I have vibration at higher RPMs. Engine alignment is pretty good. New strut and cutless bearing. Can worn out engine mounts contribute, and if so, how are they checked? 1979 Hunter 30 with Yanmar 3GMD.
How Can I Tell if Engine Mounts are Shot?
- Thread starter tomgrigsby
- Start date
- Status
Other than cracked or broken, the other less obvious is compressed or bottomed-out. Look at a new one for comparison to see how they should look.
The attached pictures show mounts that were removed from my H37C with Yanmar 3QM30 engine. In the first one you can see the rubber block on the left side has bulged and distorted. The other one is in better condition.
The boatyard guru (who I trust) told me this is a sure sign that mount was going to give problems.
Suggest you do as mentioned above - see what a new one looks like and then with a good light and maybe a mirror inspect yours.
The boatyard guru (who I trust) told me this is a sure sign that mount was going to give problems.
Suggest you do as mentioned above - see what a new one looks like and then with a good light and maybe a mirror inspect yours.
Attachments
You have a 1979 boat. How long have you owned it? Do you have records from POs? Have the mounts ever been replaced? If no records, assume probably not and work from there. Also, visually watch you engine while it's running under load - either use your autopilot on a weekday, or have someone else drive the boat, and look at what's happening.
I have heard that the clearance between the bottom metal piece of the mount- the hockey stick shaped piece- and the end of the top piece that covrs the rubber blocks- should be at least 3/16 inch ( with the mounts supporting the engine). Not sure how credible this is but maybe makes sense?
Tom:
If you replace the mounts be sure that you use OEM mounts and have someone that knows what they are doing do the installation. The mounts are made different for the forward and aft sets.
You want to be sure that they are installed in the correct position.
If you replace the mounts be sure that you use OEM mounts and have someone that knows what they are doing do the installation. The mounts are made different for the forward and aft sets.
You want to be sure that they are installed in the correct position.
Why should you occasionally check the motor mounts? .... all 'plastics' by definition are materials that slowly and permanently deform over time when stressed - called 'creep', a continual change of dimensions when under stress. Rubber although having exceptional elastic properties is still subject to 'creep' - its really by definition, a 'plastic'. (Rubber, could also be be defined as a 'liquid' with exceptionally high viscosity)
So the answer is twofold:
1. Measure precisely the engine mount dimensions 'as new' when loaded by the engine weight, and compare later on in service (trivial answer but actually the 'easiest' way.).
2. Occasionally but regularly re-align the engine and its coupling, etc. .... because the heavy engine is continually 'sinking' into its 'rubber' engine mounts simply because 'loaded' rubber eventually 'creeps' because its a 'plastic'.
Went down and took another look after seeing pictures and reading the discussions. My mounts look very good, not deformed, plenty of clearance. Motor quite firm to atttempts to shake it by hand. I recall the PO telling me the boat had been repowered, but I don't recall if he told me exactly when. From what I've read, I don't think the 79 H30 came with a 3GMD. My slip neighbor has an 81 H30 and his as a 1GM. Looks like I need to take another look at alignment to reduce the vibration. Thanks all for the input. Very helpful.
Tom,
Vibrations can be more than just coupling to shaft alignments.
* Does the shaft pass through the cutlass bearing perfectly centered?
* Is it passing though the shaft log perfectly centered?
* Is the stuffing box "side loading" the shaft in any way?
* Where are you zincs on the shaft?
* How many zincs?
* How high on the mount bolts does the engine sit?
* How long is the shaft between the strut bearing and the coupling?
* Two blade or three blade?
* Has the prop been balanced?
* Has the prop been "lap fit" to the shaft?
* Is the key in the prop "key bound"?
* Is the shaft known to be straight/true?
* Folding, feathering or fixed prop?
* Is the gear box output flange true?
* Was the coupling "fit & faced" to the shaft?
* Has the coupling ever been removed & reinstalled?
* Split coupling or solid?
* Condition of the motor mounts?
* Are the motor mounts "pre-loaded"?
Shaft coupling to gear box flange are but 1/10th of the alignment and vibration elimination equation.
You can have a perfectly smooth drive line IF proper installation procedures have been followed. They RARELY ARE FOLLOWED and is why the vast majority of sailboats vibrate horribly....
The shaft hasn't been pulled for inspection. I did try to find any run out when I did my last alignment, rotating the shaft while measuring clearances to see if they changed. Seem to be pretty consistent IIRC. I'll pull it on my next haul out. Not looking forward to trying to get the flange off the shaft.
Normally you dont have to pull a shaft to determine developed run out ... just a few places measured with a dial indicator along the length of the shaft should be 'close enough' ... but best to do with the packing/stuffing box 'released', or with the packing removed, etc.
Myself I only pull a shaft when Im suspicious of 'galling' of the shaft at the stuffing box journal area or cutless bearing journal area.
Myself I only pull a shaft when Im suspicious of 'galling' of the shaft at the stuffing box journal area or cutless bearing journal area.
When was the last time you checked your prop for a buildup of marine growth. In my area I get everything from barnacles to oysters to sponges, you name it. None of these grow symetrically for good balance. You can also get what is known as a mass moment unbalance meaning the weight is on one side of the prop blade.
My area gets such growth in one week I will have a bunch of small barnacles, two months is a major diving excursion with a plastic ice scraper. I see you are in Virginia so you probably have similar conditions for marine growth to Beaufort. Not a good time of year to go swimming but take a good look at the prop to see what's up.
My area gets such growth in one week I will have a bunch of small barnacles, two months is a major diving excursion with a plastic ice scraper. I see you are in Virginia so you probably have similar conditions for marine growth to Beaufort. Not a good time of year to go swimming but take a good look at the prop to see what's up.
Last edited:
I realigned my engine/shaft recently. Made the vibration worst than before. Later noticed one of the rear engine mount was twisted. It wasn't damaged. I must have tighten the top nut and not realising the torque had put a twist to the rubber below. That probably throw the engine out-of wrack. Upon loosening the nut, the mount springs back to normal position. Now I hold the top plate of the mount firmly before tightening the nut. No more twist and vibration gone down alot. Anyone had same experience?
- Status