Prop Shaft Coupling

[Daverw]

Hi, I have a Yanmar Gm20 fitted to our boat which is mounted on flexible engine mounts, the prop shaft runs from the directly into the stern tube and to p bracket and prop.

Boat is Kelt 29

The shaft coupling is a two part 4 hole flange with no flexible insert.

The stern gland is a rotating bellow type.

I'm seeing vibration on the prop shaft and cannot see how you can have flexible engine mounts and fixed shaft, seems the wrong way round.

Any thoughts

 

[rogerthebodger]

Hi, I have a Yanmar Gm20 fitted to our boat which is mounted on flexible engine mounts, the prop shaft runs from the directly into the stern tube and to p bracket and prop.

Boat is Kelt 29

The shaft coupling is a two part 4 hole flange with no flexible insert.

The stern gland is a rotating bellow type.

I'm seeing vibration on the prop shaft and cannot see how you can have flexible engine mounts and fixed shaft, seems the wrong way round.

Any thoughts

With a flexible mounted engine you either have a flexible mounted shaft with a rigid coupling or a flexible coupling with a rigid mounted shaft. A flexible shaft is one with a single bearing and a flexible stern seal

In the old days the engine shaft and coupling would be rigid but this then needed the alignment to be quite accurate, whit two of the elements flexible alignment is less critical and you get less transmission of engine vibration to the boat hull.

 

[Spyro]

I have a 2gm20 with flexible feet and flexible mounted shaft. I was getting quite a bit of resonating at certain revs. Before anyone told me it should be certain way I also fitted a flexible R&D coupling. It cured all vibrations and resonating noises. Been that way for years now and no problems.

 

[Plum]

Hi, I have a Yanmar Gm20 fitted to our boat which is mounted on flexible engine mounts, the prop shaft runs from the directly into the stern tube and to p bracket and prop.

Boat is Kelt 29

The shaft coupling is a two part 4 hole flange with no flexible insert.

The stern gland is a rotating bellow type.

I'm seeing vibration on the prop shaft and cannot see how you can have flexible engine mounts and fixed shaft, seems the wrong way round.

Any thoughts

That is not an uncommon arrangement and is more common on motorboats, however, the alignment of the flange coupling is critical. Remove the 4 bolts and check around the mating surfaces with a feeler gauge that there is no more than a few thousands of an inch difference around the circumference.

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