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Which engine mounts?


jessie
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5 minutes ago, BEngo said:

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

Years ago when I knew even less about diesel  engines (I now reckon i’ve Risen to the dizzy height of a little knowledge can be dangerous) I nearly bought a small yacht with a tvo kelvin,.....I am now  almost 70 and the yacht is over 100 and still to my eyes as lovely as ever but fitted with a more modern engine.

thanks for the warning about oil but though I did spend hours tracing an oil leak it was no where near the mounts. Mine are circular rand appear to have rubber covers on them

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23 hours ago, Arthur Marshall said:

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

 

Oooh no - that's not right

 

Things go wrong, and they still go on running

 

Without engine mounts

 

Richard

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11 hours ago, IanD said:

 

This means they have the same resonant frequency,

 

Do they - and is that a good thing? After all, the engine can't wag at different speeds at each end of the block - it's a single solid item

 

My guess (WARNING) is you end up with a different resonant frequency for the complete engine. And it might be that you completely upset the resonances in the setup to your advantage

 

I've never done it, I do wonder if different stiffness mounts front and back would be a good thing

 

Richard

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1 hour ago, RLWP said:

 

Do they - and is that a good thing? After all, the engine can't wag at different speeds at each end of the block - it's a single solid item

 

My guess (WARNING) is you end up with a different resonant frequency for the complete engine. And it might be that you completely upset the resonances in the setup to your advantage

 

I've never done it, I do wonder if different stiffness mounts front and back would be a good thing

 

Richard

It's probably not a good idea to have wildly  different frequencies at the two ends, this can lead to a rocking motion which can mean more movement at the back end of the gearbox where any flexible coupling is.

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On 27/08/2022 at 13:23, 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.

 

K100 mounts would be a bit hard, but the K50, K75 and K100 (shore hardness 45, 55 and 65 respectively, and may be stamped on the base of the mount) are generally similar in appearance.

K75 mounts all round if the weights are even, and are also good for up to approx 55% of the weight on the back end and 45% on the front. Should give static deflection a bit either side of 3mm.

Between 55% and 60% on the back end is a bit of a grey area given the available choice of mount, but above 60% on the back will require K50 mounts on the front.

K50, AVI SM1600-45 and Aquadrive 50212 are similar specs.

K75, AVI SM1600-55 and Aquadrive 50213 are similar specs.

K100 similar to  AVI SM1600-65 which is a 90kg mount.

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2 hours ago, Peugeot 106 said:

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?

 

 

Yes that's what it means. The bigger the deflection is (softer mounts) the lower the resonant frequency is and the better vibration isolation is.

 

But unless you have a coupling like the Aquadrive with two CV joints linked by a short shift which can cope with significant misalignment, soft mounts can cause problems -- the other cheaper flexible couplings don't like being misaligned.

3 hours 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. 

 

It's the static deflection that matters, this sets the resonant frequency. Aquadrive mounts are no different in concept to anyone else's, but they do seem to allow more deflection than most which means better isolation.

 

Others with the same deflection will work just as well and may be cheaper -- or maybe more expensive if they're painted yellow... 😉

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I quite like this image (borrowed from the AVI page) showing the optimal installation of a marine mount on what appears to be a piece of auxiliary equipment.

Its highlights why we should install the engine leg as low as possible on the height adjuster, and use a shim plate beneath the mount to achieve this.

You could of course install exactly as in the photo by using a variety of thin metal shims under the mount.

Mounting high on the adjuster is a bit like handing the engine a crow bar to lever the mount.

 

84A4801E-5545-4B97-9A43-0FABBC343A37.jpeg

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7 minutes ago, Eeyore said:

I quite like this image (borrowed from the AVI page) showing the optimal installation of a marine mount on what appears to be a piece of auxiliary equipment.

Its highlights why we should install the engine leg as low as possible on the height adjuster, and use a shim plate beneath the mount to achieve this.

