Has engine soundproofing improved in the last 30 years?

[dankilb]

For you guys that have eliminated all gaps, where/how does combustion air enter engine bay?

From factory, our Jeanneau has two approx 100mm ducts to vents in the transom (covered with the sort of ‘fascia’ sometimes seen on mobos to prevent water ingress). Both then lead forward, under the aft bunks, to the engine compartment. One has an (exhaust) fan. The other looks to have had a blower fan fitted in the past, but I plan on keeping it as a passive intake.

Even if I seal the compartment itself, air can still be drawn from the bilge both fore and aft too. I reckon this should be enough - as the exhaust fan must help create some negative pressure/in flow? But the boat has been on the hard all of our ownership so yet to test it ‘in anger’.

 

[Moodysailor]

From factory, our Jeanneau has two approx 100mm ducts to vents in the transom (covered with the sort of ‘fascia’ sometimes seen on mobos to prevent water ingress). Both then lead forward, under the aft bunks, to the engine compartment. One has an (exhaust) fan. The other looks to have had a blower fan fitted in the past, but I plan on keeping it as a passive intake.

Even if I seal the compartment itself, air can still be drawn from the bilge both fore and aft too. I reckon this should be enough - as the exhaust fan must help create some negative pressure/in flow? But the boat has been on the hard all of our ownership so yet to test it ‘in anger’.

I would advise against an exhaust only fan for engine bay air intake - usually you would want an intake fan if you only had one choice. In an ideal world, you want slight positive pressure in the engine bay. The engine will create a vacuum when it runs, so adding to that with an air exhaust fan could starve the engine of power - typically this will only become evident when you need the power (such as punching a tide around a headland). The transom is also in a natural area of low pressure, so this combination of exhaust fan + passive intake in low pressure area would not be my choice.

I used to do a lot of commissioning and new boat verification for various manufacturers, and engine bay pressure during trails was one oft he aspects we always checked. It is amazing how much hotter the engines will run if there is a vacuum and how much power they lose.

 

[dankilb]

I would advise against an exhaust only fan for engine bay air intake - usually you would want an intake fan if you only had one choice. In an ideal world, you want slight positive pressure in the engine bay. The engine will create a vacuum when it runs, so adding to that with an air exhaust fan could starve the engine of power - typically this will only become evident when you need the power (such as punching a tide around a headland). The transom is also in a natural area of low pressure, so this combination of exhaust fan + passive intake in low pressure area would not be my choice.

I used to do a lot of commissioning and new boat verification for various manufacturers, and engine bay pressure during trails was one oft he aspects we always checked. It is amazing how much hotter the engines will run if there is a vacuum and how much power they lose.

Ah okay...! That's interesting and very helpful, thanks.

TBH various posts on here and elsewhere seemed to weigh up slightly in favour of exhaust (removing hot air, mainly) over induction. But that could have just been my reading of them and/or for different applications... it certainly didn't feel conclusive!

It's obviously easy enough to switch the fan to blow, rather than suck. Perhaps a bit of experimentation will be in order?

I guess it's also notable that the manufacturer deemed it necessary to have both fans. But the unit they installed (a sort of twin-scroll type thing with a bracket to hold two blowers side-by-side) is both corroded beyond repair and also takes up loads of space. I definitely want to get away with one fan, if possible.

 

[Moodysailor]

Ah okay...! That's interesting and very helpful, thanks.

TBH various posts on here and elsewhere seemed to weigh up slightly in favour of exhaust (removing hot air, mainly) over induction. But that could have just been my reading of them and/or for different applications... it certainly didn't feel conclusive!

It's obviously easy enough to switch the fan to blow, rather than suck. Perhaps a bit of experimentation will be in order?

I guess it's also notable that the manufacturer deemed it necessary to have both fans. But the unit they installed (a sort of twin-scroll type thing with a bracket to hold two blowers side-by-side) is both corroded beyond repair and also takes up loads of space. I definitely want to get away with one fan, if possible.

Definitely worth an experiment if unsure. The manufacturer would have fitted both fans to ensure adequate airflow so removing one may have an undesirable effect. I've seen some case where it was very difficult to open the engine hatch with the engine under load due to the vacuum created - imagine how hard the engine is working in those situations....

