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Old 11-11-2022, 05:39 PM   #1
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MS 390 Vibration

Wondering if anyone else has seen this issue.

We had the marine survey yesterday on a 2000 MainShip 390, single Cat 3116 with around 1950 hours on it.

Last week the prop was serviced, the shaft alignment was checked, new bearings and dripless shaft seal.

During the survey, we had very smooth running up until around 1900 RPM. Significant vibration started at that point, then smoothed out again around 2400 RPM. WOT was around 2800 I think.

My first guess is that the prop was not seated properly. Next likely problem might be shaft alignment with transmission. Surveyor verified the rudder was in good shape before boat was launched. No obvious engine mount issues (per surveyor).

Otherwise, the boat is in very good shape. Hate to have this derail the purchase.

Any other ideas?

THANKS
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Old 11-11-2022, 06:49 PM   #2
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You said they did a shaft alignment but did they do an engine alignment?
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Old 11-12-2022, 04:35 PM   #3
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Thanks - I will have to check on that. The current owner had all of the work done, so I don't know exactly what the complete list was.
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Old 11-12-2022, 07:34 PM   #4
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Just because a marine surveyor says something doesn't necessarily mean it is fact. The survey could say the rudder is good or the engine mounts look OK but they still may be your problem.

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Old 11-13-2022, 09:36 AM   #5
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Originally Posted by Souvenir View Post
Wondering if anyone else has seen this issue.

We had the marine survey yesterday on a 2000 MainShip 390, single Cat 3116 with around 1950 hours on it.

Last week the prop was serviced, the shaft alignment was checked, new bearings and dripless shaft seal.

During the survey, we had very smooth running up until around 1900 RPM. Significant vibration started at that point, then smoothed out again around 2400 RPM. WOT was around 2800 I think.

My first guess is that the prop was not seated properly. Next likely problem might be shaft alignment with transmission. Surveyor verified the rudder was in good shape before boat was launched. No obvious engine mount issues (per surveyor).

Otherwise, the boat is in very good shape. Hate to have this derail the purchase.

Any other ideas?

THANKS
I went thru this same issue with my 2004 Mainship 400.
We balanced the prop twice (first guy didn’t do it right), shaft once, then new shaft, new coupler, new motor mounts, engine alignment 2-3 times. It got better but we Still had some vibration. However even after 2 balances when I changed the prop to a newer (used) same diameter but pitched a little higher (old one was 19.5 new one at 21 and other single MS400s had props pitched at 21-22 with different engines vs single Yanmar 370 I had) 99% of the vibration went away. I also noticed my “newer” prop the blade tips seemed thicker than the orginal. I wondered if it had anything to do with it at all. To me, having sold a lot of these 400s (I’m a yachtbroker also), This also seems to be more noticeable on the singles than twins. I’d start with prop balancing and see if that does it for you.
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Old 11-13-2022, 11:43 AM   #6
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Unfortunately, there are so many players that can be the culprit here. My neighbor hit a rock in SF Bay 3 years ago. Boat went in for repair. When it came out, yup a vibration. The boat has been in and out of the shop most of the time since, looking at all the items cited here. Hope you find the issue to resolve your vibration. Let us know what you find.
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Old 11-13-2022, 11:48 AM   #7
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Unfortunately vibrations like these are really tough to find. I think it may come down to rechecking all of the things mentioned here because you don’t know for sure that they were done correctly the first time.
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Old 11-13-2022, 08:18 PM   #8
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Unfortunately, there are so many players that can be the culprit here.
The entire driveline is the potential source. Because the vibration is at a specific RMP it's probably in resonance at that speed with something. A sure but probably not inexpensive way to find the culprit would be to use a company that specializes in vibration analysis. They would put some accelerometers in various places and graph the vibrations on a computer. Knowing the rpm, reduction ratio and the number of prop blades they could then determine if the vibration is blade rate, shaft rate, firing rate or gear rate. That should narrow the potential culprits.
You might post here and on Mainship FB page to ask to borrow someone's spare prop, or maybe a prop on a boat hauled out for winter. At least that "might" eliminate the prop

