Prop rattle after fitting flexible coupling.

[Draughty]

Hi all,

I recently fitted an R&D flexible coupling which has reduced NVH up to about 1900rpm but above that quite a nasty rattle/knock sets in from the prop/cutlass area when under way. With the boat tethered to a dock the noise is much reduced; almost to not there.
I've been down and everything seems tight and clean; prop, blades, cutlass and shaft anode.
This issue was never evident before fitting the flex coupling, which entailed moving the engine 30mm forward, to maintain space to grease the Volvo Penta shaft seal and to preserve the same shaft overhang (about 20 mm) behind the P-bracket
Engine is a Beta 30hp with TMC60A gearbox and Slipstream S4 2-blade folding prop. All have done 1700hrs since new 2013.

Any ideas out there?
Thanks

[misfits]

Make sure the flex coupling is running true where it's bolted between the trans/ prop shaft coupling. I have the same coupling on my boat & it took a few adjustments of the electric motor drop pan.

Sounds like you moved your engine forward an 1" so adjustment of your motor mounts may be necessary. You do need a feeler gauge to do this. PIA cause there's never enough room to get at that connection.

[JPA Cate]

This may be obvious to you already, so no offense intended, but if you readjust the motor mounts as suggested, you will also need to re-true the shaft. And, if you didn't replace the cutless bearing before, I would suspect it may be your culprit, especially with the noise source so far aft.

Ann

[Draughty]

Thanks Misfit; I suppose there are people out there who would do this sort of work without checking the alignment but in this case, measuring between the cone headed bolt supplied with the coupling and the face of the half coupling, at 3, 6, 9 and 12 o'clock as described by R&D there is only 4 thou between min and max which is well within tolerances.

Rgds

[Draughty]

And thanks JPA Cate

Yes the cutlass must be considered and indeed I have had this boat 15 years and never changed it; I haul out every year and it is checked, usually by an expert, sometimes by me, and it never seemed to be worn.

Rgds

[Uncle Bob]

Originally Posted by Draughty

And thanks JPA Cate

Yes the cutlass must be considered and indeed I have had this boat 15 years and never changed it; I haul out every year and it is checked, usually by an expert, sometimes by me, and it never seemed to be worn.

Rgds

Could be that the alignment is now better than before and there is now no side force on the cutlass.

[Draughty]

Thanks Uncle Bob; a bit out of left field!
BUT I asked an "engineer" a couple of years ago "how is it that I've never had to change the cutlass bearing in all this time yet I hear often of people having to replace them after 2 or 3 years?"
He said "because this engine has always been aligned properly".

Rgds

[DougR]

If I understand the situation, you have:
- a standard strut/P bracket
- a Volvo Penta rubber stuffing box
- an R&D flex coupling
- a flexible mounted engine (?)

This means you have one fixed point in the system, the strut, and then three flexible unsupported points in a row. In most engine manufacturers installation manuals, (Volvo Penta for sure), this configuration is not recommended. When the engine is working hard the prop thrust is trying to cause the drive line to buckle and here there is little in the way of support to resist it. The rubber stuffing box is too flexible to exert any control of the shaft, the R&D coupling is made to flex, and the engine flex mounts allow engine movement as well. Once this flexing begins, higher RPM ( more thrust) makes the situation worse. Also, down angle gear boxes aggravate the situation, as the shaft thrust is providing lift against the engine flex mounts instead of just horizontal thrust. When these situations get really bad the drive line starts to look like a jump rope, and frequently the noise one hears is the shaft hitting the inside of the shaft log.

The removal of the flex coupling will probably restore the system to its prior smoothness. Another option could be to retain the flex coupling and to install a steady bearing toward the front end of the shaft to control its movement.