I don't get the theory behind only having an exhaust fan, what you want is forced induction circulating air over the engine for the best cooling effect - this is what most motorboat manufacturers have done for years, either by directed vents from the outside so the forward motion of the vessel creates the airflow, or by using fans. The bigger and/or better ones will have intake and extraction fans. There is no reason to neglect this on a yacht, although a yacht with a deeper bilges/keel can probably draw enough air from the bilges. On a newer yacht with shallow bilges, fin keel and saildrive I would definitely be ensuring there was enough air getting in from external sources.

If you have an exhaust only fan, and there is not sufficient air coming in then it will actually have the negative effect of increasing temps, not reducing them. The diesel engine is just a giant air pump remember, so it needs to be fed - not starved of air.

Good luck, the experiments will be easy enough - let me know if you need any help.

 

[prv]

The diesel engine is just a giant air pump remember, so it needs to be fed - not starved of air.

I’ve had this discussion with a naval architect on a new workboat design. He was adamant about only fitting exhaust fans - though paired with large passive intake vents of cross-section calculated to be large enough.

The reason is that, as you say, the engine is a big powerful pump. If it needs 10m² per minute to run (I have no idea what the actual numbers would be) then, as long as there are vents, it will pull that 10m² in quite handily all by itself. You don’t need to help it, any more than you need a fan pointing vaguely at the intake manifold of your car.

If you add a 2m² intake fan, the engine will happily consume that 2m², and continue to suck in the remaining 8m² it needs. You don’t get any more airflow through the space than you did before, and really the fan is doing nothing. (Intake fans are often paired with ducting, for example pointing at the alternator, and that’s a useful installation. But it’s useful because of the ductwork, not the fan.)

The only way you can increase the absolute airflow through the engine space is to make the intake fan hungrier than the engine - so it sucks in 12m², the engine uses 10m² of that, and you have 2m² left over to come out of an exhaust vent somewhere. But a fan that’s more powerful than a large diesel ends up being massive - in the specific boat we were designing, it actually worked out to consume more electrical power than was available on board.

If, instead, you flip the small 2m² fan around so that it’s exhausting from the space, it works alongside the engine and between them they suck in 12m² through the intake vents - same as the monster fan. And you can duct and direct that incoming cool air to wherever you need it - it’s flowing because of the pressure differential, it doesn’t need to be pushed by a fan.

Again, this is predicated on having correctly sized intake vents - you’re absolutely right that a system with too-small intakes and a strong vacuum in the space is bad. But it explains - which I didn’t understand at first either - why putting fans only on the exhaust side of the system makes sense when you have someone actually calculating the required the airflow.

Pete

 

[Moodysailor]

I’ve had this discussion with a naval architect on a new workboat design. He was adamant about only fitting exhaust fans - though paired with large passive intake vents of cross-section calculated to be large enough.

The reason is that, as you say, the engine is a big powerful pump. If it needs 10m² per minute to run (I have no idea what the actual numbers would be) then, as long as there are vents, it will pull that 10m² in quite handily all by itself. You don’t need to help it, any more than you need a fan pointing vaguely at the intake manifold of your car.

If you add a 2m² intake fan, the engine will happily consume that 2m², and continue to suck in the remaining 8m² it needs. You don’t get any more airflow through the space than you did before, and really the fan is doing nothing. (Intake fans are often paired with ducting, for example pointing at the alternator, and that’s a useful installation. But it’s useful because of the ductwork, not the fan.)

The only way you can increase the absolute airflow through the engine space is to make the intake fan hungrier than the engine - so it sucks in 12m², the engine uses 10m² of that, and you have 2m² left over to come out of an exhaust vent somewhere. But a fan that’s more powerful than a large diesel ends up being massive - in the specific boat we were designing, it actually worked out to consume more electrical power than was available on board.

If, instead, you flip the small 2m² fan around so that it’s exhausting from the space, it works alongside the engine and between them they suck in 12m² through the intake vents - same as the monster fan. And you can duct and direct that incoming cool air to wherever you need it - it’s flowing because of the pressure differential, it doesn’t need to be pushed by a fan.

