Do I really need a flexible coupling?

[DrakeDruid]

My engine keeps rattling on its mounts no matter which way I tighten them up!

It's been suggested that I get a flexible coupling for my prop shaft but me being me I want to ensure this is the right solution, any suggestions please?

[Southern Star]

New engine mounts?

[DrakeDruid]

New engine mounts?

I think the mounts are ok the boat is only about 7 years old it's just tightening the bolts has not stopped excessive vibration!

[ditchcrawler]

Did you check the alignment when you tightened them?

[nb Innisfree]

Flexible couplings protect the prop shaft bearing from side loads from engine movements, stern gland less likely to leak and prevents a lot of engine vibrations transmitting to the hull.

[MtB]

My engine keeps rattling on its mounts no matter which way I tighten them up!

It's been suggested that I get a flexible coupling for my prop shaft but me being me I want to ensure this is the right solution, any suggestions please?

 

 

Welcome to the forum...

 

Give us a prayer of advising you well. What make and model of engine? Flexible or solid mounts? What sort of driveshaft? Is it a narrowboat or something else?

 

Photos would be so very illuminating too.

[Athy]

I looked at the thread title and wondered how many clients of Ashley Maddison are asking themselves the same thing.

[bigste]

Excess vibration means something is either clapped out, loose or out of balance, (like a flywheel). A flexible coupling will help but you are merely covering up the problem. Always better to source the cause.

[Tony Brooks]

The majority of flexible couplings will only allow for up to 2 or 3 degrees of angular misaligment and still require near perfect radial alignment. Those that use a hard plastic insert are probably less tolerant.

 

A coupling that uses two flexible members like a pair of universal joints (hooks joint) the Aquadrive, Pythondrive, an long Centaflex will accommodate both angular and radial misalignment and possible a larger angular misalignment than single points of flexibility types. The problem is that in most cases this type needs a thrust bearing (usually as part of the assembly) and are longer than single point of flexibility types. If you can not move the engine forward or cut the shaft and move the coupling back sufficiently you will not be able to fit one.

 

Have you checked the shaft and half coupling to see if they run true?

 

I echo Ditchcrawler, if the shaft is bent, has a poorly machined half coupling or the shaft alignment is not within 0.05 mm (PRM requirement) the rotating shaft will try to wag the engine about and stress the mounts. This also causes excess vibration that will loosed mounts.

 

I would suggest that once you have confirmed the above are within tolerance (check you gearbox manual for the maximum allowable misalignment) you make sure the engine is not misfiring on one cylinder and give it another go on your mounts. If it still happens the see if you can fit a flexible coupling that accepts both types of misalignment (view a variety and check their specifications). If you do not have space for such a coupling think very seriously of fitting hard wood (say oak) lengths to the top of your engine beds and then mounting the engine rigidly but for this to be a success the alignment will need to be all but perfect.

[dor]

Without a flexible coupling the shaft alignment must be spot on. Just tightening the mounts is not enough unless it is done carefully to ensure true alignment. Even then, if the diagonal mount settings is slightly out the engine can still rock excessively. You need to check the shaft is not bent then get the engine mounts set properly. Also it is quite easy to wreck engine mounts in 7 years, especially if they have been subject to excessive movement.

[Murflynn]

I have a little Yanmar.

They are notorious for wrecking the rear starboard engine mount which lies below the fuel filter.

A leaking filter thread (probably caused by over-tightening at some stage) allows fuel to drip onto the rubber insert in the mounting which then embrittles and eventually fails.

The replacement mountings are horrendously expensive.

[magnetman]

I had a lister LPW2 in a little motorsailer once which rattled horribly. I forget the engine mount manufacturer (quite small mounts) but it turned out the rubbers were nackered and the bolts were bottoming out on the engine beds. Contacted manufacturer of mounts and they went me out 4 new rubber inserts FOC in the next post which sorted it

[Chewbacka]

Just a thought, before you make big changes, but there is a warning in the beta engine manual that if the engine mount stud is too far into the mount it can touch the bearer below. Have you checked for this?

[Guest]

My engine keeps rattling on its mounts no matter which way I tighten them up!

It's been suggested that I get a flexible coupling for my prop shaft but me being me I want to ensure this is the right solution, any suggestions please?

Can you post a pic of the setup? Could it be Vetus sternegear? Arguably, and depending on who you speak to, Vetus stern glands may not need a flexible coupling (though mine has.) Vetus told me it wasn't necessary but that was 11 years ago.