You could of course install exactly as in the photo by using a variety of thin metal shims under the mount.

Mounting high on the adjuster is a bit like handing the engine a crow bar to lever the mount.

 

Amen to that, pity more boat builders don't seem to appreciate it.

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I feel I should add a bit of context. RWLP you know the engine LPWS2. After installing the engine it has run for around 550 hours without problem. At around 550 hours the stern gland started to weep more (1 pint/day). I wasn't worried about engine alignment as the engine had been out the previous year looking for a pesky oil leak and I had realigned it when I put it back. The mounts were new at 0 hours as was the propellor shaft. After querying the problem on the forum Tracy kindly suggested I check the alignment again.  I walked 5 miles to the nearest Screwfix for a pair of feeler gauges and checked the alignment which was indeed out. I rectified it with a mooring pin and a  4mm scaffold washer to within 7 thou repacked the gland and all was well for around another 150 hours the leak started again. As I was getting closer to home I just kept tweaking the gland adjusting nuts. Unfortunately they are UNC not UNF so adjustment is a bit coarse.  Its not easy to get at the offending mount but I suspect it may have sunk.

I thought I would take all the mountings off during the next month and look at them. Because of engine bracket lengths the brackets sit on top of the flexible mounts with washers underneath which makes them awkward to adjust. The brackets are 40mm x 12mm steel bar so it would be simple to get the lengths altered by a welder so that I can fit adjusting nuts.

 

So my questions here were aimed at my education but also in case I find I need more mounts (which I think I will)

 

I put the question to R&D. 2 cylinder high speed LPWS2 engine running at 2000rpm, PRM 120 gearbox 2:1. weight 112Kg + 34Kg gearbox, drive plate, casing, engine mounting legs, fluids  146Kg.

 

They said shear mounts front 800-040 front, 800-041 rear but 800-037 front, 800-010 rear be better "as there is more rubber". 

Another person at R&D  said Compression mounts 800-033 all round would be best as it is such a small engine.

 

Hence my queries about the difference between compression and shear mounts

8 hours ago, Eeyore said:

I quite like this image (borrowed from the AVI page) showing the optimal installation of a marine mount on what appears to be a piece of auxiliary equipment.

Its highlights why we should install the engine leg as low as possible on the height adjuster, and use a shim plate beneath the mount to achieve this.

You could of course install exactly as in the photo by using a variety of thin metal shims under the mount.

Mounting high on the adjuster is a bit like handing the engine a crow bar to lever the mount.

 

84A4801E-5545-4B97-9A43-0FABBC343A37.jpeg

 

8 hours ago, Tony Brooks said:

 

Amen to that, pity more boat builders don't seem to appreciate it.

I quite understand that there should be no more than 10mm stud exposed but don't think I will be welding the packers to the beds or installing a vibration monitor never mind the shiny paintwork!  R&D insist that washers on the stud in place of nuts are a no no even if they are only 6mm thick. They say that you must pack under the mounting plates if you can't use nuts which is lets face it a bit fiddley

22 hours ago, RLWP said:

 

Do they - and is that a good thing? After all, the engine can't wag at different speeds at each end of the block - it's a single solid item

 

My guess (WARNING) is you end up with a different resonant frequency for the complete engine. And it might be that you completely upset the resonances in the setup to your advantage

 

I've never done it, I do wonder if different stiffness mounts front and back would be a good thing

 

Richard

Richard you've thrown me a bit with this as R&D have come up with different rated mounts front and back. I assume they've applied the 40% front 60% rear rule. Are you saying that this resultant "different resonant frequency" could be beneficial or are you WARNING against it?

 

I'm going to send the same enquiry to AVI and see what they come up with. I've also sent off for a cheapo 100Kg spring balance (£10 ebay) and if I weigh the rear end I can subtract this off 146Kg to find the front weight. In any case I'm sure something else will rattle at the resonant frequency at the moment its the grill pan in the oven!