Again, this is predicated on having correctly sized intake vents - you’re absolutely right that a system with too-small intakes and a strong vacuum in the space is bad. But it explains - which I didn’t understand at first either - why putting fans only on the exhaust side of the system makes sense when you have someone actually calculating the required the airflow.

Pete

I fully agree with this Pete, it's all about the intake sizing being correct.

Most intake fans are there for cooling, not for bringing air in to feed the engines, hence my earlier comment about circulating the air. A lot of the machinery spaces I worked in had generators and other heat generating consumers running when the engines were stopped, so intake air was handy as there was no dynamic air flow due to vessel movement to aid cooling.

But yes, in the case you looked at, the exhaust fans are a win-win as the intakes are adequately sized.

Another reason I personally wouldn't fit exhaust fans unless required though is for fire suppression, but that is a whole other debate that would need it's own thread and one I'm not wanting to start

 

[dankilb]

It’s definitely a head scratcher...

The design of our boat doesn’t help. It’s a 42’ standard aft cockpit AWB, so the engine compartment is about as far from outside air as possible. There’s no way of going up to deck level (there’s a small bridge deck/step above, so unsuitable for any kind of vent). So the only route is via the ducting and any other gaps through which air can be drawn from the bilge.

I was largely going off the logic that the engine will suck in what it needs (4cyl/50hp Beta). I wasn’t aiming to create a negative pressure differential, more just assuming more air would be drawn in as/if one developed.

There’s no space to increase the size of the ducts (approx 4 inches maybe, so a bit bigger than the 100mm I suggested). There’s probably around the same space again - in various gaps/voids - allowing air to pass from the aft lazarettes. But this is also the exhaust route so may not be exactly ‘cold’.

The only other option would be to cut a vent into the companionway steps, but that would undermine the attempt at noise insulation. It couldn’t be much larger than a domestic air brick, either.

Having to accommodate both an exhaust and inlet fan certainly wouldn’t be the end of the world. I can definitely improve on the 1980s setup from the factory in terms of space and, hopefully, performance.

Good luck, the experiments will be easy enough - let me know if you need any help.

Thanks! Yes, that’s what I thought... quite fun to maybe test all permutations in turn and see how it performs with nothing, one/other, and both!

I had thought of sneaking a blown air heating duct through some of the remaining available space, but I won’t be doing that now!

 

[Lucy52]

Sorry for this one, but when an engine is running and drawing air through an air intake vent how much of the sound can leek out against the airflow? That is pressure waves traveling upstream. If I stand down wind I can hear things that I might not hear so well if I stand upwind. To what extent are the sound waves are able to travel against the intake airflow and would they be attenuated if they did?

 

[geem]

Having spent a lifetime designing ventilation systems for all sorts of applications including £1M packaged generator sets, I will add my 2p.
There are two reasons why you want to ventilate your engine room. One is to remove heat, the other is to remove fumes. In a hot climate heat is important. Less so in the cool UK climate but in the Tropics failure to remove heat can cause premature failure of external engine components and any other items such as fuel relays, electronics, batteries etc. On my own boat in the Tropics, the difference between ventilation running and not is an engine room temperature of 40degC or 60degC.
The other reason to provide good ventilation is the possibility of engine fumes entering the cabin space. A leaky exhaust discharging in to the cabin space can kill. For this reason the best ventilation solution for most boats and generators is mechanical extract with the exhaust air extraction point located as high as possible. This will remove heat most effectively and maintain a negative pressure on the engine room. Positive pressure fans should not be used since they will spread fumes in to the boat if used on their own. In addition, if used in conjunction with an extract fan the positive pressure fan needs to be rated such that it moves less air than the exhaust fan so as to maintain a negative pressure. I do not favour thee installation of a supply fan under any circumstances.
The best fans to install for extract are scroll fans that develop a lot of pressure. Air inlet can be achieved by providing a permanent opening to the outside. Even if this is not possible, most yachts have their engine room open to the boat bilge so they will achieve satisfactory ventilation by pulling air from the boat through any cracks and openings.

 

[Moodysailor]

Sorry for this one, but when an engine is running and drawing air through an air intake vent how much of the sound can leek out against the airflow? That is pressure waves traveling upstream. If I stand down wind I can hear things that I might not hear so well if I stand upwind. To what extent are the sound waves are able to travel against the intake airflow and would they be attenuated if they did?

