Engine mount question

[Emma b]

Hi, currently the engine (Perkins 4108) is out for overhaul.

 

It was mounted on the wooden beams and simply bolted down, albeit all 4 bolts have been loose for sometime and have elongated the bolt holes in the mounts where they bolted down into the boat.

 

My question is; would you remove the wooden beams and fit engine mounts or not?

 

There is no apparent thrust bearing and the box is a ZF Hurth 150.

 

Any advice greatly received

 

Simon

[OldGoat]

Starting the ball rolling - I would; looks like a bit of bodge job. There is what looks like a car U/J (universal joint) so all the thrust goes to the gearbox and the timber engine mounts are softwood.

I'm sure others will disagree...

[kevinl]

You should be able to get oak 2 inch square for under £10 a metre, so I guess it's a bout £30 job, seems a shame not do it while the engine's out.

As OG says it does from the pictures look like it could be softwood, if it is then I'd certainly change it for hardwood.

K

[bizzard]

Ideally the hardwood bearers need bolting down to the steel mounts similar to as they are with large plates under the bolt heads on the wood to prevent them sinking into the wood and slackening off. The engine ideally should be bolted down onto long steel plates laid on top of the wooden bearers with 4 square windows cut into the sides of the wooden bearers to take the engine mounts bolts 'nuts' again up against large steel plates located above the nuts to stop em sinking in. By mounting the engine with the mounting bolt nuts in the square cut outs will isolate the engine mass from the boats steel bearers and so will cut down some transmitted noise. All nuts need to be lock nutted or self locking nuts used.

Nothing wrong with using a vehicle propshaft coupling, but that one has not been assembled correctly, its yoke spiders are not aligned correctly with each other which could be corrected easily by relocating each half with each other at the splined sliding joint but this I reckon has been welded up solid to transmit the thrust in either direction. The present misalignment of these spider yokes would cause knuckling up and vibration if they're any more than just a little out of straight.

[ditchcrawler]

Ideally the hardwood bearers need bolting down to the steel mounts similar to as they are with large plates under the bolt heads on the wood to prevent them sinking into the wood and slackening off. The engine ideally should be bolted down onto long steel plates laid on top of the wooden bearers with 4 square windows cut into the sides of the wooden bearers to take the engine mounts bolts 'nuts' again up against large steel plates located above the nuts to stop em sinking in. By mounting the engine with the mounting bolt nuts in the square cut outs will isolate the engine mass from the boats steel bearers and so will cut down some transmitted noise. All nuts need to be lock nutted or self locking nuts used.

Nothing wrong with using a vehicle propshaft coupling, but that one has not been assembled correctly, its yoke spiders are not aligned correctly with each other which could be corrected easily by relocating each half with each other at the splined sliding joint but this I reckon has been welded up solid to transmit the thrust in either direction. The present misalignment of these spider yokes would cause knuckling up and vibration if they're any more than just a little out of straight.

If it was mine I would like a thrust bearing at the back end.

[OldGoat]

If it were me, I would prefer 'proper' / conventional rubber bonded engine mounts (but I did work at Silentbloc) and it was drummed into to me elsewhere that good quality flexible engine mounts were 'the things to have'. Solid mounts are the only way for large old heavy engines (as a flexi mount would be *** expensive). Having hired boats in the distant past with solid mounts - where the experience was 'uncomfortable' at least - I'd go for flexies, now that the engine is out - 'cos you won't have that opportunity again (hopefully, if you see what I mean).

That was the OP's question.

 

OTOH, it's a matter of 'cost-benefit'. I've been spoilt with quiet cruising, because that was my experience when we hired and I carried that one when I built my boat.

Other folks have different needs /uses and costs come into it. A set of flexi mounts are about £160 - which might not be acceptable.

I say 'Do It' .....

'Bolts loose, holes elongated' suggests that the original scheme is not effective. Get some rubbers in....

