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Chotu 26-09-2019 08:33

Loud Outboards - How to Reduce Sound?
 
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?

Chotu 26-09-2019 09:36

Re: Loud Outboards - How to Reduce Sound?
 
A lot of looks and no advice. I must have not made a very good post. Is there anything I can say to clear it up more? Was it confusing?

Dsanduril 26-09-2019 09:49

Re: Loud Outboards - How to Reduce Sound?
 
Your question is very clear, and I looked because I was curious what advice you might get, but I have none to offer. Have been there with boat structure turning into a drum/sound box. Sometimes putting things on flexible mounts helps, but that seems difficult with sliding outboard mounts. Good luck.

landsend 26-09-2019 10:01

Re: Loud Outboards - How to Reduce Sound?
 
Yes I know expensive but what about four strokes?So quiet you can't hear them.

S/V Adeline 26-09-2019 10:15

Re: Loud Outboards - How to Reduce Sound?
 
For noise coming through the hull I would try some type of sound deadening method of insulating around the engine opening. For sounds traveling through the air I would experiment with methods of deflection.

Chotu 26-09-2019 10:32

Re: Loud Outboards - How to Reduce Sound?
 
Thanks for the thoughts. Keep them coming. The one thing I know about four stroke versus two stroke as they are about the same decibels. At least in this modern time with new outboards.

I think what I need to do is find a way to keep this from vibrating the boat. They vibrate the bulkhead and that makes it carry through the entire boat. It’s my installation that’s bad. Not the motor itself I don’t think.

Sailmonkey 26-09-2019 10:39

Re: Loud Outboards - How to Reduce Sound?
 
Is the bulkhead able to have sound deafening material applied to it? Maybe even something like foam with mass loaded vinyl, then covered with thin plywood to maintain appearance?

thomm225 26-09-2019 10:42

Re: Loud Outboards - How to Reduce Sound?
 
Newer 2 cylinder 4 strokes have very little vibration and are quite quiet as well.

Chotu 26-09-2019 10:44

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Sailmonkey (Post 2984521)
Is the bulkhead able to have sound deafening material applied to it? Maybe even something like foam with mass loaded vinyl, then covered with thin plywood to maintain appearance?

The bulkhead the Outboards are attached to is actually in the engine room. Or what would have been the engine room if there were no wells. Forward of that, there is another bulkhead that is the watertight bulkhead for the aft part of the boat.

So I could do anything I want to it. However, this is an extremely high performance catamaran. Every pound counts. Adding weight is a big no-no. There doesn’t need to be a good appearance at all back in these engine rooms.

However, I think it is more of a transmission of the sound. It seems to shake and transmit through the whole engine room and hull back there then it gets the larger watertight bulkhead really rattling.

And you did just give me an idea. I could at least experiment by sticking a 2 x 4 between both bulkheads and jamming it in there to see if that changes the sound at all. Not a permanent fix, but an experiment.

Chotu 26-09-2019 10:46

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by thomm225 (Post 2984522)
Newer 2 cylinder 4 strokes have very little vibration and are quite quiet as well.

Actually, my two strokes are quieter than many four strokes. That’s not accurate.

That’s how it used to be. Currently it’s not like that anymore.

All Outboards That are new and on the market now are basically similar. I was just researching the heck out of that. Just to make sure.

thomm225 26-09-2019 10:53

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Chotu (Post 2984524)
Actually, my two strokes are quieter than many four strokes. That’s not accurate.

That’s how it used to be. Currently it’s not like that anymore.

All Outboards That are new and on the market now are basically similar. I was just researching the heck out of that. Just to make sure.

Wow that's a news flash.

It's common knowledge that 4 stroke outboards are quieter than 2 stokes.

Added Power: Two-stroke outboards fire for each revolution of the engine instead of every other revolution as in four-stroke engines

Fuel Efficiency: As all four stages of combustion are distributed over four steps instead of two, the engine runs smoother. This results in better fuel efficiency.

Silent Operation: Due to the above reason, four stroke outboards generally produce less noise than two-stroke outboards.

Durability: Because the speed of four-stroke engines is lower than that of two-stroke engines, these outboards last longer than the latter.

Chotu 26-09-2019 11:00

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by thomm225 (Post 2984529)
Wow that's a news flash.

It's common knowledge that 4 stroke outboards are quieter than 2 stokes.

Added Power: Two-stroke outboards fire for each revolution of the engine instead of every other revolution as in four-stroke engines

Fuel Efficiency: As all four stages of combustion are distributed over four steps instead of two, the engine runs smoother. This results in better fuel efficiency.

