Main engine sound enclosure

[Boathome]

Most generators on modern powerboats are installed inside sound enclosures - example: Northern Lights

But I haven't seen a single example of a propulsion engine enclosed to reduce noise, even on the latest designs and even when there is plenty of room for a sound enclosure - example: FPB Dream Machine (scroll down or search for "John Deere").

Even passage-making motor yachts like Nordhavns don't have them and owners typically keep a pair of ear muffs at the engine room door so they don't destroy their hearing when they have to work in the engine room under way.

I can't work out why this is so. It can't be cooling because diesel generators also produce heat. It can't be maintenance access because diesel generators need that too. If the main engine is in a sound enclosure weight could be saved by reducing the amount of sound insulation around the engine room. So why isn't this done? Anyone know?

 

[rasorinc]

All I can think of is it is easier to apply panels of sound insulation to walls, ceiling and floors than to insulate around all the angles and projections of an engine. Also, less cost most likely
as sound insulation comes in 4' x 8' sheets and you can cut it with a knife.

 

[Boathome]

@rasorinc
Thanks for your reply but the genset manufacturers don't have any trouble doing it so it can't be too difficult. They have the same angles and projections to deal with - they just make a sound insulating box and cover the genset with it. Also the amount needed to insulate an entire engine room is huge - therefore heavy and expensive. Why not enclose the propulsion engine just the same as the genset?

 

[CDK]

There are a number of reasons why it isn't done.

Soundproofing a small generator is relatively easy: a large ABS coffin packed with mineral wool, just a few hoses and cables connecting it to the outside world. The engine stands on soft rubber mounts attached to a sub-frame within the enclosure.

For a propulsion unit requirements are different. Size, weight, thrust, torque forces and the prop shaft make it an expensive project. Insulating the engine room is easier.
Also, the genset just has a small remote control panel that warns the owner if something is wrong and the generator has switched off. Annoying, but back in the marina he calls a technician who opens the lid and services the unit. The boat's main engine cannot be treated that way, it must be accessible at all times.

 

[PAR]

Insulation and sound control aren't as easy as pasting some cut up 4x8 panels of insulation board from the big box stores. You have to make some decisions about what you're trying to do (noise, vibration, heat, etc.), then use the products that work. Good insulation isn't cheap, but it works.

 

[Boathome]

@CDK - thanks for your reply. OK, I get why it isn't done. But I have a reason why I would like to cut the sound down in the engine room: almost every pump and other mechanical device, and the workbench, is going to be in there so I'm going to have to work there sometimes when the main is running.

Looking at the factors in more detail:

Size, weight - doesn't that just means a bigger enclosure?

Thrust, torque forces - not sure they matter. The engine would still be fixed to the boat in the exact same way - the enclosure would fit over it. Thrust and torque will be counter-acted as if the box wasn't there. Obviously harder to seal around the bottom of the enclosure than with a genset - but not impossible

The prop shaft - this is the hardest part and I haven't solved it yet.

The boat's main engine .... must be accessible at all times - my best guess is that this is the real reason it isn't done. People don't like having their main engine hidden away in a box. They want to see it. Heretical thought: it's superstition at play here, not logic. "If I can see (and hear) my engine hammering away and generating the most awful racket I know it's OK". But the engine I am specifying is most likely a John Deere - the same engine that takes FMBs and Nordhavns around the world. And in my business I owned a 5kva genset with good inspection doors that gave excellent access to all the bits that do need attention. I don't think a sound enclosure would restrict access to any meaningful extent if it is designed properly. What it would do is make the engine room a tolerable workspace. It would also save some weight by reducing the amount of sound insulation needed around the walls and overhead.

Does that make any sense?

 

[dinoa]

There are mega yachts that have the whole engine room sound insulated and shock mounted to a massive raft that is itself shock mounted to the ships' frame. This of course introduces the problem of prop alignment which is through a flexible coupling such as a Vulkan coupling. Naval vessels use the same technology to minimize their sound emissions and to isolate their vitals from blast shock.

Dino

 

[raysugar]

The propulsion engines are much more stressed than the other. Gensets for their entire working life will give power always at the same rpm, power etc. You can imagine how differently propulsion engine work profile are: manoeuvring, acceleration and also axis vibration, thrust that act on it etc (as CDK pointed out), thus nobody will design a case like ones for gensets. Furthermore any codes and regulamentations give precise rules about insulating, fire fighting etc i.e. you will always need to insulate your engine room in the same way, you would have done without insulating the propulsion engine.
In my opinion, it isn' worth design such case

 

[keysdisease]

I have seem propulsion engines in enclosures on one vessel. The vessel was very quiet but not any quieter than many yachts with good and well executed engine room insulation plans. The engineer hated it, visibility and access was difficult, the panels were large, heavy and difficult to manage. Once you had them off to access the engine they had to go somewhere, (in the way) and there is never enough room in an engine room. This was a 100ft + vessel, I can't imagine this even being workable on anything smaller.

There are several more and much larger penetrations in a propulsion enclosure than a generator, and not all of the connections are "soft" as with a generator (shaft.) Exhaust systems for propulsion engines also need to be addressed. Exhaust noise is one of the 3 types noise engines produce, airborne, structure-borne and exhaust. Exhaust noise is just the other side of the wall thickness of the exhaust piping. This is serious noise and would need to be addressed from the enclosure until the piping exits the engine room or vessel. The air intakes to these enclosures are also problematic as they have to be large enough to supply combustion air which allows noise to escape. This air path is typically baffled which reduces the noise that escapes the enclosure, and that "escaped" noise typically becomes the baseline for enclosure design.