Edited November 9, 2015 by OldGoat

[bizzard]

If it were me, I would prefer 'proper' / conventional rubber bonded engine mounts (but I did work at Silentbloc) and it was drummed into to me elsewhere that good quality flexible engine mounts were 'the things to have'. Solid mounts are the only way for large old heavy engines (as a flexi mount would be *** expensive). Having hired boats in the distant past with solid mounts - where the experience was 'uncomfortable' at least - I'd go for flexies, now that the engine is out - 'cos you won't have that opportunity again (hopefully, if you see what I mean).

That was the OP's question.

 

OTOH, it's a matter of 'cost-benefit'. I've been spoilt with quiet cruising, because that was my experience when we hired and I carried that one when I built my boat.

Other folks have different needs /uses and costs come into it. A set of flexi mounts are about £160 - which might not be acceptable.

I say 'Do It' .....

'Bolts loose, holes elongated' suggests that the original scheme is not effective. Get some rubbers in....

If they did fit rubber mounts that vehicle propshft would need assembling correctly.

[David Mack]

It looks as if the bolts ran through the engine mounting flange, wood and steel plate attached to the engine beds. That will (if the bolts are tight) give a direct metal to metal contact which will transmit vibration to the hull. As Biz says the engine should be bolted down to the (hard) wood and the wood should be separately bolted to the engine bed. That way the wood will damp some of the vibration.

 

The yolks of the UJs should also be aligned i.e. the two middle pairs of 'ears' should be opposite each other, and likewise the two outer pairs of 'ears'. That will minimise the transmission of torsional vibrations. With this setup the gearbox output shaft should be offset from the stern shaft, so that the two UJ's are operating at a slight angle to straight ahead. That causes the UJ parts to move relative to one another, spreading oil and evening up the wear. If necessary, this can be achieved by using a thicker or thinner piece of hardwood as appropriate.

[bizzard]

If it was mine I would like a thrust bearing at the back end.

Or do away with the vehicle propshaft and fit a longer single shaft and connect up half coupling to half coupling direct and accurately aligned in the conventional manner, but the mounts need to be proper as I described.

[ditchcrawler]

If it were me, I would prefer 'proper' / conventional rubber bonded engine mounts (but I did work at Silentbloc) and it was drummed into to me elsewhere that good quality flexible engine mounts were 'the things to have'. Solid mounts are the only way for large old heavy engines (as a flexi mount would be *** expensive).

I didn't know how heavy that engine is,

With this setup the gearbox output shaft should be offset from the stern shaft, so that the two UJ's are operating at a slight angle to straight ahead. That causes the UJ parts to move relative to one another, spreading oil and evening up the wear. If necessary, this can be achieved by using a thicker or thinner piece of hardwood as appropriate.

What about the trust if the prop, shaft, gearbox are not in a direct line? I appreciate that the UJ's should run at an angle to lubricate but they are not normally under compression as in this case when the boat is being driven forward.

[DHutch]

Am I right in thinking, in your installation, there bolts go right through the wood from the engine to the mounts on the hull. Rather than the engine being bolted to the wood, and the wood being bolted to the hull. As said, a direct metal connection from the engine to the hull is what your trying to avoid.

 

Either way, the obvious question is, was there anything wrong with the installation before it was removed? Are you looking for an improvement in vibration levels, or just looking for extra work?

 

 

 

Daniel

[PeterG]

The bearers look like pressure treated softwood I have used for the joists in my shed, hence one of the reasons for the elongated holes.

[BEngo]

Engine bearers should be softwood. Softwood damps vibrations much better than hardwood. Douglas Fir/Oregon Pine or Western Red Cedar are the stuff to have. Then, as above, bolt the engine to the bearers and the bearers separately to the hull.

 

In this case the engine is transmitting the thrust, so there should be some chocks or vertical stops against the engine feet to transfer the thrust back into the boat.

 

 

N

[nb Innisfree]

To prevent holes elongation I would use large washers that are a close fit on the mounting bolts, drill 3 or 4 holes in the washers and screw them to the bearer, or better still try to seat the washers into a close fitting recess in the wooden bearer. Or maybe some toothed washers to grip the wood.

[Emma b]

Many thanks for your replies and apologies for not replying before now,been very busy.

