Which engine mounts?

[jessie]

Hi all, When choosing engine mounts do you assume that the capacity of each needs to be somewhere above 1/4 of the engine size as there are 4 mounts, but movement will add some weight occasionally?

I have vetus 38hp engine weighing 185kg. The mounts currently in place appear to be k100, ie a capacity of 100kg (based on their dimensions), but in the spec for these mounts it says suitable for small engines upto 80hp. 

So, will the K100 mounts be ok? I can't say for certain that this is what is already fitted, it just looks like it.

 

[Tony Brooks]

40 minutes ago, jessie said:

Hi all, When choosing engine mounts do you assume that the capacity of each needs to be somewhere above 1/4 of the engine size as there are 4 mounts, but movement will add some weight occasionally?

I have vetus 38hp engine weighing 185kg. The mounts currently in place appear to be k100, ie a capacity of 100kg (based on their dimensions), but in the spec for these mounts it says suitable for small engines upto 80hp. 

So, will the K100 mounts be ok? I can't say for certain that this is what is already fitted, it just looks like it.

 

 

It depends, normally the gearbox overhangs the rear mounts and is at the same end as the heavy flywheel, so often the rear mounts need to be stronger. I am afraid it should be read the manual or consult a dealer. Maybe @Eeyore can give a more4d efinatve answer because he seems very genned up on Vetus.

 

In the majority of cases the mounts will also have to absorb the fore-aft stresses caused by prop thrust as well as the weight of the engine.

[Eeyore]

@Tony Brooks Only because I spent a long time peering at Vetus and Sole documents trying to resolve mounting issues on my Westerbeke marinised Mitsubishi engine of the same core type!
Mine did in fact come with different rated front and rear mounts, and that was the principle cause of the issues I was having.

There was a variation in weight between the mounting points, but not sufficient that different rated mounts from any of the ranges investigated could be justified. The importer eventually relented and swopped the two softer mounts for two more of the harder ones.  

 

Edited August 27, 2022 by Eeyore

[Peugeot 106]

5 hours ago, Eeyore said:

@Tony Brooks Only because I spent a long time peering at Vetus and Sole documents trying to resolve mounting issues on my Westerbeke marinised Mitsubishi engine of the same core type!
Mine did in fact come with different rated front and rear mounts, and that was the principle cause of the issues I was having.

There was a variation in weight between the mounting points, but not sufficient that different rated mounts from any of the ranges investigated could be justified. The importer eventually relented and swopped the two softer mounts for two more of the harder ones.  

 

Out of interest what was the problem you had? Engine mounts seems another black art with the different ones available eg compression or shear 

[Eeyore]

It was what is generally referred to as a challenging installation. The Westerbeke 44B used the  Mitsubishi S4L2 engine as did the Vetus M4.17 (M4.45) and the Sole Mini 44. Sitting it on four identical mounts like it’s contemporaries was a major improvement. But the biggest issue was some thing I was told when considering the purchase “it’s a Mitsubishi, you can’t go wrong with a Mitsubishi”, big mistake!

 

[Arthur Marshall]

12 minutes ago, Eeyore said:

It was what is generally referred to as a challenging installation. The Westerbeke 44B used the  Mitsubishi S4L2 engine as did the Vetus M4.17 (M4.45) and the Sole Mini 44. Sitting it on four identical mounts like it’s contemporaries was a major improvement. But the biggest issue was some thing I was told when considering the purchase “it’s a Mitsubishi, you can’t go wrong with a Mitsubishi”, big mistake!

 

Bit like everyone says about Listers. They go on for ever and nothing ever goes wrong...

[Peugeot 106]

Ok but what was actually wrong with the mounts? I’m interested.

I know what you both mean. But you can get fond of your engine. I ran Seagulls for years. Not very reliable but easy to fix. I guess these days if you know what make engine you’ve got it means it’s probably not very reliable especially if you know the model too! Plenty of people when questioned tell me “it’s a diesel” 

[Eeyore]

5 minutes ago, Peugeot 106 said:

Ok but what was actually wrong with the mounts? I’m interested.