Consider the environment we are talking about. The propulsion engines will only be operating while the vessel is underway, and underway the vessel will experience wind and water noise as well as driveline noise it will not experience while at anchor or dockside. The goal on a well executed insulation plan for a vessel underway is to reduce the airborne engine noise to a level equal to or slightly lower than driveline + wind and water noise while underway at typical cruise speed (or speed designated by the project.)

It is at anchor (or dockside) that the vessel is expected to operate at its quietest, and therefore a generator in an enclosure (and perhaps double isolated) in an insulated engine room is appropriate and provides the quietest "at anchor" operation.

Engine room insulation is not that difficult to do well and correctly. The real attention needs to be only on the forward bulkhead and overhead, and perhaps the aft bulkhead depending on the arrangement. The structure of these surfaces bring considerable mass to the table as an insulation plan is developed. For the hull sides, insulation is typically optional, and usually if insulation is installed only absorption is installed without a noise barrier.

Propulsion engine enclosures can certainly be done, and have been. I believe why you don't commonly see it is that when this seemingly good idea is fully weighed against conventional engine room insulation, the conventional method wins.

Steve Moyer
Soundown Ft Lauderdale
www.soundown.com

 

[gdavis]

On one of my more recent jobs I had to cut in an access panel into the engine room of a 85' swan. There was never much thought put into access to the alternator. There was two layers of 2" foam with a composite sound deadening layer in the middle of each one. I'm pretty sure it was soundown or was very much like it.When I was done and all the hatches and panels were closed we could barely hear the engine even standing right next to the engine room. And while standing at the helm all you could hear was the low rumble of the exhaust. So it can be done and is done. I have been on many new builds where the right sound insulation and special attention given to vibration isolation resulted in extremely low levels of engine noise. If you want to know how to achieve this talk to someone who's business is putting the hush on marine engine noise.(I know for a fact that the good folks at soundown are about the best you'll find) It's hard to beat a man at his own game!........peace out.....g.....ps, you owe me one steve

 

[keysdisease]

Thanks G Davis, checks in the mail. Steve

 

[michael pierzga]

@CDK - thanks for your reply. OK, I get why it isn't done. But I have a reason why I would like to cut the sound down in the engine room: almost every pump and other mechanical device, and the workbench, is going to be in there so I'm going to have to work there sometimes when the main is running.

Looking at the factors in more detail:

Size, weight - doesn't that just means a bigger enclosure?

Thrust, torque forces - not sure they matter. The engine would still be fixed to the boat in the exact same way - the enclosure would fit over it. Thrust and torque will be counter-acted as if the box wasn't there. Obviously harder to seal around the bottom of the enclosure than with a genset - but not impossible



The prop shaft - this is the hardest part and I haven't solved it yet.

The boat's main engine .... must be accessible at all times - my best guess is that this is the real reason it isn't done. People don't like having their main engine hidden away in a box. They want to see it. Heretical thought: it's superstition at play here, not logic. "If I can see (and hear) my engine hammering away and generating the most awful racket I know it's OK". But the engine I am specifying is most likely a John Deere - the same engine that takes FMBs and Nordhavns around the world. And in my business I owned a 5kva genset with good inspection doors that gave excellent access to all the bits that do need attention. I don't think a sound enclosure would restrict access to any meaningful extent if it is designed properly. What it would do is make the engine room a tolerable workspace. It would also save some weight by reducing the amount of sound insulation needed around the walls and overhead.

Does that make any sense?

Ive never seen a boxed main engine. I suppose it could be done. The size and weight of the box panels would make it clumsy to service the engine. Air in and air out is a substantial volume . Ventilation will be an issue.

On a boat very much noise is transmitted into the boat through the propshaft and engine mounts. A generator will be double soft engine mounted...the engine on soft mounts, then the generator package on another set of soft mounts . Main engines are set on hard mounts.

 

[CDK]

Thrust, torque forces - not sure they matter. The engine would still be fixed to the boat in the exact same way - the enclosure would fit over it. Thrust and torque will be counter-acted as if the box wasn't there. Obviously harder to seal around the bottom of the enclosure than with a genset - but not impossible

The generator I am familiar with looks like a giant Samsonite case, kept in place with 4 lugs at the corners. The interior has its own sub-frame within the box and all external connections are flexible so no vibrations are transferred to the hull. The owner asked me to check the circuit because the microwave didn't work and the green light on the control panel was off. It turned out to be a corroded ground terminal in the box so it was quickly fixed.
With the cover closed and clamped the only way of knowing the set was operating was the green light and water leaving the thru hull, no sound at all.

With an engine on classic engine mounts, taking tons of thrust it is simply impossible to reach that level of soundproofing.

 

[michael pierzga]

Yes indeed. The bonding strap on a generator is a very important detail and its prone to failure because generators are so softly mounted that the jump around. Exhaust and plumbing also need a good look.

Noise reduction is a skilled business.. So many components transfer noise and vibration into a hull.

 

[gdavis]

I think you'll find that a lot of the big boats with big power being built these days have soft mounts and flexible couplings with thrust bearings. Very little vibration being transferred to the engine beds, stringers and hull. The engines are not in boxes but the entire engine room is sound insulated to the max. Trust me, people paying millions of dollars for these boats will not put up with excessive engine noise. Most of the time they don't even want to hear water slapping against the hull. Uhh, why did you buy a boat??? ......No proplem steve, spent many yrs at Lm in Thomaston,me. It doesn't look like we'll ever see glory days of new builds like that again, 4 60'+ builds at a time, that was fun!...................................adios...g