Good Luck, chasing these shake n bake gremlins can be frustrating

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Old 11-16-2022, 12:35 PM   #9
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I have successfully found sources of vibrations by using a “redneck” method someone taught me.
Take a rocks size glass and fill it half full with water. Place the glass at various locations. The spot that makes the water vibrate the most is usually closest to the source.
Sometimes the glass may bounce and empty most of the water.
Place the glass on the cockpit floor above the prop. If that is the source it may be your hull “oil canning” because of too little prop to hull clearance.
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Old 11-16-2022, 03:28 PM   #10
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Sometimes a stethoscope or even a screwdriver placed on different areas make it where you can hear the vibration.
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Old 11-16-2022, 03:30 PM   #11
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I have successfully found sources of vibrations by using a “redneck” method someone taught me.
Take a rocks size glass and fill it half full with water. Place the glass at various locations. The spot that makes the water vibrate the most is usually closest to the source.
Sometimes the glass may bounce and empty most of the water.
Place the glass on the cockpit floor above the prop. If that is the source it may be your hull “oil canning” because of too little prop to hull clearance.
You want a minimum of 15% of the prop diameter clearance between the tip of the prop and the hull. 20% is better.
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Old 11-23-2022, 11:47 AM   #12
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Quote:
Originally Posted by jleonard View Post
I have successfully found sources of vibrations by using a “redneck” method someone taught me.
Take a rocks size glass and fill it half full with water. Place the glass at various locations. The spot that makes the water vibrate the most is usually closest to the source.
Sometimes the glass may bounce and empty most of the water.
Place the glass on the cockpit floor above the prop. If that is the source it may be your hull “oil canning” because of too little prop to hull clearance.
I really like this idea, simple and easy. I would just improve a bit. There are now so many cheap CCTV systems out, I would mount a camera viewing each glass. So, you can go on a run, record each station, including your tach/speed/instruments and record the whole thing to sit, playback and analyze back at the shop/home. This would be a great tool. You could send data to any number of experts (like the guys on this forum) for your answers. I like it!
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Old 11-25-2022, 05:05 PM   #13
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Engine to shaft or shaft to bearings misalignment rarely causes vibration because it is constant rather than eccentric. While misalignment is an issue that should be checked, it's a rabbit hole one should probably not go down when diagnosing a vibration.

If the prop is binding, i.e., it is not fully engaged with the shaft taper, that can cause a vibration but it usually gets worse as rpm increases.

As others have said, there are many possibilities. Usually things that are eccentric, a bent prop or shaft or not fully seated prop, increase in intensity as rpm increases. However, a motor mount issue, a collapsed mount or mounts that are causing the engine to teeter diagonally, can cause resonance at a narrow band of rpm. I would at least check the mounts closely, make sure they are not collapse, and that they are not teetering (better mounts have an indicator to determine loading, so they can be evenly loaded, Barry Controls is one brand that has this feature, but it's not mandatory). Prop balance is another obvious culprit and one shop can get it wrong, so that could be re-checked too.

Beyond that, you could throw a lot of money at resolving a vibration using the guess and check approach. After the obvious culprits have been ruled out, having a vibration analyst come in to pinpoint the source is often money well spent.

Some potentially useful articles...