 

If I were to fit flexible mounts how would a thrust bearing be incorporated and would this mean changing the current propshaft?

I cannot argue with the term 'bodge' in an earlier post. It is a lovely Mel Davis 50Ft shell but the engine and general engine bay fit out has been somewhat uncared for hence its removal and current overhaul.

I have attached images that show how the engine mount holes have elongated. This seems to be primarily due to all of the bolts being loose. Amazingly there was no apparent excessive engine vibration. The shanks of the bolts were too long so when tightened were not actually clamping anything down.

It would be easier to keep the current layout but alter the single bolt per corner to the ideas of separate bolts for the engine mounts and then the bearers to the boat mounting point.

The image of the prop shaft shows the misalignment although as it does not turn as such would this be such an issue?

Many thanks

Simon

[nicknorman]

So it sounds as though the problem was loose bolts. Not too surprising then, that the holes have become elongated. If you were quite happy with the vibration level before, why not put it back as it was (minus the elongated holes and inappropriately long shanked bolts of course) and just keep an eye on the bolt tension from time to time. Wood is obviously subject to dimension charges and compression over time.

[ditchcrawler]

keep an eye on the bolt tension from time to time. Wood is obviously subject to dimension charges and compression over time.

Good point, I will nip mine next time I am on the boat

[nicknorman]

Good point, I will nip mine next time I am on the boat

It's always a tricky one because if you "nip them" too much the wood gets crushed. I work on our Robin DR400 tow plane which is a wooden airframe with a steel engine mounting cradle. It is always tricky at the interface between steel and wood. The maintenance manual offers no help in terms of a torque setting, rather is says "tighten in accordance with the art". Bloody French! For the wooden propellor there is a torque setting for the bolts and interestingly, although you have to check the torque from time to time, you do that only by re-applying the correct torque, as opposed to the normal technique with steel bolts into metal which would be to slacken first and then reapply the torque.

[Emma b]

Thanks nicknorman, this would be the easiest route. I will repair the mounting holes as a matter of course and the correct bolts with appropriate washers is easy to resolve too.

Now I am aware of this the bolts are fairly accessible so periodic inspection would be the norm.

[bizzard]

You need to lay full length steel plates of at least 3mm thickness on top of the wooden bearers to spread the load and to prevent the mounting feet sinking into the wood. Wide 3mm or 6mm thick angle iron would be good for this and drill through for the feet.

An angle grinder used carefully might cut through the weld on that prop shaft to enable it to be moved onto the correct spline position and then re-welded. I would certainly keep it solid mounted.

[ditchcrawler]

It's always a tricky one because if you "nip them" too much the wood gets crushed. I work on our Robin DR400 tow plane which is a wooden airframe with a steel engine mounting cradle. It is always tricky at the interface between steel and wood. The maintenance manual offers no help in terms of a torque setting, rather is says "tighten in accordance with the art". Bloody French! For the wooden propellor there is a torque setting for the bolts and interestingly, although you have to check the torque from time to time, you do that only by re-applying the correct torque, as opposed to the normal technique with steel bolts into metal which would be to slacken first and then reapply the torque.

I do that the advantage, or disadvantage depending on which way you look at it of having oak bearers

[Emma b]

OK Bizzard thanks, both of those are also easy to do also I would say.

[ditchcrawler]

OK Bizzard thanks, both of those are also easy to do also I would say.

Just to elaborate on Bizzards post, you can see on this engine the long flat steel that spreads the load onto the wood.

[bizzard]

Just to elaborate on Bizzards post, you can see on this engine the long flat steel that spreads the load onto the wood.

Yes Brian that's the sort of thing but by just laying the plates or angle iron on the wooden bearers and drilling through for the feet would do fine Also the bolt heads underneath in windows cut into the sides of the wooden bearers or in large rebates underneath also need as larger plates as possible fitted to stop the bolt heads sinking in.

If using the window method threaded stud is used for the four main mounting instead of bolts, with nuts top and bottom.

Edited November 16, 2015 by bizzard

[Emma b]

Ok great many thanks. A soft or hard wood?