 

Has anyone changed compression to shear mounts or vice versa or had a bad experience with different rated mounts front and rear? Has anyone good or bad experience with R&D?

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10 hours ago, Peugeot 106 said:

 

Richard you've thrown me a bit with this as R&D have come up with different rated mounts front and back. I assume they've applied the 40% front 60% rear rule. Are you saying that this resultant "different resonant frequency" could be beneficial or are you WARNING against it?

 

I'm warning against my idle, untested speculation.

 

From the bits of theory I remember the engine is one mass/spring/damper system with the damping coming from mainly from the engine mounts. This system is going to have a (or more likely many) resonant frequency which is determined by all of those. You cannot change the mass, the springing and damping come from the engine mounts

 

So, in theory, if you've got an engine that's going into resonance at an undesirable cruising speed, you could change the frequency by having different types of mount front and rear

 

On the other hand - I'm speculating idly and R&D do this for a living. Go with the R&D set up

 

Richard

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2 hours ago, RLWP said:

 

I'm warning against my idle, untested speculation.

 

From the bits of theory I remember the engine is one mass/spring/damper system with the damping coming from mainly from the engine mounts. This system is going to have a (or more likely many) resonant frequency which is determined by all of those. You cannot change the mass, the springing and damping come from the engine mounts

 

So, in theory, if you've got an engine that's going into resonance at an undesirable cruising speed, you could change the frequency by having different types of mount front and rear

 

On the other hand - I'm speculating idly and R&D do this for a living. Go with the R&D set up

 

Richard

Thanks Richard. I seem to remember that you too are an HND man. 50 years ago vibration degrees of freedom and fourier theorems completely defeated me and along with calculus were filed away as "Black Arts". The mounts I've had have been fine but have no markings on them otherwise I'd probably just replace them. Since I've got bugger all else to do these days (Joke) I thought I'd find out more about them which as usual led me into the total unknown and wastelands. On the the other hand no one else seems exactly clued up. I will send the enquiry to AVI out of interest. 

I hope all is going well with yourself, Sue and of course Tawny Owl

Robert

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2 hours ago, RLWP said:

 

I'm warning against my idle, untested speculation.

 

From the bits of theory I remember the engine is one mass/spring/damper system with the damping coming from mainly from the engine mounts. This system is going to have a (or more likely many) resonant frequency which is determined by all of those. You cannot change the mass, the springing and damping come from the engine mounts

 

So, in theory, if you've got an engine that's going into resonance at an undesirable cruising speed, you could change the frequency by having different types of mount front and rear

 

On the other hand - I'm speculating idly and R&D do this for a living. Go with the R&D set up

 

Richard

 

The problem is that -- as you say -- there are many modes of resonance as well as the obvious ones.

 

If you want to get technical, a long heavy object like an engine/gearbox suspended on four feet has a potential problem if the mass/stiffness ratio (resonant frequency) is different for the front and rear feet, due to the way the impedance of the resonant circuits change with frequency -- in electrical terms, they're inductive below resonance and capacitive above it. At a point in between the two resonant frequencies (e.g. front and rear feet) these reactances cancel out, and you can get a nasty torsional resonance where the front of the engine swings one way and the rear the other -- which means the tail of the gearbox waggles from side to side or up and down. Needless to say, this is A Bad Thing.

 

This is *especially* bad for a 3-cylinder engine which is statically balanced but torsionally imbalanced, so tends to waggle like this anyway rather than bounce up and down or from side to side -- 4-cyl are better for this.

 

A 2-cylinder engine with the cranks 180 degrees apart (like the LPWS2) has an even bigger problem than 3-cyl since the rocking moment is larger.

 

Whether this is a problem in real life depends on a lot of things including how far apart the feet are and what the mass on each is, but suffice it to say that there can be very good reasons for having different stiffness feet at the two ends... 😉

Edited by IanD
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On 28/08/2022 at 23:33, Eeyore said:

K100 mounts would be a bit hard, but the K50, K75 and K100 (shore hardness 45, 55 and 65 respectively, and may be stamped on the base of the mount) are generally similar in appearance.