https://stevedmarineconsulting.com/t...aft-alignment/

https://stevedmarineconsulting.com/c...g-etiquette-2/

https://stevedmarineconsulting.com/w...llation-02.pdf

https://stevedmarineconsulting.com/w...ignment-05.pdf
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Old 11-25-2022, 05:43 PM   #14
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How long is the shaft does it have 2 struts. I had a 2000 430 and it had a pounding shaft whip from day one. Had it not been for another 430 owner that was and engineer it would have never been fixed. Turned out they sent 2 engineers to his boat to look at his boat. I was there. It was a whole day with a oscilloscope and a vibration puck. They even pulled the boat and did some tests. At the end of the day they admitted the boat needed a second strut the shaft was stretched to far. According to ABYC. This was after a year of fighting with them. After the second strut was installed it was very smooth.
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Old 11-25-2022, 07:14 PM   #15
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Complicated but most likely the prop cavitation, I went through a number of props until I got ones I like. 3 blade, 4 blade finally a 5 blade a inch bigger with more pitch. It runs fine to 1,600 RPM. Then it "whistles" a high pitch ringing sound that goes away at 2200 RPM, This is a sweet spot with low vibration and best performance. Many experts offered their opinions in the process, It is expensive to experiment.
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Old 11-25-2022, 08:06 PM   #16
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How long is the shaft does it have 2 struts. I had a 2000 430 and it had a pounding shaft whip from day one. Had it not been for another 430 owner that was and engineer it would have never been fixed. Turned out they sent 2 engineers to his boat to look at his boat. I was there. It was a whole day with a oscilloscope and a vibration puck. They even pulled the boat and did some tests. At the end of the day they admitted the boat needed a second strut the shaft was stretched to far. According to ABYC. This was after a year of fighting with them. After the second strut was installed it was very smooth.
I have encountered that as well. Max allowable shaft length between supports is 40 shaft diameters.
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Old 11-25-2022, 08:47 PM   #17
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I have encountered that as well. Max allowable shaft length between supports is 40 shaft diameters.
Mainship sent me 2 5 blade props it did nothing. I put 2 Auqamet 22 shafts in.
When the guy came to put in the 2 struts he couldn't believe how close i had the alignment. When you were on the bridge it felt like somebody was tapping on the floor with a hammer. The shaft was whipping. I know they monitored the Mainship site I mentioned it in there one day and 2 days later I had a certified letter in the mail. They didn't want that out.
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Old 11-26-2022, 09:43 PM   #18
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I had a similar experience and did not purchase the Mainship.
If the problem were caused by misalignment anywhere in the engine-gearbox- shaft then the vibration would be evident at ALL rpm and ALL speeds.
I ran a sea trial of a Mainship some years ago.
During sea trial with surveyor (and seller owner) on board, experienced the same issue. This was a Mainship with single large Cummins. The vibration is caused by cavitation.
Mainship built the vessel with a semi immersed tunnel in which the propeller is mounted. This tunnel does not pass forward sufficiently enough and is the cause of the problemm As the revs and speed increase, the bow lifts and stern lowers. At a critical angle, the water cannot flow into the front of the tunnel and causes cavitation by starvation of water entering the propeller disc. At increased rpm and increased speed the bow will lift higher. With increased planing angle and increased attitude this changes the angle of the tunnel sufficiently to allow water to enter the semi tunnel and the propeller can bite smoothly into the water. For this reason I did not purchase this boat.
I had a 46 ft Jenneau which has twin Cummins 485 hp engines - also with the propellers set into tunnels in the hull. This was a full planing hull. When the throttles were opened, the boat would accelerate onto the plane - however as the speed increased and the angle of the boat lifted to full planing attitude - there was always a noticeable vibration from the propeller due to the same issue - starvation of water unable to enter the tunnel. This was a very minor (but noticeable) event - however with a planing hull you are at displacement speed or at planing speed and the transition through the vibration speed was reasonably quick.
The Mainship is a semi planing hull and it is possible to hold rpm and speed where the starvation of propeller is significantly ongoing.

One other problem I found with the Mainship was significant instability of the boat at higher speeds. It felt very twitchy and tender when run at speed. In a heavy sea with the bow raised and at speed the bow could easily fall off is a sideways wave action.
Cavitation caused by the semi immersed propeller in the tunnel is the cause of vibration.
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Old 11-27-2022, 04:20 AM   #19
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Could be the rear bearing on the Velvet drive failing at high RPM, but your low hrs is on your side
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Old 11-28-2022, 11:01 AM   #20
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I had a similar experience and did not purchase the Mainship.
If the problem were caused by misalignment anywhere in the engine-gearbox- shaft then the vibration would be evident at ALL rpm and ALL speeds.
I ran a sea trial of a Mainship some years ago.
During sea trial with surveyor (and seller owner) on board, experienced the same issue. This was a Mainship with single large Cummins. The vibration is caused by cavitation.
Mainship built the vessel with a semi immersed tunnel in which the propeller is mounted. This tunnel does not pass forward sufficiently enough and is the cause of the problemm As the revs and speed increase, the bow lifts and stern lowers. At a critical angle, the water cannot flow into the front of the tunnel and causes cavitation by starvation of water entering the propeller disc. At increased rpm and increased speed the bow will lift higher. With increased planing angle and increased attitude this changes the angle of the tunnel sufficiently to allow water to enter the semi tunnel and the propeller can bite smoothly into the water. For this reason I did not purchase this boat.
I had a 46 ft Jenneau which has twin Cummins 485 hp engines - also with the propellers set into tunnels in the hull. This was a full planing hull. When the throttles were opened, the boat would accelerate onto the plane - however as the speed increased and the angle of the boat lifted to full planing attitude - there was always a noticeable vibration from the propeller due to the same issue - starvation of water unable to enter the tunnel. This was a very minor (but noticeable) event - however with a planing hull you are at displacement speed or at planing speed and the transition through the vibration speed was reasonably quick.
The Mainship is a semi planing hull and it is possible to hold rpm and speed where the starvation of propeller is significantly ongoing.

One other problem I found with the Mainship was significant instability of the boat at higher speeds. It felt very twitchy and tender when run at speed. In a heavy sea with the bow raised and at speed the bow could easily fall off is a sideways wave action.
Cavitation caused by the semi immersed propeller in the tunnel is the cause of vibration.
The 390 has no tunnel underneath it is flat you must have been testing a 30 or 34
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