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Old 20-12-2015, 19:55   #1
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FLEXIBLE OR SOLID MOUNTS

We have developed a leak at out stern tube and suspect that the mounting bolts at the tube have worked loose. Our 3GM30F is mounted with original Yanmar flex engine mounts. The mounts are probably 12 years old and on startup the tranny does move about a small bit, shaking the shaft, but no more than in the past. It smooths out as things warm up. I guess it may be time to replace the mounts. Today a mechanic looked at the system and suggested replacing the mounts with solid units to remove the shaking. Is that even possible or recommended. I always thought FLEXIBLE was essential especially on diesels. If so what happens to the vibes and movement which must certainly still be there? Should I find a new mech?
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Old 21-12-2015, 03:48   #2
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Re: FLEXABLE OR SOLID MOUNTS

Time, oil and water all cause Yanmar mounts to soften. Solid mounting is hard to live with in pleasure Craft due mechanical noise transfer and vibration.
1. Is there a shaft flexible coupling ?
2. What size shaft? Could be to small.
3. Length of unsupported shaft between Gland and Gearbox Coupling? Could be over Spec For Shaft material & Dia.
Take a look at the polyflex mounts web site they make direct change over mounts and couplings to suit, they are much stiffer than the original yet still dampen the nasties.
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Old 21-12-2015, 04:16   #3
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Re: FLEXABLE OR SOLID MOUNTS

I just swapped our hard mounts for soft mounts and I sure wouldn't go back.

I'd be replacing the old mounts with the proper new soft mounts on the basis that it used to work fine and it should do again.


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Old 21-12-2015, 04:52   #4
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Re: FLEXABLE OR SOLID MOUNTS

Like GIlow said, if the first ones lasted 12 years the next ones should too.
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Old 21-12-2015, 05:39   #5
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Re: FLEXIBLE OR SOLID MOUNTS

Take Gilow's advice... Consider them a consumable part like a belt, and when they aren't it's simply a bonus...
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Old 21-12-2015, 08:07   #6
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Re: FLEXABLE OR SOLID MOUNTS

Quote:
Originally Posted by shakey doug View Post
Time, oil and water all cause Yanmar mounts to soften. Solid mounting is hard to live with in pleasure Craft due mechanical noise transfer and vibration.
1. Is there a shaft flexible coupling ?
2. What size shaft? Could be to small.
3. Length of unsupported shaft between Gland and Gearbox Coupling? Could be over Spec For Shaft material & Dia.
Take a look at the polyflex mounts web site they make direct change over mounts and couplings to suit, they are much stiffer than the original yet still dampen the nasties.
Thanks to all for your replies.

Shakes Doug,
1. There is no flex shaft mount, it is hard mounted to tranny.
2. The shaft is stainless and is
1 1/4"
3. The shaft is right at 35" from trans coupling to wet stuffing box.
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Old 21-12-2015, 09:27   #7
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Re: FLEXIBLE OR SOLID MOUNTS

No way solid mounts! the vibration and harmonics produced will cause cracking along with noise as the hull acting like a sound board. I recently replaced my engine mounts with K50's on the back and K-75's on the front, runs like a dream.
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Old 21-12-2015, 09:41   #8
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Re: FLEXIBLE OR SOLID MOUNTS

Never heard of using solid mounts on any engine. Otherwise you could just bolt the engine directly on the bed. Mounts are suppose to absorb the vibrations. Nothing particularly wrong with going to solid, stationary mounts with regards to the engine. perhaps he was concerned about vibrating stresses on the drive shaft and prop shaft, but you should have a connector with a slip disk between the two.

Downside will be more noise and vibrations being transmitted from the engine to the entire hull. However maybe your bed has built in absorbers that will reduce this problem.

In any event, ask him why solid and not flexible. He might have a good reason.
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Old 21-12-2015, 11:59   #9
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Re: FLEXIBLE OR SOLID MOUNTS

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Originally Posted by reed1v View Post
Never heard of using solid mounts on any engine. Otherwise you could just bolt the engine directly on the bed. Mounts are suppose to absorb the vibrations. Nothing particularly wrong with going to solid, stationary mounts with regards to the engine. perhaps he was concerned about vibrating stresses on the drive shaft and prop shaft, but you should have a connector with a slip disk between the two.

Downside will be more noise and vibrations being transmitted from the engine to the entire hull. However maybe your bed has built in absorbers that will reduce this problem.

In any event, ask him why solid and not flexible. He might have a good reason.
Many commercial boats bolt their engines solid to steel stringers which are in turn bolted solidly to hull stringers.This prevents any movement of engine or shaft,especially during hard accel/deccel maneuvers common in work boats. You would be replacing flex mounts regularly if used.
However,there are ripples in your teacup & some teeth chattering at certain RPM.

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Old 21-12-2015, 12:25   #10
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Re: FLEXIBLE OR SOLID MOUNTS

Quote:
Originally Posted by reed1v View Post
Never heard of using solid mounts on any engine. Otherwise you could just bolt the engine directly on the bed. Mounts are suppose to absorb the vibrations. Nothing particularly wrong with going to solid, stationary mounts with regards to the engine. perhaps he was concerned about vibrating stresses on the drive shaft and prop shaft, but you should have a connector with a slip disk between the two.

Downside will be more noise and vibrations being transmitted from the engine to the entire hull. However maybe your bed has built in absorbers that will reduce this problem.

In any event, ask him why solid and not flexible. He might have a good reason.
Many, many commercial/fish boats have solid mounts, and have had since the first engines. Try talking an old timer into changing to flexible mounts...

A friend of mine who works only on fish boats did a test using dial indicators after switching a Detroit 6-71 to flex/soft mounts and he said the movement was negligible. And the benefits were well worth the change.
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Old 21-12-2015, 12:46   #11
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Re: FLEXIBLE OR SOLID MOUNTS

I had a 3GM mounted to the stringers directly. There was not enough room to use the flex mounts in the repower. It may have been a bit louder, but it worked fine for the 15 years I owned the boat.
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Old 21-12-2015, 13:01   #12
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Re: FLEXIBLE OR SOLID MOUNTS

Quote:
Originally Posted by deblen View Post
Many commercial boats bolt their engines solid to steel stringers which are in turn bolted solidly to hull stringers.This prevents any movement of engine or shaft,especially during hard accel/deccel maneuvers common in work boats. You would be replacing flex mounts regularly if used.
However,there are ripples in your teacup & some teeth chattering at certain RPM.
:

This was exactly how we were set up and talk about teeth chattering, at some revs it could remove your fillings. I agree it was a good simple and reliable way of attaching the engine but it was truly horrible to live with.



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Old 21-12-2015, 20:03   #13
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Re: FLEXIBLE OR SOLID MOUNTS

I tried the R&D Mounts and went back to new Yanmar mounts. The old mounts and cutlass lasted 3000 hours and the harmonics are so much better with the factory mounts.
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