Engine mounts - Fixed v flexible

[Bloomsberry]

Can't find any info on the net for this so I've decided to post it here.

The question is although flexible engine mounts have the advantage of reducing engine vibration getting transmitted to the hull, doesn't this also make everything attached to the engine vibrate too. This includes gearbox, shaft, fuel & exhaust connections, electrical connections to the alternator, the alternator itself etc. And won't this vibration cause stress and eventual wear/ fracture ?

So aren't fixed mountings more favorable due to the engine being held in place and hence all the connections to it are more stable as a result ?

Or am I missing something ?

[RLWP]

Do all the flexible connections on your car fracture?

 

Richard

[luctor et emergo]

Not many cars with Lister engines. Or Gardners...

[Charlotte Burnett]

Yes, you are right, however, solid mounts are also much more noisy. You work out your compromise. What engine have you got?

[PhilR]

Can't find any info on the net for this so I've decided to post it here.

 

The question is although flexible engine mounts have the advantage of reducing engine vibration getting transmitted to the hull, doesn't this also make everything attached to the engine vibrate too. This includes gearbox, shaft, fuel & exhaust connections, electrical connections to the alternator, the alternator itself etc. And won't this vibration cause stress and eventual wear/ fracture ?

 

So aren't fixed mountings more favorable due to the engine being held in place and hence all the connections to it are more stable as a result ?

 

Or am I missing something ?

If a marine engine is perfectly aligned on solid mounts there should be minimum wear on the stern gear. When an engine is on flexible mounts it is essential to have a good quality flexible coupling between the gearbox and prop shaft + flexible fuel pipes etc.

If you are happy with the extra vibration and noise that is usually associated with solid mounts you will probably have an installation that is more reliable in the long term.

 

It is horses for courses ..........

[RLWP]

'Reliable' and 'long term' are very loose terms

 

Richard

[Bloomsberry]

My engine is an old BMC 1.5 in an old narrowboat that is fixed mounted (the engine , not the narrowboat !) .

So I assume the flexible mounts eventually wear and need replacing as will the flexible shaft coupling ? So what kind of timespan and expenditure are we talking of here and what damage can be done if they fail prematurely ? Does alll this justify the need for a quieter boat ?

[RLWP]

I'm not really clear why you are asking these questions.

 

Fitting flexible mounts to a boat with a fixed BMC 1.5 is going to be a complicated job because of having to provide sufficient height for the mounts between the engine and the bearers. Then you will have to have enough length between the propeller shaft and the gearbox for a flexible joint. With the cost of the components and modifications, I don't think you will want to do this job

 

Personally, I wouldn't buy a boat with a modern four cylinder engine that didn't have flexible mounts

 

Richard

[nicknorman]

... Does alll this justify the need for a quieter boat ?

Yes. Not just quieter but less vibration inside as well. I want to be able to relax inside when its windyrainy, causing mayhem on CWDF, whilst Jeff is outside steering.

 

But I think it depends on the type of engine. The energy in vibration is related to the frequency so if you have an old thumper, the vibration is much less offensive than a faster revving engine. That's why vintage engines tend to be on solid (wooden) bearers whilst modern jap tends to be on flexi mounts. BMC s seem to fall somewhere in the middle but if I had one I would want it flexi.

[luctor et emergo]

 

Snip..

 

Personally, I wouldn't buy a boat with a modern four cylinder engine that didn't have flexible mounts

 

Henk

Edited May 5, 2013 by luctor et emergo

[Bloomsberry]

I'm not considering changing the mounts to my engine, I'm just curious as to why flexible mounts are preferred to fixed these days.

 

If it is just for a quieter running then fair enough.

 

No doubt if they weren't successful then everybody would have fixed mountings.

Thanks for the replies.

[Arthur Brown]

Flexible mounts will absorb vibration BUT the possible flex needs to suit the transmission. The transmission thrust needs to be transmitted to the hull at some point either in the gearbox or at the stern gland.

[RLWP]

I'm not considering changing the mounts to my engine, I'm just curious as to why flexible mounts are preferred to fixed these days.

 

If it is just for a quieter running then fair enough.

 

No doubt if they weren't successful then everybody would have fixed mountings.

 

Thanks for the replies.

 

They are used for exactly the same reason as in your motor car - refinement

 

Richard

[Tony Brooks]

I am not saying the poster is wrong about his BMC having fixed mounts but the original Newage marinisation had flexible mounts that looked very much like fixed mounts until you looked closely. In fact we had a thread about this a while ago where I think several people saw a photo and said it was fixed until I jogged memories.

 

A photo of a rear foot would soon make sure.

[Mike Jordan]

Having tried both systems in years past on the 1.5 I found that the flexibly mounted engine with a separate thrust bearing and flexible drive shaft is far superior in terms of reduced noise and vibration. Its also advantageous to use the front engine mounts from a road vehicle that uses the 1.5 since these are designed to match the engine. The flexible mounts sold by the chandlers are not very good but do make it look easy to align the engine and drive shaft, something you don't need to do with the flexible drive shaft.

