Loud Outboards - How to Reduce Sound?

[ronstory]

That looks just to be sorbothane. Just figure out the hardness you need and then order from Amazon. You will need to drill your own holes.

https://www.sorbothane.com/Data/Site...othane-SPG.pdf

[1affiah]

If it were my boat, I would: 1) Mount the aluminum rails with a thick rubber washer at each bolt under the nuts and washers each side, and a layer of the same rubber pad under the entire length of the rails, and 2) Insulate the inside of each engine compartment with foam sound deadening insulation.
Take a look at www.mcmaster.com for these types of materials.

[44'cruisingcat]

Originally Posted by thomm225

I gave you the reason above why the 2 strokes are louder......"they fire every stroke and are higher reving " but if you still want to believe two stroke are quieter there's not much I can say to change your mind

I guess if you have a belief it's hard to change that

But here's more:

Pro’s and Con’s:

So, which is ‘better’? Here are a few of the pro’s and con’s to both engine designs:
•As far as efficiency goes, the 4-stroke certainly wins. This is due to the fact that fuel is consumed once every 4 strokes.
•Four-stroke engines are heavier; they weigh upwards of 50% more than a comparable 2stroke engine.
•Typically, a 2-stroke engine creates more torque at a higher RPM, while a 4-stroke engine creates a higher torque at a lower RPM.
•The 4-stroke engine is also much quieter, a 2-stoke engine is significantly louder and has a distinctive, high-pitched “buzzing” sound.
•Because 2-stroke engines are designed to run at a higher RPM, they also tend to wear out faster; a 4-stroke engine is generally more durable. That being said, 2-stroke engines are more powerful.
•Two-stroke engines are a much simpler design, making them easier to fix. They do not have valves, but rather ports. Four-stroke engines have more parts, therefore they are more expensive and repairs cost more.
•Two-stroke engines require pre-mixing of oil and fuel, while the 4-strokes do not.
•Four-strokes are more environmentally friendly; in a 2-stroke engine, burnt oil is also released into the air with the exhaust.

You're talking about the normal pre-mix carbureted two stroke. The e-tec Evinrudes have direct fuel injection and are as clean and efficient as four strokes. (Unfortunately they're also about as heavy as four strokes) They're allowed to be sold in places normal two strokes are banned.

Two strokes generally don't rev higher than four strokes. Because they fire twice as often, they Can SOUND like they rev higher though.

[44'cruisingcat]

Originally Posted by Chotu

I have a catamaran. It’s Fiberglass and foam.

It has Outboard Wells at the Stern. These wells are ahead of the rudders. Where a sail drive would normally go. So the prop sticking down in the water is essentially in the exact same position that a sail drive is on all the other catamarans.

These work pretty well. No complaints except that they are currently under propped. I need to get higher pitch.

The problem is the noise. They are excruciatingly loud to me once they rev up.

They currently hang off of mounts made from very thick plywood. Like you would do for a transom. These mounts are about the width of the outboard itself. These mounts slide up and down in an aluminum track.

The aluminum track is bolted to a bulkhead in the hull.

These are 30 hp fuel injected, modern two-stroke engines.

The sound is pretty much deafening in each of the hulls. It seems to act like a drum and amplify the sound and vibration of the outboard. And it’s a buzzing sound almost like a chainsaw at full throttle mid cut. And it’s about as loud as if you were holding that chainsaw cutting through a big log. Very very loud. You can’t even yell over it and hear someone in parts of the hulls.

it’s pretty darn loud up on the bridge deck as well. But most of that sound seems to come from outdoors through the open aft door and into the main salon room.

Any tips for stopping all of this noise? How would you attach the Outboard differently in order to stop the noise?

It's probable that there's movement between the aluminium tracks and the slides that run on them. The vibration of the motor could be amplified by this movement.

[Chotu]

Originally Posted by Dsanduril

Don't know anything about it, but there's a product called Vibra-Stop (Google it) that says it reduces noise from outboard vibration transmitted to the boat through the mountings. They make a pad for under the clamps and some bushings for additional isolation.

