pro shaft alignment?

[madmat99]

Hello all.
Reinstalling a Yanmar 3gm in to a 30ft sail boat.
I have had the shaft filled with stainless weld. turned down to size and straightened.
it all seems to be going well but the rubber/plastic plate that goes between the back of the engine and the coupling on the shaft seems to add about 15thou of oscillation.
the back of the gearbox is true when i spin it with a dial on. i cant find anything in the yanmar manual as that piece is not covered.
is 15thou to much? should i be getting a new middle plate?
would it be bad to take it out and just bolt the two couplers together making sure they are true?
the coupler is a R&D 910 i think

any advice is appreciated!

Mat

 

[Laminar Flow]

15 though of an inch is about 0.4mm which I would consider too much of a misalignment.

First step is to take the coupler apart again and to try fitting it back together using different hole settings i.e. different pairs by rotating the two pieces against each other; it is possible that you did not match the original (correct) hole pairs on assembly.

The flexible plate is essential to compensate for very(!) slight misalignment of shaft to gearbox/end bearing. Do not remove it.

Also, on reinstallation the engine needs to be properly realigned as well. It is easy enough to do.

 

[Catalina36]

15 thou is shedloads.

 

[madmat99]

The coupler was never removed from the gearbox. I have tried the pro shaft in all four rotations and the 15thou remains the same. I think the coupler maybe the issue!

 

[Laminar Flow]

The coupler was never removed from the gearbox. I have tried the pro shaft in all four rotations and the 15thou remains the same. I think the coupler maybe the issue!

Well, they do not last forever and the plastic does harden over time. Any impact taken on the prop shaft may cause them to misalign.
You cannot have flexible engine mounts without a flexible coupler.

 

[Beneteau381]

Hello all.
Reinstalling a Yanmar 3gm in to a 30ft sail boat.
I have had the shaft filled with stainless weld. turned down to size and straightened.
it all seems to be going well but the rubber/plastic plate that goes between the back of the engine and the coupling on the shaft seems to add about 15thou of oscillation.
the back of the gearbox is true when i spin it with a dial on. i cant find anything in the yanmar manual as that piece is not covered.
is 15thou to much? should i be getting a new middle plate?
would it be bad to take it out and just bolt the two couplers together making sure they are true?
the coupler is a R&D 910 i think

any advice is appreciated!

Mat

My view on flexible couplings. Why! Its a 3 cylinder engine? Inherently well balanced. Given that the majority of production boats come with flange to flange, given that the engine can wobble on its mounts a lot more than 15 thou, given that the hull can flex, do it flange to flange and set the alignment in the water and all should be fine. Just make sure that the boss of the prop doesnt get too close to anything if you remove the plastic thing.

 

[Beneteau381]

Well, they do not last forever and the plastic does harden over time. Any impact taken on the prop shaft may cause them to misalign.
You cannot have flexible engine mounts without a flexible coupler.

I beg to disagree. Most production boats dont have a flexible coupler!

 

[Laminar Flow]

I beg to disagree. Most production boats dont have a flexible coupler!

Each to their own.
I re-engined recently and replaced a forty year old BMC four cylinder with a new engine. While I did the job myself it was done under the supervision of the Dutch yard where we kept out boat and they specialize in this kind of thing. The chief mechanic insisted that if you had flexible engine mounts then you needed a flexible coupling to absorb the vibration, in particular if you have an inside shaft bearing and you would like to save your transmission's end bearing.

I've only ever been on one boat that did not have flexible mounts or a flexible coupling. In idle the 2 cylinder Lister sent ripples across the harbour.

The OP's boat came with it and probably for a good reason, particularly if , as you claim, contemporary boats are so soft and flexible. (?)

 

[rogerthebodger]

With a prop shaft drive system or any drive system its there the items are to be flexible mounted you only need 2 of the 3 items to be flexible.

In fact I would not have all 3 flexible as it could set up a residence vibration.

In most cases including mine I have a rigid prop shaft with a flexible coupling and a flexible mounted engine.

If the prop shaft has a single bearing and a flexible stern seal the you would need either a flexible coupling and a rigid engine of a flexible mounted engine and a rigid coupling.

My prop shaft does have a flexible PSS stern seal but I have 3 bearings on the shaft itself.

 

[Plum]

Hello all.
Reinstalling a Yanmar 3gm in to a 30ft sail boat.
I have had the shaft filled with stainless weld. turned down to size and straightened.
it all seems to be going well but the rubber/plastic plate that goes between the back of the engine and the coupling on the shaft seems to add about 15thou of oscillation.
the back of the gearbox is true when i spin it with a dial on. i cant find anything in the yanmar manual as that piece is not covered.
is 15thou to much? should i be getting a new middle plate?
would it be bad to take it out and just bolt the two couplers together making sure they are true?
the coupler is a R&D 910 i think

any advice is appreciated!

Mat

If your forward bearing of the propshaft is of a flexible type then you could eliminate the plastic drive plate. A common arrangement , particularly on French boats. Otherwise you need it. Is your 15 thou oscillation angular? If so, surely the plastic coupler will compensate for that?

www.solocoastalsailing.co.uk

 

[madmat99]

Well, they do not last forever and the plastic does harden over time. Any impact taken on the prop shaft may cause them to misalign.
You cannot have flexible engine mounts without a flexible coupler.

 

[CAPTAIN FANTASTIC]

The flexible coupling, provided is in good condition, should be able deal with your 15 thou misalignment. I have re-engined 3 of my boats and always used flexible coupling to absorb any small misalignment. The engine will get "misaligned" regardless when in the water and over time due to engine vibration. However, if you can adjust the engine to reduce your current misalignment, will be even better.

 

[madmat99]

The setup is. Prop. 6inch. A 5 inch cutlass bearing. Roughly 3 feet of unsupported shaft. Then the stuffing box that is supported by a 3 inch rubber fitting. Then around 14 inch of shaft onto the back of the engine.
the 15thou is side to side on the prop shaft. When the engine is staring it bounces around like a mad thing. Probably over an inch of movement. It settles down nicely when it’s running. So all the movement seems to be coming from the flex coupler. It may just be old or had something sat on it for the 10 years it was stood before I got it. I would like to remove the flex as I think I would be able to align the engine much better without it!

mat

 

[rogerthebodger]

The setup is. Prop. 6inch. A 5 inch cutlass bearing. Roughly 3 feet of unsupported shaft. Then the stuffing box that is supported by a 3 inch rubber fitting. Then around 14 inch of shaft onto the back of the engine.
the 15thou is side to side on the prop shaft. When the engine is staring it bounces around like a mad thing. Probably over an inch of movement. It settles down nicely when it’s running. So all the movement seems to be coming from the flex coupler. It may just be old or had something sat on it for the 10 years it was stood before I got it. I would like to remove the flex as I think I would be able to align the engine much better without it!

mat

If I read the above correctly with a single bearing and a flexible (rubber) stuffing box you do not need a flexible as well. In fact as I posted it is not desirable to have all three flexible.

It's the rigid connection to the gearbox that in fact supports the forward end of the prop shaft.

 

[madmat99]

thanks for all the great advice on this.
It came down to two problems. The flex coupler had sagged forward and was pushing the shaft out of line by a small amount. But the bigger problem was the cast half coupler. There was no way of bolting this to the shaft and the pair staying true. On inspection it looks like the shaft has spun inside the coupler at some point as it was around 20 thou bigger than the actual shaft.
after a quick ring to the lovely people at r and d new bits were ordered and all is now smooth!