DougR

[Draughty]

Thanks DougR,
I think you have hit the nail on the head probably.
The flex coupling was supplied new with the engine/gearbox.
Beta say a flexible coupling "is strongly recommended in almost every case".
I would have thought that my arrangement was very common on this sort of boat size/ displacement/horsepower.
I did not fit the coupling originally because Volvo-penta say do not use a flexible coupling with their rubber cone shaft-seal. Interestingly R&D told me the other day that volvo penta refer only to their own flex coupling here because it does not guarantee concentricity between the two half couplings as do R&D and I think probably others which register into the half couplings.
About a year ago with 1300 hours up the ATF started leaking out the back of the gearbox; not out the shaft seal but out along the spline/thread where the output coupling slides onto the output shaft.
I had it "fixed" twice by mechanics, both fixes lasted only a few hours.
I then fixed it myself using the sealant (Threebond 1211) recommended by Twindisc Aust and was really patting myself on the back until it started leaking again after about 350 hours!
Out of frustration I decided to try fitting the flex coupling, hoping it might stop this leakage.
I think I will do as you suggest and just take it out, which is a pity because up to 1900 rpm it did improve NVH.
R&D suggested similar to you that a half cutlass brg at the front end of the shaft log may help; that the length of shaft unsupported was a little longer than recommended, and the shaft may be "whipping".
Could such a brg just be pushed into the shaft log?

The LAST question is does anybody know how to stop these gearboxes leaking oil out the output shaft spline.

Thanks for your responses.

[coastalexplorer]

The bearing at the inboard end of the stern tube is called a Stave bearing, They look very similar to a Duramax Cutless bearing except they normally have a flange. However I found that they are only available in larger sizes. (as per propeller shafts over 2 " diameter) I only have a one inch diameter shaft 43" long.


So I drilled and tapped into the side of the stern tube and fitted an appropriate sized Cutlass bearing. (actually ended up being a different one to the outboard end) It was a tight fit anyway, but I inserted the stainless steel Allan grub screws to secure it anyway because of the absence of the flange.

[Draughty]

Thanks Coastalexplorer

I will look at fitting a cutlass brg in the front end of the stern tube (i've been calling this a shaft log; it is just a bronze tube; about 2 or 3mm wall thickness: I believe some builders use plastic now) when I haul out next march or April.

This tube is heavily glassed into the hull but whether it was ever engineered to have a brg in it I don't know.
The Volvo-penta shaft seal absolutely requires water for lubrication so I'll have to watch that.
In the meantime I will remove the flex coupling and move the engine back again: glad I haven't filled up the old holes yet!

Rgds

[NYSail]

Just a simple question: if he engine was running smooth and you had virtually no wear on he cutlass, why even consider the flex coupling?

Greg

[Draughty]

Hi,
The reason IS buried back there!
I hoped it would stop the recurring oil leak through the output shaft spline.
Oil in the bilge is a PIA.

Rgds

[coastalexplorer]

So Curios to know why you you have a Volvo shaft seal? did you have a Volvo and ripped it out to install a Beta/Kabota from Gloucestershire UK. ?



The old traditional shaft log was usually either one piece of wood that was bored, or two matched pieces of wood with a half round groove typically made on a wood shaper. That bronze fittings were sealed and bolted too at each end. water flowed up it to drip out the gland and cool it.



Since you obviously have a bronze stern tube? Do you have enough of it protruding past the fiber glass lay-up of the hull to fit a PYI drip-less gland? They all come with a connection to pipe water from the mixing elbow of the diesel; So that warm water is pushed through the stave bearing down the stern tube, through the Cutless and out the back of the vessel. Thus the water is flowing in the opposite direction to conventional. and cooling/lubricating the bearings.

[Draughty]

When I bought this boat 5 yrs ago it had a Volvo Penta MD11D in it.
When I retired and was going cruising full-time I thought it wise to replace the old seawater cooled Volvo, which was still running OK but a bit smokey, hours and histoty unknown.
The Beta seemed the best option at the time; they are sold all over the world, and I was quite keen to get a Kubota based engine.
Having seen the demise of British heavy manufacturing as a teenager going British did take a bit of self-convincing: this was compounded by the Italian gearbox: think Alfa Romeo.
Anyway, the boat also had a VP shaft seal which I only replaced a couple of years ago, because I thought it was getting a bit old, not because there was any fault with it. I think they are brilliant, cheap, easy to fit, longlasting and they work as claimed.
These seals just clamp over the stern-tube in the same way as PYI or similar seals do.
I've only got 4" between the stern tube and the half coupling on the shaft which is enough for the VP seal and a bit of room to lubricate it: I looked at various "dripless" seals not that long ago and they seem to require about 7"; which would require moving the engine even further forward which is problematic, and a new propshaft.
I've seen on the web what looks to be a VP seal with a water feed added but I don't think they are VP made; I will look further if I decide to add a brg in the stern tube.
Rgds