Engine mounts and coupling alignment: Warm or cold?

[Krusty]

I understand that anti-vibration mounts (and flexible couplings) soften a little as they come up to operating temperature in the engine bay.
My AVM's are the pedastal type; rubber bonded under a rectangular metal hood, and have now done 1200 hours over five seasons.
During the past year I have become increasingly aware that when my engine (BD722, 3-cyl, 20hp) is started from cold and put in gear, it sounds a bit rough and there is some vibration, which decreases as everything warms up. Once under way it is all fairly smooth (though I have known far smoother ones) : it does vary a bit with the amount of backstay tension, as one might expect.
I am about to investigate more thoroughly by breaking the coupling, running the engine in isolation, and checking alignment at the coupling. I have spare mounts, and will probably fit them.
QUESTIONS: Do the mounts 'sit down' a fraction as they warm up? And should alignment checks be done warm or cold?

 

[Bilgediver]

It is difficult to make sensible comment as you do not state wether or not you have a flexible shaft coupling fitted on your boat.

I must say I have never sailed on a yacht where the vibration reduced as backstay tension was applied!!!! I can understand how on some poorly designed hulls this could affect shaft alignement!

 

[oldharry]

Rubber absorbing vibration does warm up and become more pliable, so it sounds reasonable that your engine mounts may be more efficient when warm, though I never noticed it. More likely the injectors need attention and are making the engine run a bit ragged until it warms up.

 

[Bilgediver]

More likely the injectors need attention and are making the engine run a bit ragged until it warms up.

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My thoughts exactly but waiting for him to confirm if a flexible shaft coupling is fitted.

 

[Krusty]

Yes I do have a flexible coupling: sorry for the omission, but I have never come across a flexibly-mounted engine with a rigid coupling.
You might be right about the injectors, but the engine runs sweetly enough after start-up until it is put in gear; which says to me it is an alignment issue.
By the way, I don't think the van de Stadt design team would be too happy about your inference that the Pionier 10 is a poorly designed yacht.

I would most grateful for any helpful answers to my specific questions.

 

[Mrnotming]

Hi Piota, good luck with your alignment session. The backstay on my ron Holland Club Shamrock was hydraulic and did affect the shape of the boat when on hard.If you are replacing all the mountings, whilst the shaft half coupling is detached from the gearbox, it's own weight will cause the stuffing box packing to sag towards the bottom of the stuffing box. support the flange at the stuffing box end with wood wedge packing before you disassemble .then when you offer up the gearbox and engine to mate, the shaft will be central in the stuffing box.(does it have a length of rubber hose on it?) If it does have this rubber hose and clips, it is best to dismantle everything and visually check with the stuffing box removed that the shaft is as central as possible in the stern tube.Taking care of these points will ensure that there are not knocking noises developed.Best do a check again if not afloat as the boat will change shape a bit after launching.Engine mounts deterioriate quite quickly especially if any oil gets spilled near the area.Good luck with the job.

 

[wotayottie]

Any ic engine is more lumpy when it is cold, but you dont feel it in a modern car because of balancer shafts and very compliant tuned mountings.

A quick way to check your alignment is to get the boat under sail with the prop revolving but the engine out of gear. Does the engine move about at all? Yes - there is an alignment issue. No - the lumpiness you are feeling is simply engine vibration.

As for the mounts, the temperature should make no real difference. After all the engine moves about in use anyway.

Tightening the backstay to bend the boat not the mast? I thought that only happened with beneteau! /forums/images/graemlins/grin.gif

 

[Krusty]

Thanks, 'polly' and 'wotayoty'; some good tips there.
Perhaps I should have mentioned that I have installed three engines and sterngear in my time, and aligned the whole thing several more, so I do know something of the problems and procedures.
As for bending boats or masts: no problem on fractional rigs, the mast has to bend. But on a masthead rig with a hefty mast, tensioning the forestay-backstay 'bow' by any means inevitably causes some deflection of the hull. (Even concrete is elastic and deformable: ask any civil engineer).
Is it of any significance for the engine-beds/sterntube/P-bracket geometry? On some designs it is (and not just bennys!), and should be allowed for; I knew one 72 footer where it was so considerable that the coupling had to be split every time she settled in her drying berth!
I'm still hoping to learn a bit more about flexible engine mounts!
There is some valuable stuff to be found on this forum, but as one forumite has commented; ''it is often like panning for gold''

 

[Bilgediver]

Some boats with very flexible mountings do not have flexible couplings and as a result this can cause certain problems.

Your set up is OK IE flexible coupling and flexible mountings however we have to accept with this set up that the engine is free to move about and in fact at certain low speeds and especially when cold will move around a lot. The flexible couppling is tolerant of relatively poor misalignment and so anything you do with the back stay should not affect what you feel.

The mounts stiffness should not change appreciably within the temperatures for which they are designed to operate .

It is pssible that the engine vibration creates resonanve in the hull and this may change with tensioning the backstay.

Your problem could quite simply be due to one or more dodgy injectors so I would suggest these be taken to a reputable service centre . Poor injectors can produce the syptoms you describe and in some circumstances even under power if bad enouogh.

One problem with this set up is that it is possible to set up perfect alignment with the shaft running out of centre of the hole through the hull with the result that the shaft knocks the hull when the engine rocks excessively.

When checking the alignment the first job is to estaablish where the shaft should lie so it is centralised in the hole through the hull. This can be done accurately enough if the shaft gland is flexibly mounted by moving the shaft coupling from port to starb and up and down and ensuring the coupling moves about the same distance offset from the engine coupling in all directions. If the shaft is almost lined up in any position even when pushed to the extreme sideways position then there is a risk of the shaft hitting the hull when it rotates.

Pack the shaft in the central position and then align the engine to this. .

Mr VAn de stad would be upset if a bit of back stay tension caused alignment problems and would probably fix under warranty /forums/images/graemlins/laugh.gif.