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Old 04-29-2021, 08:08 PM   #1
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When to Replace Engine Mounts on Yanmar 370on

I have a vibration problem on our 34' Mainship Pilot with a single Yanmar 370. Had the cutlass bearing replaced, re-bedded the rudder post, balanced the prop, and aligned the engine. Ran great for the rest of the year but the following season the vibration returned. . A boat yard recommended changing the engine mounts and stated on the Yanmars this should be done every 5 -7 years. Is replacing engine mounts a scheduled maintenance item like he indicated? Thanks!!
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Old 04-29-2021, 10:57 PM   #2
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I can't comment about that engine, specifically. But, in many cases, failure or impending failure is really obvious from the condition, position, and/or angle of the rubber part.

Any chance you can post pictures?

I don't know of anyone who swaps mounts every 5-7 years. Or, even close. I've seen boats with 40+ year old mounts.

But, it depends upon a lot of things.. engine and shaft alignment, cutlass and stern bearings, how the engine is run.

Get enough things pulling, pushing, or wobbling in different directions hard enough and they can wear out quick. Ditto if they were installed off at build, which happens.
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Old 04-29-2021, 11:43 PM   #3
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Do a search for Mainship Pilot 34 vibration. Apparently it's a known issue.
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Old 04-30-2021, 02:16 AM   #4
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This is from Rubber Design, a Dutch manufacturer of marine isolation mounts

"The life expectancy of the rubber elements will be approx. 20 years in ideal circumstances. Unfortunately ideal circumstances are not feasible, therefore the (working) life expectancy will be approx. 10 years. The life expectancy of the rubber elements is dependent on the environmental circumstances (weather influences, contaminants, etc)."

This does not mean the mounts will "fail," it means they will no longer isolate vibration within specification with any reliability 10+ years out.

You might check the mount under the injection pump as leaking fuel may have contaminated it and caused premature failure. Just sticking your finger into the rubber will tell you enough.

What Porman posted is true, some 34 Pilots do have a history of vibration, I believe centered around the prop tunnel and tip clearance, which is not uncommon in boats with tunnels

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Old 04-30-2021, 06:59 AM   #5
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What rpm do you get the vibration? The flexible mounts on that engine really only come into play at dead idle. The engine then has a "shake" from firing pulses. Above about 1000rpm those pulses diminish. At cruise rpm that engine is inherently balanced and basically creates very little vibes. The magic of a straight six.

So if you are getting vibes at power, it is caused by something other than the mounts. Unless one is obviously broken.

Mainships do have a reputation of harsh prop noise at planing/semi-planing speed.
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Old 04-30-2021, 08:01 AM   #6
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I owned a Pilot 34 for several years and researched the prop vibration problem and tried one fix that didn't work. But first do this simple test:

Run the boat up to fast cruising speed, say 14-15 kts at 2,800 rpms. Open the engine hatch and put your hand an some part of the engine that you can stand the heat. You should feel little vibration, just a buzz. As Ski notes above, a straight six is inherently smooth.

If that test is successful, ie no significant vibration, then run the speed up and down and tell us where it vibrates the most. For the ubiquitous prop cavitation vibration it will be at higher rpms- 2,800+.

On my boat that vibration would occur when the bottom or prop was a little fouled. A good diver cleaning job would solve it for a while.

I tried fairing the dead wood ahead of the prop with hopes that smoother water flow would solve it. It didn't. Some have reported it did help.

Some have tried a 5 blade prop I think but I have never heard reports of it working.

Good luck!

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Old 04-30-2021, 11:06 AM   #7
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Yes,

And especially w Yanmar mounts.
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Old 04-30-2021, 02:26 PM   #8
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Thanks for all of t he comments. The bottom hadn't been cleaned in a year so that may be a factor. Going to splash it next week and have an appointment at Zimmermans, Mathews yard. I guess they know what they are doing. Thanks!
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Old 05-18-2021, 11:55 AM   #9
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Just an update. When changing the zinc, the technician found that both prop nuts were barely handtight. They tightened the nuts and on a test run there was no vibration. Not sure if this was human error (unlikely) or if something eventually caused the prop nuts to loosen. A side note, the boat yard said I should be able to turn the shaft by hand where it is exposed below the transmission, which I couldn't budge it, and was hard to turn the prop when on the travel lift. Is this normal? Going to post this question under the engines topic. Thanks!
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Old 05-18-2021, 01:59 PM   #10
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new cutlass bearings will probably result in a tight clearance so hand turning shaft will be stiff.
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Old 05-19-2021, 05:53 AM   #11
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Quote:
Originally Posted by sammy999 View Post
A side note, the boat yard said I should be able to turn the shaft by hand where it is exposed below the transmission, which I couldn't budge it, and was hard to turn the prop when on the travel lift.
The boatyard is right
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