Silent Operation: Due to the above reason, four stroke outboards generally produce less noise than two-stroke outboards.

Durability: Because the speed of four-stroke engines is lower than that of two-stroke engines, these outboards last longer than the latter.

You should Google some of that. Completely wrong. It’s all about how many decibels the Outboard makes at different RPMs. The modern two strokes fall right in line with the fouras. They are the same thing, sound wise. They’re right in the middle. I guess that might be a newsflash for you, but the rest of the Internet already knows this.

Just look up the decibel rating on them. There are lots and lots of posts everywhere on other types of fishing forums that go through the stuff in minutia.

You are thinking of the old days. It’s not like that anymore.

And I don’t wanna get too aggressive here, because you are trying to help me out, but you’re wrong on the fuel efficiency as well. Times have changed. Evinrude E tec

My installation is the problem. That’s what needs the attention. It doesn’t matter what vibrating thing I hang on there, it’s going to be loud.

thomm225 26-09-2019 11:03

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Chotu (Post 2984534)
You should Google some of that. Completely wrong. It’s all about how many decibels the Outboard makes at different RPMs. The modern two strokes fall right in line with the fouras. They are the same thing, sound wise. They’re right in the middle. I guess that might be a newsflash for you, but the rest of the Internet already knows this.

Just look up the decibel rating on them. There are lots and lots of posts everywhere on other types of fishing forums that go through the stuff in minutia.

You are thinking of the old days. It’s not like that anymore.

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.

Dsanduril 26-09-2019 11:29

Re: Loud Outboards - How to Reduce Sound?
 
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.

Dsanduril 26-09-2019 11:32

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Boating Magazine examined the popular myth that 4-stroke outboard engines are more fuel efficient than 2-stroke engines. After analyzing data, Boating concluded that this claim is simply untrue.

Comparisons are often made between older, carbureted two-strokes and newer fuel-injected four-strokes. In those cases, the enhanced economy comes from the fuel delivery system, not from the number of revolutions in a power cycle. But that’s not the case when you compare Evinrude E-TEC’s new direct injected outboards to the modern four-strokes.

Karl Sandstrom, product manager for BRP’s Evinrude Outboard Division, explains how Evinrude E-TEC’s direct injected 2-stroke engines can compete head-to-head and out-perform 4-stroke engines on fuel economy. He gives some great insight into the benefits of the Evinrude E-TEC’s version of the 2 stroke engine.
It's a sales pitch, but....

ronstory 26-09-2019 11:36

Re: Loud Outboards - How to Reduce Sound?
 
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 26-09-2019 11:49

Re: Loud Outboards - How to Reduce Sound?
 
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 26-09-2019 12:55

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by thomm225 (Post 2984538)
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 26-09-2019 13:00

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Chotu (Post 2984433)
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 26-09-2019 13:05

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Dsanduril (Post 2984545)
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 26-09-2019 13:07

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by ronstory (Post 2984551)
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 26-09-2019 13:08

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by 1affiah (Post 2984556)
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 26-09-2019 13:09

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by 44'cruisingcat (Post 2984610)
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 26-09-2019 13:13

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by thomm225 (Post 2984538)
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 26-09-2019 13:40

Re: Loud Outboards - How to Reduce Sound?
 
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 26-09-2019 13:46

Re: Loud Outboards - How to Reduce Sound?
 
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 26-09-2019 14:18

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Jim Cate (Post 2984647)
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 26-09-2019 14:43

Loud Outboards - How to Reduce Sound?
 
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 27-09-2019 02:38

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by a64pilot (Post 2984697)
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 27-09-2019 04:08

Re: Loud Outboards - How to Reduce Sound?
 
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

Chotu 27-09-2019 04:22

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Rapanui (Post 2984928)
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

Thank you very much for lending your experience and thoughts to this thread. These are the general ideas I was heading toward. What I can say is the aluminum tracks and bulkhead the Outboards are mounted on are extremely stiff. They don’t move at all. Not even the slightest bit. It’s all heavy duty stuff. The aluminum tracks are actually 1/4" thick U channel, bolted every 6" to a 1/2" backer/doubler. Absolutely no give at all. They needed to be strong like this in order to support the power from 30 hp outboards.

So, at least I should be in good shape to isolate the motor vibration from this very stiff track.

I should probably add the entire boat is extremely stiff as well. It uses 1 inch foam as a core. Of course there is plenty of glass over that bulkhead as well. And it is supported by steps all around it. So it’s extremely stiff in the area that the Outboard’s connect.