There is rarely enough velocity in the intake airflow to make any difference, so designing an intake that has baffles to remove sound is important for the best sound attenuation.

 

[Moodysailor]

Having spent a lifetime designing ventilation systems for all sorts of applications including £1M packaged generator sets, I will add my 2p.
There are two reasons why you want to ventilate your engine room. One is to remove heat, the other is to remove fumes. In a hot climate heat is important. Less so in the cool UK climate but in the Tropics failure to remove heat can cause premature failure of external engine components and any other items such as fuel relays, electronics, batteries etc. On my own boat in the Tropics, the difference between ventilation running and not is an engine room temperature of 40degC or 60degC.
The other reason to provide good ventilation is the possibility of engine fumes entering the cabin space. A leaky exhaust discharging in to the cabin space can kill. For this reason the best ventilation solution for most boats and generators is mechanical extract with the exhaust air extraction point located as high as possible. This will remove heat most effectively and maintain a negative pressure on the engine room. Positive pressure fans should not be used since they will spread fumes in to the boat if used on their own. In addition, if used in conjunction with an extract fan the positive pressure fan needs to be rated such that it moves less air than the exhaust fan so as to maintain a negative pressure. I do not favour thee installation of a supply fan under any circumstances.
The best fans to install for extract are scroll fans that develop a lot of pressure. Air inlet can be achieved by providing a permanent opening to the outside. Even if this is not possible, most yachts have their engine room open to the boat bilge so they will achieve satisfactory ventilation by pulling air from the boat through any cracks and openings.

Your experience is well noted, but may I ask if you have experience designing ventilation for vessels? The reason that I ask is that your recommendations, whilst they do make some sense, would not be permitted in the design guidance of most engine manufacturers and would also need an automated shutdown system in case of fire.

I also have some experience with packaged gensets, but not as much as you as I was involved from the engine and alternator suppliers perspective, not the system designers.

It's very interesting to me, but I must apologise for detracting against the OP. The question was regarding sound insulation, on that regard packaged gensets are some of the best examples around so some good lessons could be applied from those.

 

[geem]

There is rarely enough velocity in the intake airflow to make any difference, so designing an intake that has baffles to remove sound is important for the best sound attenuation.

If you install square ducting instead of round and install a couple of 90deg bends you will get surprisingly good sound attenuation but these need to be put outside the engine room since sound will simply break back in to the ducting if it is in the engine room space unless you add acoustic insulation to the duct. The alternative is to use round ducting and add a simple galvanised sheetmetal silencer. A 2D silencer ( twice the duct diameter) can be purchased for not a lot of money and simply plumbed in to your fresh air inlet duct

 

[Moodysailor]

If you install square ducting instead of round and install a couple of 90deg bends you will get surprisingly good sound attenuation but these need to be put outside the engine room since sound will simply break back in to the ducting if it is in the engine room space unless you add acoustic insulation to the duct. The alternative is to use round ducting and add a simple galvanised sheetmetal silencer. A 2D silencer ( twice the duct diameter) can be purchased for not a lot of money and simply plumbed in to your fresh air inlet duct

A 'periscope' type baffle arrangement lined with sound insulation is the simplest. Soundwaves prefer to go in a straight line, so if you can incorporate a change of direction along with some absorption material this will greatly reduce airborne noise.

The caveat though - if this intake is also designed as a fire extinguisher port then this safety feature needs to be retained.

Great minds

 

[geem]

Your experience is well noted, but may I ask if you have experience designing ventilation for vessels? The reason that I ask is that your recommendations, whilst they do make some sense, would not be permitted in the design guidance of most engine manufacturers and would also need an automated shutdown system in case of fire.

I also have some experience with packaged gensets, but not as much as you as I was involved from the engine and alternator suppliers perspective, not the system designers.

It's very interesting to me, but I must apologise for detracting against the OP. The question was regarding sound insulation, on that regard packaged gensets are some of the best examples around so some good lessons could be applied from those.

