Hard vs Soft Engine Mounts

[GILow]

Hello Cruisers,

I have recently purchased a Swanson 42 with a Nissan ED33 diesel, 96HP (marinised by Diecon). The engine has been inspected and found to be in excellent condition and only has 2000 hours. The boat was setup by somebody who really knew what he was doing, but something they have done has me a bit confused.

The engine is HARD mounted to the boat. There are absolutely NO flexible mountings anywhere on the drive train. Needless to say, a 96HP 4 cylinder diesel produces a staggering amount of vibration, even in an 18 ton boat. The gearbox is a BorgWarner Velvet drive (can't remember the model), it too is hard mounted.

I am not about to rush and change anything here, since it is all working very well, but for future planning I would consider soft mounting the engine unless there is a good reason not to. Yes, I do know it will involve a lot of work, but I have tackled and completed much bigger engine jobs than this.

But WHY did he do it that way? I am really reluctant to change anything by this guy, everything has been so well thought out (but OMG, the vibration!), so there must be a good reason. Does anyone know why he might have done it this way?

Matt

[djmarchand]

Commercial fishing boats are often mounted hard like yours, but recreational boats almost always have soft mounts. Yes a 4 banger produces a lot of vibration.

Check with Barry Controls. They are a big supplier of soft mounts. You will probably have to cut down your stringer to make room for the mounts and probably have to fabricate an aluminum mounting plate.

David

[sailorboy1]

maybe start with a flex coupling between the shaft and engine?

[djmarchand]

A flex coupling won't do anything for a hard mounted engine and won't help a soft mounted engine that is properly aligned.

David

[SkiprJohn]

If you do soft mounts for the engine then use a flex coupling. If you keep your hard mounting then don't do a flex coupling. If you mix the two then something will eventually become misaligned and each time the driveline moves a bit and the engine remains stable there is a bit of wobble that really is bad for bearings.
If you can stand it just remain with the hard mounting until it fails then change to soft. Sometimes the vibration you have might be due to misalignment. Have you checked that?
If the extreme vibration only occurs while the gear is engaged then there might be an alignment problem. If it occurs while you are in neutral as well then maybe not.
kind regards,

[zeehag]

use hard mounts so your driver line doesnt disintigrate. i learned the hard way..no, no flexible coupling. not going there--i will stiffen my mounts

[GILow]

Hi all,

Thanks for the replies, but sorry, I did not make myself clear. I am really trying to work out WHY he did it this way. The system is rock solid, and er... SkiprJohn... if the current hard mounts fail it would imply the stringers had failed. There are no engine mounts to fail, the engine is bolted straight to the stringers! The engine itself is, according to the diesel engineer (and I KNOW he knows his stuff, you should see his CV!) running exactly as it should, not vibration except exactly what should be there. No problems with the prop shaft or gearbox either, been through the whole RPM range and everything feels as it should, you just FEEL every bit of it.

Aside from this, there seems to be a few pros and cons on using a soft coupling between the engine and the driveshaft, but that's a whole separate question. I just feel I must be missing some really good reason for hard mounting the engine, the guy who set this boat up did everything for a good reason.

Matt

[GILow]

Zeehag, sorry, I did not see your reply.

Maybe it as simple as that, maybe he was going for robustness... but I feel that a well maintained set of mounts and coupling should be very reliable. Perhaps in a boat where access is a problem I could understand avoid adding maintenance items but the access to this one is pretty good. Still, he was belting all over the Pacific in this thing, and away from home port for months at a time, so maybe you have picked the issue.


Matt

[Artif]

If you could see how much a boat moves and flexes you would never fit rigid engine mounts or solid couplings.
Just altering the rig tension can throw your alignment out.
It will be a costly exercise but flexible mounts/coupling are definitely preferred.

[GILow]

Very interesting thought... but I would be quite suprised if a Swanson 42 flexed enough in that area to cause problems. They are built like the proverbial ----house (sail about as fast too ). I can certainly imagine a less solidly built boat flexing though.

Matt

[Artif]

RNLI Arun class life-boats moved over 0.005" on the coupling, going from a well supported cradle to floating, and I've yet to see a yacht built anything nearly as heavy as they are.
And yes they use flexi couplings and mounts.

[SkiprJohn]

Sorry for the misinterpretation. Thought you were asking for a way to feel less vibration and wanted to caution you against adding a flex coupling on a hard engine/trans installation. Don't know why he did it that way except that he didn't care about vibration and wanted a more secure mount.
kind regards,

[delmarrey]

Originally Posted by GILow

Very interesting thought... but I would be quite suprised if a Swanson 42 flexed enough in that area to cause problems. They are built like the proverbial ----house (sail about as fast too ). I can certainly imagine a less solidly built boat flexing though.

Matt

I think I would be checking your mountings often for loose/sheared bolts. How is the noise factor? It's a good thing it's not a steel/alum hull.

[GILow]

Originally Posted by Artif

RNLI Arun class life-boats moved over 0.005" on the coupling, going from a well supported cradle to floating, and I've yet to see a yacht built anything nearly as heavy as they are.
And yes they use flexi couplings and mounts.

OK, I conceed the point! Even a Swanson is probably not that well built!

[GILow]

Originally Posted by delmarrey

I think I would be checking your mountings often for loose/sheared bolts. How is the noise factor? It's a good thing it's not a steel/alum hull.

Funnily enough, noise is not too bad. I wouldn't want to run the thing at night, but that's mostly because the layout of the boat clusters the sleeping arrangements around the engine, so unless you plan to sleep in the dodger, you will be at most three feet from a lumpy great diesel.

I suppose the thick GRP soaks up a bit of the noise on the rest of the boat, but yeah, a steel hull would ring like a bell with the current setup.

The mountings all look good and tight, and more than capable of supporting the engine if we get rolled.