Never used them, don't know effectiveness or cost, but might look at them even if just to see if you can engineer something similar.

Thank you for the information about Vibra stop. This is probably where I should start. It’s the most simple thing.

[Chotu]

Originally Posted by ronstory

That looks just to be sorbothane. Just figure out the hardness you need and then order from Amazon. You will need to drill your own holes.

https://www.sorbothane.com/Data/Site...othane-SPG.pdf

And thank you for this link to Sorbothane. This type of solution is the first place to start. If it doesn’t work, I can keep going. Trying more things.

[Chotu]

Originally Posted by 1affiah

If it were my boat, I would: 1) Mount the aluminum rails with a thick rubber washer at each bolt under the nuts and washers each side, and a layer of the same rubber pad under the entire length of the rails, and 2) Insulate the inside of each engine compartment with foam sound deadening insulation.
Take a look at www.mcmaster.com for these types of materials.

Also a good idea. I was thinking the isolation technique would at least help with the conduction of the sound through the boat.

[Chotu]

Originally Posted by 44'cruisingcat

It's probable that there's movement between the aluminium tracks and the slides that run on them. The vibration of the motor could be amplified by this movement.

Another good tip. Thank you again. Maybe if I can use an aluminum plate there with some Delrin or something to slide inside of the aluminum track that might be a tighter fit.

[Chotu]

Originally Posted by thomm225

I gave you the reason above why the 2 strokes are louder......"they fire every stroke and are higher reving " but if you still want to believe two stroke are quieter there's not much I can say to change your mind

I guess if you have a belief it's hard to change that

But here's more:

Pro’s and Con’s:

So, which is ‘better’? Here are a few of the pro’s and con’s to both engine designs:
•As far as efficiency goes, the 4-stroke certainly wins. This is due to the fact that fuel is consumed once every 4 strokes.
•Four-stroke engines are heavier; they weigh upwards of 50% more than a comparable 2stroke engine.
•Typically, a 2-stroke engine creates more torque at a higher RPM, while a 4-stroke engine creates a higher torque at a lower RPM.
•The 4-stroke engine is also much quieter, a 2-stoke engine is significantly louder and has a distinctive, high-pitched “buzzing” sound.
•Because 2-stroke engines are designed to run at a higher RPM, they also tend to wear out faster; a 4-stroke engine is generally more durable. That being said, 2-stroke engines are more powerful.
•Two-stroke engines are a much simpler design, making them easier to fix. They do not have valves, but rather ports. Four-stroke engines have more parts, therefore they are more expensive and repairs cost more.
•Two-stroke engines require pre-mixing of oil and fuel, while the 4-strokes do not.
•Four-strokes are more environmentally friendly; in a 2-stroke engine, burnt oil is also released into the air with the exhaust.

Oy.... Do we really need to have a copy and paste war so that you can understand the current situation of Outboards? Why are you living in the past? The stuff you are posting is from the 1990s. Extra hilarious that you are writing about having to premixed fuel. LOL. The oil injected have been around for decades. Never mind the fact that mine is completely fuel injected as well.

Here goes a copy and paste of the actual data. Not just some made up set of guidelines for people without experience. Scientific data. Decibels.


Powerboat Reports has published test results comparing six different 150-HP engines:

Four-Stroke
Mercury Verado 150
Suzuki DF150
Yamaha F150
Honda BF150
Two-Stroke
Mercury Optimax 150
Evinrude E-TEC 150
In terms of sound at idle speed of 1,000-RPM, the quietest motors:
dB-SPL / Motor
63.5 -- Suzuki
64.0 -- Verado
65.5 -- Yamaha
66.5 -- Honda
69.0 -- E-TEC
72.0 -- Optimax
However, pump the speed up to 1,500-RPM, a typical No-Wake speed, and the order changes to
dB-SPL / Motor
70.0 -- Yamaha
70.5 -- Honda
71.0 -- Verado
71.5 -- Suzuki
75.0 -- E-TEC
79.0 -- Optimax
Jump ahead to cruising speed, and the order changes again. This time we are going to look at boat speed, not engine speed. This helps the two-strokes because they won't be running the engine quite as fast. Now the results are:
dB-SPL / Motor / MPH
84.0 -- E-TEC -- 29-MPH
84.0 -- Yamaha -- 27.9-MPH
84.5 -- Honda -- 27.6-MPH
87.0 -- Verado -- 27.8-MPH
89.5 -- OptiMax -- 27.7-MPH
90.5 -- Suzuki -- 30.3-MPH
At wide open throttle, the ranking changes some more. Again we compare at boat speed not engine speed:
dB-SPL / Motor / MPH
92.5 -- Honda -- 42.9-MPH
96.0 -- Verado --45.1-MPH
97.5 -- E-TEC -- 48.8-MPH
99.0 -- Suzuki -- 44.0-MPH
101.8 -- OptiMax -- 45.0-MPH
102.0 - Yamaha -- 45.3-MPH




So as you see, for the most part, at my cruising speed, the E tec is right in the middle of all the four stroke outboard’s. So why don’t you take that weird argument you are having with me somewhere else please. I can’t believe I just had to go through this. Ridiculous.

I was asking how to silence the engine. Not looking to have a debate about what engine is the loudest. Especially when you don’t know what you’re talking about.

[Jim Cate]

Chotu, do you have the feeling that most of the noise is direct, as in coming to you from the engine itself, or from vibration transferred to hull structure?

I think you would use different methods of suppression for each situation, ie the lead loaded foam on the walls of the "engine room" wouldn't do much for transferred vibration but would for radiated sound from the engine itself.

Query: if you put your hand on the bulkhead(s) that are most directly attached to the engines can you feel vibration? You might use a sounding rod (shade tree mechanic's stethoscope) to find which panels were vibrating most and use that info to direct your attention for mitigation.

You have my sympathy... it sounds (!) like an awful situation!

Jim

[1affiah]

Lastly: Install a kick-ass stereo & crank it up when motoring!

"I asked you what time it was, I didn't need you to build me a clock"

Cheers

[Chotu]

Originally Posted by Jim Cate

Chotu, do you have the feeling that most of the noise is direct, as in coming to you from the engine itself, or from vibration transferred to hull structure?

I think you would use different methods of suppression for each situation, ie the lead loaded foam on the walls of the "engine room" wouldn't do much for transferred vibration but would for radiated sound from the engine itself.

Query: if you put your hand on the bulkhead(s) that are most directly attached to the engines can you feel vibration? You might use a sounding rod (shade tree mechanic's stethoscope) to find which panels were vibrating most and use that info to direct your attention for mitigation.

You have my sympathy... it sounds (!) like an awful situation!

Jim

Thanks, Jim. Yes. I really think it is being carried through the hull. At least in the two hulls that’s what’s happening. It seems to be creating vibration throughout the hull, and then the water tight bulkhead acts like a giant drum. Just amplifying it. Or at least passing it through without much loss.

That is one of the issues. And that’s the worst one.

Outside, they are pretty loud as well at Close to full throttle cruising speed.

I’m going to have to test this stuff out when I get back to the boat. I can’t quite answer your questions at this time. I am currently at my other boat. Yes, I need to get rid of one of these.

[a64pilot]

As it’s most likely vibration induced, it’s similar to something acting like a drum head and actually being the source of the noise itself.
You can try to fight it two ways or most likely both.
Try to limit the transmission of sound / vibrations by soft mounting or something to try to absorb the vibrations at their source
Or by stiffening up bulkhead or whatever is vibrating, maybe even try to change its resonance frequency.
I think you attempt with the 2x4 is so far the best I’ve heard, cheap easy and may point to the right direction, or not.
Then go the rubber sheet mounting or whatever, but do one thing at a time to judge effectiveness.
I think in truth all you can do is minimize the issue, I’d be amazed if you eliminated it.

You may just have to live with it.

Curious why do you say your underpropped? Do you have a tach on the motors? Only rarely is a displacement hull powered by an outboard underpropped.

