When do you replace motor mounts?

[Richard Bryer]

I am about to replace the prop and shaft on our H34. I have had a past problem with a thumping noise that only happens after we have run under power for some time- 1 hr or so. happens when I throttle down then pick back up to ~2000rpm. Only way to get rid of it is to rev up through it, then back down. The shaft is old and when I had it checked a few years ago it was borderline, I have the shaft out, and although I thought the thumping sound was the shaft bumping on the shaft tube, there is no marks on the shaft at that point. The cutlass bearing appears evenly worn.

I am thinking about replacing the motor mounts as one other possible culprit. However the months for the Yanmar 3GMF are $+200 each so I want to be fairly sure I need new ones. How can I tell?
Thanks

 

[Don S/V ILLusion]

It is sometimes difficult to tell by visual inspection while at rest if one or more is gone. Often any cracks are very subtle and hard to see without actually removing them. One option is to inspect the "rubber" cushions to make sure one or more of the mounts is not bottomed out. It may also be possible to identify a mount problem by running the engine under load while sticking your head in the compartment to see what is actually happening.

 

[BarryL]

Hi,

I thought the best way to inspect mm's was to place the engine in gear and quickly throttle up. If the engine moves more than a little, the mounts are bad.

My surveyor did an 'emergency stop' by having the boat moving ahead under power at 5 kts or so, and then quickly shifting in reverse and applying full power. He said that this put the most strain on the mounts. My engine didn't move and the mounts are fine.

I did have thumping from the shaft if I left the engine in gear and we sailed at 5 kts or so. That was caused by a worn out cutless bearing. I changed the bearing (and shaft and coupler and shaft log and packing) and no more thumping.

Barry

 

[Steve Dion]

Richard:

I think that the OEM mounts are more like $140 ea (Torresen Marine). If you are not going to do it yourself you may want to ask your mechanic to check them out. Quite often you only need to replace one set (2 ea) and not all four.

 

[Joe]

Mainesail posted a very detailed article related to this issue a few days ago... check it out... lots of pictures.... and, as always, sound advice.

 

[Allan12210]

My engine was doing the thumping thng too. While running I would grab the heat exchanger and just pull the engine to the side...voila the sound would go away. Bad motor mounts. I wrote an article regarding how I changed mine as well as an alternate (read less expensive) motor mount. They look like the same one in Maine Sail's article of recent times. You can find my discussion at:

http://hunter.sailboatowners.com/in...&cat_id=1000&aid=7569&page=article&mn=General Boat Ownership

Allan

 

[Bob Mack]

Richard:

A few points about the Yanmar mounts:

As I remember, from my research and replacement of the 2GM mounts:
1. Beware of trying to use cheaper alternatives than the Yanmar mounts. I have read of several people who have had unsatisfactory results. You'll note the Yanmar mounts are much more "lively" than many alternate mounts.
2. If I remember correctly the aft and fore Yanmar mounts are unique and care has to be taken to install the correct mounts in the proper location.

 

[RichH]

I check the integrity of my engine mounts yearly. I use a small scissors jack and *slightly* lift each mount and watch for any separation of the rubber 'pad' versus the steel mating parts. If the rubber has become separated (bonding has come loose) from the steel mating parts, I replace immediately. PS - dont break the mount by applying too much 'lift' with a jack.

 

[rhythmsurfer]

Two of mine are broken on the boat I just bought. Both are on the starboard side. I personally would go out with them in bad shape. You an cause alot of damage to the shaft.

 

[Richard Bryer]

Thanks for all the input, guys. I will take a close look at them come some warmer weather. Steve: this discrepancy in US vs Cdn pricing is just one of many we experience up here, even with the $ at nearly par.
It sounds Like I need to have a close look while the engine is running. I was wondering if there was a way I could tell if the rubber had become "dead". I have not seen the mounts bottoming, and other than the situation I described, the motor runs extremely smoothly.