Edited to add my Vetus engine used to shake its mounting nuts loose until I replaced them with m16 nylocks

 

Vetus sterngear looks like this

Edited August 20, 2015 by Guest

[Tony Brooks]

As long as the engine mounts do not allow the engine to move vertically or laterally to a degree greater than the movement allowed in the flexible stern gland no flexible coupling is required as long as there is just bone shaft bearing and it has a suitable degree of compliance like a Cutless rubber bearing. The Vetus stern gear has this.

 

The problem comes when the engine waves the shaft about to a degree that exceeds the compliance in the bearing or so the shaft bangs against the stern tube.

 

Given close to perfect alignment with no/little chance of the engine settling on its mounts I think that Vetus are correct but as it seems all engine mount rubbers settle over time I would prefer a twin point of flexibility coupling to minimise vibration and potential gland/tube/bearing wear.

[Guest]

As long as the engine mounts do not allow the engine to move vertically or laterally to a degree greater than the movement allowed in the flexible stern gland no flexible coupling is required as long as there is just bone shaft bearing and it has a suitable degree of compliance like a Cutless rubber bearing. The Vetus stern gear has this.

 

The problem comes when the engine waves the shaft about to a degree that exceeds the compliance in the bearing or so the shaft bangs against the stern tube.

 

Given close to perfect alignment with no/little chance of the engine settling on its mounts I think that Vetus are correct but as it seems all engine mount rubbers settle over time I would prefer a twin point of flexibility coupling to minimise vibration and potential gland/tube/bearing wear.

Fair point if it is rigidly mounted. A bit hard to know really without further info and pics...

[DrakeDruid]

Hello and thank you for the help I have managed to upload a video that may give a better picture of what I mean!

https://m.youtube.com/watch?feature=youtube_gdata_player&v=buWveNYdTuo

Thanks.

[Guest]

Hello and thank you for the help I have managed to upload a video that may give a better picture of what I mean!

https://m.youtube.com/watch?feature=youtube_gdata_player&v=buWveNYdTuo

Thanks.

It is Vetus sterngear. Some say it should have a flexible, some say not. I have seen many without, but my vetus sterngear has an R and D flexible coupling. Some 11 years ago, Vetus UK told me not to fit one. Some members on here have had the same reply, but one member recently spoke to Vetus and was told that a flexible should be fitted.

My advice would be to email Vetus. They are normally very helpful in my experience.

 

VETUS Ltd.

5 Compass Point,

Ensign Business Park, Ensign Way,

Hamble, Southampton SO31 4RA

Tel. +44-23 8045 4507

Fax +44-23 8045 4508

E-mail sales@vetus.co.uk

Edited August 20, 2015 by Guest

[MtB]

Hello and thank you for the help I have managed to upload a video that may give a better picture of what I mean!

https://m.youtube.com/watch?feature=youtube_gdata_player&v=buWveNYdTuo

Thanks.

 

 

Well that certainly sounds to me like a busted engine mount, allowing the engine to contact the steel of the boat at one specific resonant engine speed.

 

Can you feel that harsh buzz through your feet (standing on the metal of the boat) when it happens?

[Guest]

 

 

Well that certainly sounds to me like a busted engine mount, allowing the engine to contact the steel of the boat at one specific resonant engine speed.

 

Can you feel that harsh buzz through your feet (standing on the metal of the boat) when it happens?

Can hear it better now on laptop. It does sound like you describe. Another possibility is the propshaft could be touching the stern tube at certain revs. Not uncommon with a neoprene cutless bearing, when the engine has gone out of alignment.

[Alan Saunders]

My Vetus M3.10 (Mitsubishi K3D) has been known to rock around excessively on its flexible mounts at minimum throttle, even in neutral. Replacing the defective fuel, servicing the injectors and then setting minimum rpm to the recommended 850 rpm solved my problem.

 

HTH, Alan

[DrakeDruid]

It's does just do it at certain revs but not all the time.

Is it possible to visually identify a broken mount as nothing appears obviously broken ?

Is there a specific model R&D coupling I could use?

[Dyertribe]

...... and I thought this was another Ashley Maddison thread

[Guest]

It's does just do it at certain revs but not all the time.

Is it possible to visually identify a broken mount as nothing appears obviously broken ?

Is there a specific model R&D coupling I could use?

I always tap the mounts with a big spanner or similar, a dull sound means a loose or broken mount usually.

 

This is the type on mine: http://www.randdmarine.com/flexiblesc.asp

[DrakeDruid]

Where can I get new engine mounts from as it seems two of mine are in a bad way?