Unfortunately, I can’t close the wells. The airborne noise I may have to tackle a different time. If it still is a problem. These outboards are retractable. They come up out of the water when not in use. So, closing off the top of the well would prevent that from happening. They actually come up out of the well. Which allows you to be able to service them in place. They stay on the track and ride way up.

There have been a lot of good comments on here. Plenty of things for me to try out. Based on your post, I feel like one of the best things I can do to quiet this thing down is to make sure my rig and sails are good in light air. Ha ha.

wsmurdoch 27-09-2019 04:32

Re: Loud Outboards - How to Reduce Sound?
 
While most of this thread has focused on vibrations transmitted through the engine mounts and while I know little about your particular engines, is there a chance that you have an exhaust problem? My outboard motors exhaust into the air below the crowl when at idle and into the water through the center of the propeller or through the lower unit when at speed. Is there a chance that the underwater route is either blocked or that because of the mis-propping is not effective, and your engines are continuing to exhaust into the air at operating speeds? Also, is there a chance that you have a loose joint or a missing gasket in the exhaust system?

Rapanui 27-09-2019 04:57

Re: Loud Outboards - How to Reduce Sound?
 
I understand the constraints you are working with. One point I have is that the dymamic stiffness at say 1 kHz (which is likely to be the highest noise component) is very different to the static stiffness of a structure. At such frequencies, the structure breaks up into a series of resonances. This will be worse for a high performance cat with high strength to weight structure, stronger but not necessarily stiffer at acoustic frequencies. To give you some idea when we were designing a mine hunter, we actually measured the actual dynamic stiffness at acoustic frequencies of every mounting of the 50 or so equipment that were critical, and despite designing to specific acoustic criteria and finite element analysis, a significant proportion had to be reworked to meet the isolation criteria.
What I am trying to say is that despite the existing thick channel and sub structure, you will probably need something with a very high stiffness (moment of inertia) behind the sliding channel, off the top of my head 100mm x 6mm T, angle or channel to allow the existing outboard internal mountings and any additional external mounts to work effectively.

Sailmonkey 27-09-2019 05:37

Re: Loud Outboards - How to Reduce Sound?
 
Would mounting the tracks on a series of mounts like this be a possible solution?

https://www.mcmaster.com/5823k27

The only major problem I can see with the linked mounts would be running the engine astern as the mounts are not rated for anything other than compression and shear.

a64pilot 27-09-2019 06:25

Re: Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Chotu (Post 2984906)
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.


If your hitting the rev limiter, your underpropped for sure.
First you will need a tachometer.
I use this one on my outboard, cost is $22 and it tracks operating time too so I know when to change oil etc. you can’t really prop an engine without a tach.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Then you look up on line to find the operating range for your motor, on my Suzuki it happens to be 5200 to 6200. That is where your engine should max out at full throttle speed stabilized. Boat speed will just be whatever it ends up being, it really doesn’t have anything to do with propping a boat.

I’d shoot for the lower end of your operating range, as often high RPM means more noise.
Be sure to be running both motors wide open when you try this of course.

You just have to try a higher pitch prop, maybe a shop will let you try different props if they know your buying from them.
I’d try increasing two inches of pitch, that’s quite a bit really, but it will tell you a lot, if for example it drops your RPM by 1,000 then you know that 1 inch means about 500 RPM.
I say two inches since your banging the rev limiter and I assume you want to be at the bottom end of the range.

If you were a planing boat and were looking for max performance, then you would want to be close to max RPM.

Since RPM will determine the vibration frequency, and since your noise may be mostly due to vibration, I’d get the props right first, it may be that is enough to make it tolerable.

Then if it were me and I would plan on motoring at no more than 2/3rds max RPM, so it it turned 6,000, then I’d motor at no more than 4,000. I think your noise will be more tolerable and you motors last longer that way.

bruceb 27-09-2019 10:25

Re: Loud Outboards - How to Reduce Sound?
 
A few thoughts. Part of the problem with outboard's noise issues is the type and pitch of the sound they make. The outboard itself doesn't have enough mass to reduce vibrations very much so it seems noisier than it really is. Outboards have a lot of induction noise that is fairly high pitched, some exhaust noise and of course vibration of the engine.
Each part of the total noise needs to be reduced in different ways. Vibration needs to isolated so the soft pads need to be where the engine fastens to the mount. The less mass you have vibrating the better. Induction noise is usually through the air, so redirecting it and/or absorbing it before it contacts a surface works best. If sound waves have to turn corners the energy is greatly reduced. Maybe a removable lid or blanket over the engine wells? If the thru the hub exhaust is getting uncovered, maybe a larger cavitation plate (like a "Dole" fin) might help direct the sound aft.

