hard mount or rubber mount?

[Russellb]

I posed this question on another forum and the response was to use 2inch thick sheet led which makes sense to absorb the vibrations.

any other suggestions?

My recent purchase of a glass/ply twin hull 10mx4.5m to suit for wheelchair access has led to a question about hard mounted twin isuzu diesels 75KW(not verified yet) and shouldn't they be properly rubber mounted?

boat pics http://www.aussiecommunity.com.au/pics/twin-hull/twin.htm


When questioning the previous owners the response was basically they asked around which was better and then decided the easy way was to hard mount them.

Of course when doing the sea trial the noise is horrific to say the least. well noisy to me anyway, and vibrations.

Although take into consideration there is zero insulation anywhere on the boat so the noise will be louder anyway.

they are penta legs 19inch props on them.

anyone know if its common to hard mount engines? I will endeavor to have a mechanic to look at it and get them rubber mounted but wanted to know of any opinions on this.

thanks

russ

[Lew Barrett]

Marine engines were essentially hard mounted for many years, but if you are after noise and vibration isolation, using tuned engine mounts is the way to go. Now it has to be said that isolating your hull from noise and vibration requires more than just thinking about how you mount your engine. Of course there is the issue of silencers/mufflers and making a decision about how you will couple the tail shaft to the rest of the boat. If you want maximum isolation from vibration, and a degree of flexibility in alignment, you can also consider some sort of Python or Aqua drive. All of this is strictly optional, but to maximize the benefits of soft mounts, this is the program. If you go this route, you will also require a thrust bearing.

Rubber mounts by themselves only provide a small percentage of the solution. Think of noise and vibration reduction in system terms.

Barium/lead filled insulation should be part of the solution as well, but "2 inches of lead" is not the answer there AFAIK. Look to specific, multi-layer material with a membrane as your ER material, tuned mounts and a drive isolation system if you want to go the whole way. Or, go in steps and report back to us. Start by installing the insulation, which is the easiest job for a DIYer.

[Russellb]

Hi, thanks for the info.

What are tuned mounts? and tail shaft? I am not familiar with what i have accept that it is a volvo penta leg attached to the engine.

The assumption was that the expense of putting rubber mounts in may come from possibly having to change the system to align properly with the leg like exhaust, fuel or whatever. Of course i haven't had a good enough look to see what may or may not need changing to the system.

I will also sound proof the engine bay itself as there isn't any.

But yes ideally i do want to get rid of vibration. I want my wife to be able to sit in her wheelchair on the back deck whilst we putter along the waterways. At the moment i think the vibration would shake her off the chair!.

i found some things here that looks like might be useful http://www.acoustica.com.au/damping-plate.html

[nedL]

Lew has good information, I would just add that these four cylinder diesels may be pretty loud & rough (vibrations) regardless what you do. I'm sure things can be improved, but I wouldn't expect them to ever purr quietly.

[Lew Barrett]

When manufacturers supply an engine these days, the mounts are part of the package. In a well designed system, the degree of flexibility. type and amount of elasticity, is designed into the mounts in consideration of the engine's characteristics. This is what I meant when I said "tuned" mounts. If you are replacing the mounting system, be sure to check with the manufacturer or an experienced marine engineer to determine what they feel is a good recommendation. There are several manufacturers of after-market mounts that are intended for this sort of solution, but this sort of work is something of a specialty.

These days 4 bangers usually incorporate a counterbalancing system to alleviate the worst of the primary and secondary vibrations and the rocking couple that is typical of 4 cylinder engines, but I'm not familiar with your Volvo.

Mounts alone won't accomplish the full job. The tail shaft (or stub shaft) delivers thrust to your prop shaft and through it to the propellor. It is attached by a flange to your engine. In some applications, the shaft may be a single piece running straight back to the wheel, but in most designs where vibration isolation is accounted for, the shaft will be in two runs; the stub or tail shaft, and the actual "shaft" itself. While the engine and stub shaft may be free to rock a bit by dint of your newly installed mounts, unless some additional movement is allowed, the direct coupling through any additional supports (including the stuffing box/packing gland assembly) will limit the effectiveness of the mounts as such will equally limit the degree of movement available. In addition, thrust from the wheel will be transmitted back to the engine mounts, as this is "the hard point" that actually receives the drive from the propellor. Hence, soft mounts alone will not be the total solution since their ability to convert vibration to heat will be limited to the extent that their freedom to move will be limited by the fixed mounting points in the system.

Thus, some sort of "universal joint" or isolating drive mechanism will be required to make full use of the new mounts. In order for that device to do it's job, thrust must be isolated from the engine mounts and returned to the boat via a "thrust bearing," which is, simply put, a fixed point in the structure of the boat that takes the propellor's thrust and applies it at a point other than the engine mounts. This is the very point of an isolated drive system, and the integral "hard point" that is required to both drive the boat and allow the engine free movement.

I don't know if I've done a good job of describing this to you, but once you see such an application, the physics, which are relatively straightforward, will
become clear. Such an installation has of course a significant disadvantage: cost. But this is the now traditional solution to vibration and alignment control.
Noise is dealt with by proper installation of mufflers and noise absorbent materials, and then you are good to go.

Try reducing noise first by selective addition of noise absorbing materials and see where that lands you. You will need a water lift muffler as well, if one is not already installed.

[nedL]

Very well said, & good advise Lew.

[Russellb]

I think there is a slight misunderstanding on my description.

The volvo penta is the leg only. The inboard motors (twins) are isuzu 75KW, not sure how old.

I don't know much about the setup yet but i assume there is some sort of gearbox that the outside leg attaches to. so as far as i know there is no stuffing box or shafts to worry about.

I have to get it slipped and antifouled and surveyed for insurance yet which i hope to do within the next 2 weeks. Then i will see if the marine mechanic can align the engine in such a way with mounts or whatever to reduce vibration.

But yes I will sound proof the engine bays first and see how much that reduces the noise effect.
rough it
I do understand what your saying though whats involved in reducing noise/vibration as I just went through it with my bay cruise and spent 7K on the V-Drive system and its still has noise lol but no where near as bad as it was.

[kc8pql]

The volvo penta is the leg only. The inboard motors (twins) are isuzu 75KW, not sure how old.

I think he means he has I/O.

http://www.aussiecommunity.com.au/pics/twin-hull/twin3.JPG

[Russellb]

yes thats correct :), so many different terminolgy these days :)

[Lew Barrett]

That's an entirely different beast.

[Russellb]

Thanks Lew, its all the same principle though with what you are saying. so its all good info and helpful.

will update with any progress in the next couple of weeks.