ordered by vdo gauge from them and it had a 8 week lead time. Use is rather simple, balance the needle. This is where calibrating everything comes into play. If you did a good job you are almost always in sync anyway. Finally, I used to sync by ear like tuning a guitar. That works for higher revs like a charm. Lower revs harmonics I was surprised to find were quite off from my ear. Because the % difference in revs is inversely logarithmic the instrument is quite sensitive to low revs and so easier to finely tune as the needle moves more.
 
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