I know what you both mean. But you can get fond of your engine. I ran Seagulls for years. Not very reliable but easy to fix. I guess these days if you know what make engine you’ve got it means it’s probably not very reliable especially if you know the model too! Plenty of people when questioned tell me “it’s a diesel” 

There wasn’t anything wrong with the mounts other than the manufacturers insistence that it required mounts of a different rating for the back and front.

[Peugeot 106]

10 hours ago, Eeyore said:

There wasn’t anything wrong with the mounts other than the manufacturers insistence that it required mounts of a different rating for the back and front.

What was the consequence of having different mounts front and back  and what was the difference when you changed them to all the same? Do you have a mechanical (I presume relatively lightweight ) or hydraulic gearbox?

 

[IanD]

11 hours ago, Eeyore said:

There wasn’t anything wrong with the mounts other than the manufacturers insistence that it required mounts of a different rating for the back and front.

 

If the weight of the engine on the front and rear flexible feet is widely different -- for example, because of a heavy protruding gearbox -- then they should be different. The front ones should be stiffer, they should compress by about the same amount when loaded. This means they have the same resonant frequency, otherwise the engine can rock more.

 

Whether this is done properly is another issue, but there is a good reason for it...

[Ex Brummie]

My boat was built with what I subsequently found out were R&D mounts. When one failed, a respected Chandlery recommended a new set which I never was able to eliminate vibration, which I'd never experienced before. after discovering the make of the originals, I phoned R&D who wanted details of engine make and model, gearbox and transmission. They then specified front mounts and heavier rear mounts. When I suggested I might as well have a set of the heavier mounts, I was 'advised' in very polite terms, that I might as well not have bothered consulting the experts. 

Granted they were twice the price of the Chandlery one size fits all, but they more than do the job.

11 hours ago, Peugeot 106 said:

 

I guess these days if you know what make engine you’ve got it means it’s probably not very reliable especially if you know the model too! Plenty of people when questioned tell me “it’s a diesel” 

Knowing the make and model of your engine does not necessarily mean it is unreliable. If you self service and maintain it, then surely it is second nature. Too many boaters these days are not so diligent regarding maintenance and it shows up through some of the questions that are asked on the forum.

[Peugeot 106]

14 minutes ago, IanD said:

 

If the weight of the engine on the front and rear flexible feet is widely different -- for example, because of a heavy protruding gearbox -- then they should be different. The front ones should be stiffer, they should compress by about the same amount when loaded. This means they have the same resonant frequency, otherwise the engine can rock more.

 

Whether this is done properly is another issue, but there is a good reason for it...

Thank you . Do you know the difference between compression and shear mounts? I know that R&D make them but are there any other good supplier/manufacturers?

[Eeyore]

1 hour ago, IanD said:

 

If the weight of the engine on the front and rear flexible feet is widely different -- for example, because of a heavy protruding gearbox -- then they should be different. The front ones should be stiffer, they should compress by about the same amount when loaded. This means they have the same resonant frequency, otherwise the engine can rock more.

 

Whether this is done properly is another issue, but there is a good reason for it...

Having found the front to rear balance point on mine to be more or less midway between the front and rear legs it became something of a mystery as to why the manufacturer specified differently rated mounts.

[Eeyore]

Has anyone else noticed that the kettle only dances off the hob at cruising revs? 😎

[IanD]

1 hour ago, Eeyore said:

Having found the front to rear balance point on mine to be more or less midway between the front and rear legs it became something of a mystery as to why the manufacturer specified differently rated mounts.

 

They shouldn't have -- but not the case for all engines (depends where they squeeze the mounts in to avoid auxiliaries), especially ones with a big heavy gearbox hanging off the end.

[IanD]

2 hours ago, Peugeot 106 said:

Thank you . Do you know the difference between compression and shear mounts? I know that R&D make them but are there any other good supplier/manufacturers?

Most marine engine mounts are a combination of compression and shear, and also often have different stiffness laterally (most vibration) and longitudinally (prop thrust). If you have a flexible coupling which allows misalignment like an Aquadrive than you can use much softer mounts for lower  vibration, they supply a suitable range with various stiffnesses but I suspect they're expensive...