Commercial vessel ventilation systems are considerably more complex than pleasure vessels. The main difference is fire separation between engine room and adjacent space. In addition, the active use of high pressure water mist extinguishing systems are almost universal on modern ships. In pleasure vessels there is no fire separation between the engine room and the accommodation. They rely on adequate means of escape in the event of an engine room fire.
My own Perkins engine installation guide describes the natural intake free area requirement for it. Dont forget that diesel engines are in fact high powered pumps. Their suction effect is tremendous. The pressure an extract fan get exert on the inlet to a diesel engine when their is a free air inlet is not even measureable
I first started working on HVAC installations on Frigates in my 20s as a young designer.

 

[UK-WOOZY]

i have poly flex engine mounts on my 2gm20 , definitley helped the dampening, and they are fuel, oil tolerent

 

[Moodysailor]

Commercial vessel ventilation systems are considerably more complex than pleasure vessels. The main difference is fire separation between engine room and adjacent space. In addition, the active use of high pressure water mist extinguishing systems are almost universal on modern ships. In pleasure vessels there is no fire separation between the engine room and the accommodation. They rely on adequate means of escape in the event of an engine room fire.
My own Perkins engine installation guide describes the natural intake free area requirement for it. Dont forget that diesel engines are in fact high powered pumps. Their suction effect is tremendous. The pressure an extract fan get exert on the inlet to a diesel engine when their is a free air inlet is not even measureable
I first started working on HVAC installations on Frigates in my 20s as a young designer.

If you haven't already, you may want to read some of the earlier posts on this topic, a lot of this has already been covered.

Not disagreeing regarding the commercial vessel requirements (this is the field I work in), but I never suggest to compare the two due to the distinct and significant differences in space, application, regulation and useage.

 

[LittleSister]

. . .
The reason is that, as you say, the engine is a big powerful pump. If it needs 10m² per minute to run (I have no idea what the actual numbers would be)

It’s definitely a head scratcher...
. . .
I was largely going off the logic that the engine will suck in what it needs (4cyl/50hp Beta). I wasn’t aiming to create a negative pressure differential, more just assuming more air would be drawn in as/if one developed.

A naturally aspirated 4 stroke engine will presumably require half of its displacement multiplied by its maximum revs per minute.

So in the case of dankilb's Beta 50, which is 2,197cc and revs to 2,800rpm -
2.197 litres /2 x 2800 = 3, 076 litres per minute

= 51 litres per second.

That's a lot of air for your intake to handle without it being restricted. Lining an existing intake with sound insulation might not be such a good idea in all cases.

 

[LittleSister]

Im sure the foam itself is vastly improved in type and efficiency, and the heavy lead barrier on the higher end insulation now replaced by some sort of plastic whilst all of it being heat resistent and flame retardent.

Some old sound insulation is actually flammable.

 

[dankilb]

I know the thread has drifted somewhat - but it’s all interrelated to the soundproofing question...

So on the ventilation issue, for those like us with the option for fans/ducting, it sounds like:

Extraction -
Pros: removes heat and fumes

Cons: won’t move significant volume of air when compared to the motor itself (‘giant air pump’!); can create fire suppression issues (sucking out the suppressant); baffles required to attenuate noise through the duct

Induction
Pros: can direct cooler air towards components, like alternator

Cons: unlikely to make a huge difference to ventilation; if used without extraction, might push fumes into cabin; more complexity

Passive
Pros: simple and engine will do most of the work of moving air itself; no additional fire suppression concerns

Cons: heat could build in the engine compartment, especially in warmer climes

Have I got that about right? (In credit and deference to our resident experts of course!)

 

[Moodysailor]

I know the thread has drifted somewhat - but it’s all interrelated to the soundproofing question...

So on the ventilation issue, for those like us with the option for fans/ducting, it sounds like:

Extraction -
Pros: removes heat and fumes

Cons: won’t move significant volume of air when compared to the motor itself (‘giant air pump’!); can create fire suppression issues (sucking out the suppressant); baffles required to attenuate noise through the duct

Induction
Pros: can direct cooler air towards components, like alternator

Cons: unlikely to make a huge difference to ventilation; if used without extraction, might push fumes into cabin; more complexity

Passive
Pros: simple and engine will do most of the work of moving air itself; no additional fire suppression concerns

Cons: heat could build in the engine compartment, especially in warmer climes

Have I got that about right? (In credit and deference to our resident experts of course!)

Good summary