Prop pitch can have a large effect on vibrations as your changing the frequency of the vibration by changing RPM.
I’d prop to the min RPM, for example my little Suzuki should be propped from 5200 to 6200 RPM at max throttle, I’d go for 5200 to reduce the frequency of the vibration.
I assume you cruise at 2/3 max RPM or so? Maybe 4,000 RPM max?

[Chotu]

Originally Posted by a64pilot

As it’s most likely vibration induced, it’s similar to something acting like a drum head and actually being the source of the noise itself.
You can try to fight it two ways or most likely both.
Try to limit the transmission of sound / vibrations by soft mounting or something to try to absorb the vibrations at their source
Or by stiffening up bulkhead or whatever is vibrating, maybe even try to change its resonance frequency.
I think you attempt with the 2x4 is so far the best I’ve heard, cheap easy and may point to the right direction, or not.
Then go the rubber sheet mounting or whatever, but do one thing at a time to judge effectiveness.
I think in truth all you can do is minimize the issue, I’d be amazed if you eliminated it.

You may just have to live with it.

Curious why do you say your underpropped? Do you have a tach on the motors? Only rarely is a displacement hull powered by an outboard underpropped.

Prop pitch can have a large effect on vibrations as your changing the frequency of the vibration by changing RPM.
I’d prop to the min RPM, for example my little Suzuki should be propped from 5200 to 6200 RPM at max throttle, I’d go for 5200 to reduce the frequency of the vibration.
I assume you cruise at 2/3 max RPM or so? Maybe 4,000 RPM max?

I think this is the plan for the time being. Some vibration isolation. And changing the fundamental vibration frequency of the bulkhead. But yes. I am reaching full RPMs way too early. This is an a normal boat. This is a very high performance catamaran. So I’m at close to 8 kn and hitting the rev limiter. I should be getting somewhere closer to 12 to 14 kn. I used prop prop that the Outboard shop sold me. They didn’t know what they were talking about. They treated me like a pontoon boat. I also didn’t know what I was doing because I had never bought any other Outboard except for a dinghy before. So I didn’t really know how to match the prop to the boat. I figured the low-end power was more important. But since I am under propped I am wasting fuel.

[Rapanui]

I spent most of my working life on ship noise and vibration reduction, and your problem sounds (sorry) difficult.
The airbourne noise component directly radiated from the engine can be reduced by:

• lining the whole well(s) with a proper acoustic absorption material which would need to be at least 25mm thick but 50mm or better. Given that the material is porous, it will need a thin impermeable layer such as mylar to prevent it wicking water, oil and fuel.
• If the well is open to the sky so that the sound can be directly radiated to say the cockpit, introduce some form of baffling, lined again with acoustic absorption material to prevent direct radiation. Ideally the the well should be closed off completely and other means used to feed fresh air into the enclosure.
• The airbourne noise transmitted through the engine bulkhead can be reduced by lining the inside face of that bulkhead with acoustic transmission loss material which is a material comprising a resilient heavy layer with a springy foam base. The heavy layer should be around 1 lb/sq ft or 5kg/m2. This may not be effective until you address the structureborne noise component first

These 2 things should give a noticeable reduction in the airborne noise above decks and may have some effect reducing the noise in the interior.


The structureborne noise component, which is high frequency engine vibration is more difficult. Once the engine vibration gets into the structure, then there is virtually nothing that will work apart from lining every internal surface with transmission loss material - not good for a performance cat. So you have to isolate the engine at its mounting or improve the effectiveness of the outboards internal engine mounts.

• If the outboard is mounted on a flexible structure i.e. the dynamic stiffness of the structure is too low, the engine mounts will not work effectively. if it possible, some form of added stiffening such as a large section aluminium T or angle behind the aluminium tracks and bolted through. Extend such stiffening to where the bulkhead meets the hull or deck to maximise it.
• You could consider adding vibration mounts either between the aluminium track and the bulkhead, or having a second outboard pad resiliently mounted to the existing one.

Good luck