[Eeyore]

3 hours ago, Peugeot 106 said:

Thank you . Do you know the difference between compression and shear mounts? I know that R&D make them but are there any other good supplier/manufacturers?

You can have a look here https://www.avindustrialproducts.co.uk

No connection with them, just happen to have their page open to research a non boating application.

Plenty of tech info for you to browse.

Edited August 28, 2022 by Eeyore

[IanD]

4 hours ago, IanD said:

Most marine engine mounts are a combination of compression and shear, and also often have different stiffness laterally (most vibration) and longitudinally (prop thrust). If you have a flexible coupling which allows misalignment like an Aquadrive than you can use much softer mounts for lower  vibration, they supply a suitable range with various stiffnesses but I suspect they're expensive...

 

[Peugeot 106]

4 hours ago, Eeyore said:

You can have a look here https://www.avindustrialproducts.co.uk

No connection with them, just happen to have their page open to research a non boating application.

Plenty of tech info for you to browse.

Thanks that is interesting. I didn’t realise that engine speed was so important.

 

1 hour ago, IanD said:

 

Thanks but I have the r and d flexible coupling not the Aquadrive. Does anyone know if the coupling has a Lifespan?

 

[Eeyore]

19 minutes ago, Peugeot 106 said:

Thanks that is interesting. I didn’t realise that engine speed was so important.

Yes, the static deflection is a compromise for a variable speed engine.

The figure of 3mm static deflection mentioned in the document posted by @IanD equates to approx 70% vertical isolation at 1200rpm. In layman's terms that means you can sit watching tv whilst running the engine to charge the batteries without your fillings being vibrated out of your head.

AVI, R&D and others manufacture mounts, whilst I suspect Aquadrive will sell you a mount in a box with their name on. The  data is presented differently, but they appear functionally similar. 

 

[BEngo]

Just be grateful you don't have to buy yellow painted mounts in a yellow box with a Dutch name on.......

 

N

 

 

[Peugeot 106]

9 minutes ago, Eeyore said:

Yes, the static deflection is a compromise for a variable speed engine.

The figure of 3mm static deflection mentioned in the document posted by @IanD equates to approx 70% vertical isolation at 1200rpm. In layman's terms that means you can sit watching tv whilst running the engine to charge the batteries without your fillings being vibrated out of your head.

AVI, R&D and others manufacture mounts, whilst I suspect Aquadrive will sell you a mount in a box with their name on. The  data is presented differently, but they appear functionally similar. 

 

So does 3mm static deflection mean that when you fit the engine the mounts should squash down 3mm? I know the weight of my engine + gearbox and could use a spring balance to find the weight on the rear mounts. My cunning plan is to subtract this off the total weight of the engine to get the figure for the front as I can’t easily use a spring balance at the front. 

I can then give this info to the manufacturers. Does the coupling stay flexible or does the plastic have a useful lifespan before hardening?

 

 

[Peugeot 106]

6 minutes ago, BEngo said:

Just be grateful you don't have to buy yellow painted mounts in a yellow box with a Dutch name on.......

 

N

 

 

Mine are nameless. I haven't clue what type, make or size they are. I’m going to take them off and check then as one looks a bit distorted and the opposite corner was down 3mm when I checked the alignment. 

 

I did have a Dutch yellow engine but swapped for a green British one. And I think spares for mine are expensive!

[Eeyore]

1 minute ago, Peugeot 106 said:

 

I did have a Dutch yellow engine but swapped for a green British one. And I think spares for mine are expensive!

Green British = Lister

Green Japanese = Beta

[BEngo]

1 minute ago, Eeyore said:

Green British = Lister

 

Green Scottish = Kelvin  and if you think Lister parts are expensive......

 

  On a more serious note, oil is absolute death to rubber engine mounts.  It causes them to go all soft and gooey.  The weight of the engine/box then causes collapse and misalignment.  Moral is to keep them clean and oil free, or if they are in the line of distribution of a continuous flow corrosion  prevention system (aka oil weep/seep/leak),  fit a cover. These can be improvised from beans or sardine tins with suitable bolt holes added.

N