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Drive train clonking in gear

Dear all,
I have a beta marine 38, with a PRM delta gearbox (no input shaft spline wear) driven by an R & D 'extra dampening' drive plate. Coupled via a double-cardan/constant velocity joint (from a london taxi propshaft), through a plumber's block to the stern bearing.
When engaged in gear, at tick-over (wound down to 550rpm), there is a distinct 'clonk-clonk-clonk' coming from the region of the gearbox.
At higher engine speeds, this noise seems to go away, although that could just be the engine drowning it out.
The shaft/cv join moves freely (with the force expected to drive a prop in water) by hand in both directions. The engine is angled down slightly on its mounts (adjustable screw-nut ones) so that the double-cardan input is slightly off-set.
My question is - should I worry about this? I have no nylon coupling between the gearbox and the prop/cv-join input shaft, so is this likely to just be 'clonky plumbers block'?
Should I raise the tick-over? It was wound down when the engine control lever insisted on increasing the rpm prior to engaging gear - there is now a correct control lever. I think the rated tickover is 750rpm. I will increase it.
Many thanks,
Patrick Vale

Asked by: Patrick Vale  | 4.32pm, Monday 3 June

WW says:

At very low input speeds, there is the remote possibility that not enough hydraulic oil pressure is present to keep the clutch engaged- however this is not that likely, but a possibility.
It is more likely that you may be at the range that the engine is resonating, which makes things worse, carrying vibration through to the gearbox and mounts, where the low speed accentuates the firing of the cylinders- BMC engines running slowly, for instance, are very prone to this "clonking" noise.
However, there is another possible angle. The symptoms you describe are similar to when an oil operated gearbox has low oil levels. If the oil level is normal, and it behaves properly at around 750RPM, I would leave it there. It is also worth ensuring that the gearbox is not running hot- ideally not above 70C when running.
Most engines do not respond well to running at an RPM below their designed tickover- a quick word to Beta Marine would probably confirm this as well. If the engine is all set-up properly, then low tickover is probably the cause.
However, it might be worth checking that the plummer block is fully lubricated via the grease nipple- if one is fitted (most do).
Also, check that the engine mounts are not loose- depending on the mounts fitted, check that not have shimmied down, causing any localised extra movement. Sometimes movement on the mounts only is noticeable at very low engine revolutions.
If the boat is behaving normally at cruising speed, and, on careful close inspection of the noise, say when in gear (tied up) then all is probably well...

Mark Langley  | 9.32PM, Monday 3 June

I think the clunking noise might be from the cardan shaft bearing, if as you say it is working through a slight angle.
The middle bearing from a double cardan shaft is designed to be secured to the chassis, with each shaft working independently. If yours is spinning freely it might not be working correctly with one side doing all the work. If the bearing was not rebuilt when fitted to the boat, it might be worn giving a clicking noise.

Rupert Smedley  | 10.24AM, Tuesday 4 June

To grease the plummer block, you will need to screw a grease nipple into the threaded hole.
Your mention of needle rollers implies that the cardan shaft has Universal Joints as well as a CV joint. UJs do not run smoothly when orking at an angle which might also give rise to vibration.

Rupert Smedley  | 10.50AM, Wednesday 5 June

Readers say:

Hi Mark & thank you for your answer - I also suspected low oil, and will check this out. The plumber block has a screw threaded input on the top - I assumed this was for greasing, but I'm not sure what I need to 'inject' grease into it? The engine mounts are a little tired - I will also replace these.
Rupert thank you too for your answer - the double cardan is 'loose' between the gearbox and plumbers block - it has had one side and the middle bearing replaced as the needle rollers were worn just this year - it made a terrible noise before that! This is more of a 'clonk-clonk' that seems to be in time engine rpm. I'm not sure if it's ideal to have this 'free' joint, but it does seem to alleviate the problem of engine miss-alignment - at least in that it's the CV joint rather than the stern bearing which gets worn!

Patrick Vale  | 2.24PM, Tuesday 4 June

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