B

Cheechako 27-09-2019 10:43

Re: Loud Outboards - How to Reduce Sound?
 
Sound deadening material, especially on the bulkhead style outboard mount. It's likely acting like a sound baffle enhancing the noise like a drum head. Pics would help a lot though. Deadening material comes in various thickness, and unfortunately is heavy. But usually works well. The best used to come with a thin sheet of lead in the layers of dense foam.

thomm225 27-09-2019 10:45

Re: Loud Outboards - How to Reduce Sound?
 
1 Attachment(s)
Quote:

Originally Posted by Dsanduril (Post 2984790)
Sorry, but I don't know of any 8HP that meet your requirements. The smallest Evinrude makes (that I am aware of) in the DFI market is a 15HP. So you're looking for a different beast.

I've owned and operated two-stroke outboards for something like 50 years. Seagull, Johnson, Evinrude, Mercury, Tohatsu, and a couple of others. Until an Evinrude they were all carbureted, noisy, stinky beasts. I loved their power/weight ratio and hated just about everything else about them. They left a sheen. They were loud. Couldn't always remember if I'd mixed the spare gas can. Four-stroke was so much simpler in so many ways, just heavier.

But modern two-strokes simply aren't your father's Johnson. They are available and sold in the US. They meet all emissions requirements and then some. They are just as fuel efficient as four-strokes. They aren't banned. They're quiet. Can you get them in a 2,4,6,8,9.9 HP? Probably not. But not every outboard is a small engine.


That's good news actually, but my Father never owned an outboard!

I wish they made the lower horse power ones so I could get a larger light weight outboard (maybe 8hp -9.9hp) for my boat even though my 5hp 4 stroke does a fine job. (I refuse to put a heavy 110 lb 9.9 outboard on the back of my boat on a bracket)

My first outboard was a 40 hp Johnson anyway which was mounted on a 16' Chincoteague Scow. I bought the boat, motor, and trailer for $300.00 in 1971 when I was around 16 years old

Over the next 3 months, I went thru three more boats, motors, and trailers being a teenager buying, selling, and trading.

I had an old Mercury 35 at one point that summer with a white case (shows you how old that one was) 1960 I think)

In the end, I wound up with a 25 Evinrude on a 14' Aluminum V bottom boat. It would push that boat to near 25 knots. I took it no further than maybe 15 miles offshore to Tangier Island Chesapeake Bay

Later still I had another 40 HP Johnson. This one said add one quart to 6 gallons of gas on the pop down lid. This in 1980

All these were 2 stroke and I had the one 4 stroke in 1976 and now the new one I bought in 2011. Mercury 5 hp 4 stroke 25" extra long shaft. Very quiet

https://www.sandeace.com/waali/WaaliMotors.html

morribabes 27-09-2019 11:36

Re: Loud Outboards - How to Reduce Sound?
 
I doubt if changing to 4 stroke engines would improve the situation. I think you've got a resonance problem due to the lightweight construction of your boat. I would suggest trying various ways of bracing the engine mounts to reduce the amount of movement. Alternatively, try introducing rubber into the mounts to dampen the vibration.

a64pilot 27-09-2019 12:49

Loud Outboards - How to Reduce Sound?
 
The new 2 strokes are likely built on the orbital engine technology, it was an Aussie adaption that quite a few motors tried before most gave up and went to a four stroke.
A direct injected engine using the orbital engine technology, is not a simple or inexpensive thing.

I believe but am not sure that Mercury’s Optimax engine’s used the Orbital injection system.
https://archive.maas.museum/australia...ticle_id=10041

It has been trying to bring back two strokes for at least 30 years, but for some reason, it just hasn’t happened. Motorcycles would be a big market. Aprillia had I believe a very good two stroke that came out about the time I was Road Racing motorcycles, but for some reason wasn’t successful.
RS 250 maybe? I saw a few on the track, really a good bike, but out of my budget at the time
https://en.m.wikipedia.org/wiki/Aprilia_RS250

I believe modern two strokes are becoming a lot like modern road Diesels, that is they are becoming more and more expensive and complex, gaining weight and meanwhile the spark ignition motor is becoming more and more maintenance free, they are sort of morphing into each other, and I’d expect soon we may even see a gasoline engine that operators at least some of the time in a Diesel cycle.

I wouldn’t expect to see a small orbital injection two stroke, likely more complex and expensive than a four stroke. But. That’s my guess.


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