K75 mounts all round if the weights are even, and are also good for up to approx 55% of the weight on the back end and 45% on the front. Should give static deflection a bit either side of 3mm.

Between 55% and 60% on the back end is a bit of a grey area given the available choice of mount, but above 60% on the back will require K50 mounts on the front.

K50, AVI SM1600-45 and Aquadrive 50212 are similar specs.

K75, AVI SM1600-55 and Aquadrive 50213 are similar specs.

K100 similar to  AVI SM1600-65 which is a 90kg mount.

Thanks Eeyore, I went back to look at the mounts again, and phoned a vetus dealer to check and they are indeed K75 mounts, the inverted metal cup over the rubber is slightly different on the different models.

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Wow, this thread has taught me a lot, and there is much more to this than i am likely to learn.

One thing that confuses me is that my engine mounts look fine, the rubber isn't sticky, I can still fit my finger beneath the metal cup over the rubber, yet the engine is lower than it was, (or the prop shaft is higher) but cannot be adjusted higher as the bolts are at the top of the thread.

 

So, i am looking at putting steel shims in to raise it so that

a) it can be aligned correctly and

2) the bolts can be further down the threaded rod.

 

But, how did it drop? it's mounted on steel.

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28 minutes ago, jessie said:

Wow, this thread has taught me a lot, and there is much more to this than i am likely to learn.

One thing that confuses me is that my engine mounts look fine, the rubber isn't sticky, I can still fit my finger beneath the metal cup over the rubber, yet the engine is lower than it was, (or the prop shaft is higher) but cannot be adjusted higher as the bolts are at the top of the thread.

 

So, i am looking at putting steel shims in to raise it so that

a) it can be aligned correctly and

2) the bolts can be further down the threaded rod.

 

But, how did it drop? it's mounted on steel.

 

Rubber creeps under sustained load (especially shear stress), so the engine mounts drop over time as the rubber gradually deforms. In addition it can be degraded and weaken under continuous vibration and the rubber can change with age (stiffen and crack) due to material ageing, the mounts effectively wear out over time, so eventually need replacing even if they haven't had any fuel spilled on them.

Edited by IanD
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26 minutes ago, jessie said:

Wow, this thread has taught me a lot, and there is much more to this than i am likely to learn.

One thing that confuses me is that my engine mounts look fine, the rubber isn't sticky, I can still fit my finger beneath the metal cup over the rubber, yet the engine is lower than it was, (or the prop shaft is higher) but cannot be adjusted higher as the bolts are at the top of the thread.

 

So, i am looking at putting steel shims in to raise it so that

a) it can be aligned correctly and

2) the bolts can be further down the threaded rod.

 

But, how did it drop? it's mounted on steel.

Wow indeed, I don't think I could manage that with my fat fingers 😇.

The mounts should/would have been readjusted after the first 48 hours or so, did that happen?

Sounds like the shim plate will be quite thick. In this case weld them in and drill and tap to allow the use of set bolts to hold the mount down. This makes them much easier to position during the set up as you don't need to grow a third arm to hold the nut under the beds!

If other people do the servicing is it possible that someone did you a favour by tightening a loose mount; by tightening the top nuts instead of the bottom one. 

 Other than that everything goes south with age as @IanD mentioned.

 

Edited by Eeyore
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2 minutes ago, jessie said:

This is one of the rear mount, they are both at the same height, there is a lot more room for upwards adjustment on the front mounts.image.jpeg.962b5b48431aad7776712abd7a9cd0fa.jpeg

Scary 

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52 minutes ago, Eeyore said:

Wow indeed, I don't think I could manage that with my fat fingers 😇.

The mounts should/would have been readjusted after the first 48 hours or so, did that happen?