Many of the earlier conversions of the 1.5 don't have enough weight in the flywheel to smooth the vibrations, this is normally the result of removing the original clutch assembly and bolting a drive plate straight to the flywheel with no added weight to replace that of the clutch.

In years gone by most boat builders bolted the engines down to the steel with a timber bearer between, it was quick and cheap and only stopped happening when boaters demanded quieter boating.

The rule I apply is that the steerer should be able to hear the sound of the water from the propeller over the sound of the engine. If you can obtain a copy of a book called Marine Conversions by Nigel Warren it has all you need to know about quiet boating.

Mike.

[Tony Brooks]

snip

 

Many of the earlier conversions of the 1.5 don't have enough weight in the flywheel to smooth the vibrations, this is normally the result of removing the original clutch assembly and bolting a drive plate straight to the flywheel with no added weight to replace that of the clutch.

 

snip

 

Mike.

Actually all the early official BMC conversions used bulk rings bolted to the flywheel which in my experience were far heavier than the clutch assembly. Also I think the industrial version of the engine had a heavier flywheel as standard. It was the "Freds in sheds" conversions that did not use a bulk ring or heavier flywheel - especially when the likes of Meaks of Marlow started to retail their own marinisation parts.

[Bloomsberry]

I am not saying the poster is wrong about his BMC having fixed mounts but the original Newage marinisation had flexible mounts that looked very much like fixed mounts until you looked closely. In fact we had a thread about this a while ago where I think several people saw a photo and said it was fixed until I jogged memories.

 

A photo of a rear foot would soon make sure.

I'll take a photograph next time I'm at the boat but I"m pretty sure the legs are just bolted to the boat bottom, in fact the front 2 legs are attached to the gearbox (borg warner velvet drive) not the engine, is this common ? . There is no issue with it,

 

I was just curious as to the pros & cons of both fixing methods.

 

Edited May 6, 2013 by Bloomsberry

[gazza]

I'll take a photograph next time I'm at the boat but I"m pretty sure the legs are just bolted to the boat bottom, in fact the front 2 legs are attached to the gearbox (borg warner velvet drive) not the engine, is this common ? . There is no issue with it,

 

I was just curious as to the pros & cons of both fixing methods.

 

The mounts won't be on the box, they will be on the adaptor plate.

My money is you have a thornycroft 90 with a Borg warner velvet drive 70c box, is it painted light blue hammerite?

[Tony Brooks]

I have never seen a Velvetdrive with the rear engine mounts on it. In fact the only gearbox I know that has mounts was the BMC DCB and the two mounts on the gearbox gave a total of 6 mounts.

 

Do the rear mounts have a sort of steel tube about 2 to 3 inches high stuck up with a nut or bolt holding it down in the middle? If so they are standard Newage flexible mounts but by now have probably collapsed the foot right down hard on the bed.

[lexicon]

I am not saying the poster is wrong about his BMC having fixed mounts but the original Newage marinisation had flexible mounts that looked very much like fixed mounts until you looked closely. In fact we had a thread about this a while ago where I think several people saw a photo and said it was fixed until I jogged memories.

 

A photo of a rear foot would soon make sure.

 

Hi Tony

 

think that the engine mounts in question were mine.

they had collapsed and one front mount had well fallen apart (split in half.)

 

after the excellent work of primrose engineering (cant recommend highly enough)

they were replaced and the difference in noise and vibration was astounding.

[Bloomsberry]

I have never seen a Velvetdrive with the rear engine mounts on it. In fact the only gearbox I know that has mounts was the BMC DCB and the two mounts on the gearbox gave a total of 6 mounts.

 

Do the rear mounts have a sort of steel tube about 2 to 3 inches high stuck up with a nut or bolt holding it down in the middle? If so they are standard Newage flexible mounts but by now have probably collapsed the foot right down hard on the bed.

Hope this works, the gearbox brackets are in the background, top left. It appears that the brackets sit on some round spacers before the floor of the boat.

 

http://s1329.photobucket.com/user/Graham_Jackman/media/IMG_3037_zpsd681a8b4.jpg.html?sort=3&o=0

 

And yes the engine is a light blue colour

[RLWP]

 

Richard

[gazza]

That's a crude set up!

Your brackets are bolted to the casing using a single bolt, the side we can see is mounted infront of the optional neutral inhibitor switch cover (3 bolt cover) the opposite side is just infront of the dipstick/filler, the rating plate will be above the bracket correct?

You may be able to get the mounts from rs components or similar.

Our setup is mounted from the sae adaptor plate, which was what I was expecting to see on yours. The mounts tony is talking about are nothing like yours.

[Tony Brooks]

From the little I can see I very much doubt its a Velvetdrive.

 

A better photo of the gearbox would help.

[Timleech]

This might be a bit clearer:-

 

 

Tim