Sounds like the shim plate will be quite thick. In this case weld them in and drill and tap to allow the use of set bolts to hold the mount down. This makes them much easier to position during the set up as you don't need to grow a third arm to hold the nut under the beds!

If other people do the servicing is it possible that someone did you a favour by tightening a loose mount; by tightening the top nuts instead of the bottom one. 

 Other than that everything goes south with age as @IanD mentioned.

 

Not sure about after first 48 hours but they have been adjusted several times when the boat was serviced by an engineer i really trust to know his stuff. He told me to never tighten the top nuts and always the bottom ones so i'm pretty sure that hasn't happened. The difference in height between a new mount (to the top of the metal cup), and this one,  is just 4mm.

2 minutes ago, Eeyore said:

Scary 

I know, it definitely wasn't quite that high when bought, but i think maybe  20mm shims all round should work ok.

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6 minutes ago, jessie said:

This is one of the rear mount, they are both at the same height, there is a lot more room for upwards adjustment on the front mounts.image.jpeg.962b5b48431aad7776712abd7a9cd0fa.jpeg

Best guess is about 30mm of exposed thread (assuming 1.5 mm pitch), way too high. No wonder it required adjusting several times. 

If I "inherited" that as a maintenance job I might say something like "it needs sorting, but live with it till you've had the life out of the mounts, it will give you time to save up for the remedial works". I'm sure you've had a similar conversation.

Minimum of 20mm shim/packer on the rear mounts, although I might go for 25mm given that you already know the static deflection. That would leave you with approx 2mm of exposed thread on a new mount for a 25mm shim or 7mm for a 20mm shim.; both figures will increase as the mount is adjusted for settlement.

 

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1 hour ago, Eeyore said:

Wow indeed, I don't think I could manage that with my fat fingers 😇.

The mounts should/would have been readjusted after the first 48 hours or so, did that happen?

Sounds like the shim plate will be quite thick. In this case weld them in and drill and tap to allow the use of set bolts to hold the mount down. This makes them much easier to position during the set up as you don't need to grow a third arm to hold the nut under the beds!

If other people do the servicing is it possible that someone did you a favour by tightening a loose mount; by tightening the top nuts instead of the bottom one. 

 Other than that everything goes south with age as @IanD mentioned.

 

I too am getting a bit ancient. I had a socket welded to a bar as shown in the photo. This jams against the engine bearers  and I can tighten up without holding it. When tight I just knock it out. To tighten ( or loosen) from above I use a cheapo impact driver and socket with long extension bars. I finish off with a torque wrench. It’s worth replacing any nylocs. 

Before I used spanner’s etc and it was an awful job. It’s now quite quick and simple. I hope this simplified things fit you too. Good luck486D6424-3BD3-4F95-A426-CF418676FF55.jpeg.a614c091cb5a745aa5bb3e0f45e9b555.jpeg

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20 minutes ago, Eeyore said:

Best guess is about 30mm of exposed thread (assuming 1.5 mm pitch), way too high. No wonder it required adjusting several times. 

If I "inherited" that as a maintenance job I might say something like "it needs sorting, but live with it till you've had the life out of the mounts, it will give you time to save up for the remedial works". I'm sure you've had a similar conversation.

Minimum of 20mm shim/packer on the rear mounts, although I might go for 25mm given that you already know the static deflection. That would leave you with approx 2mm of exposed thread on a new mount for a 25mm shim or 7mm for a 20mm shim.; both figures will increase as the mount is adjusted for settlement.

 

Great, I feel like I'm on the right track.

Where is the best place for buying 20-25mm flat bar in small pieces? Approached one place but they weren't interested in cutting small.

 

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55 minutes ago, jessie said:

Great, I feel like I'm on the right track.

Where is the best place for buying 20-25mm flat bar in small pieces? Approached one place but they weren't interested in cutting small.

 

Give a location, I’m sure someone in your area will know of somewhere that supplies off cuts.

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