Loud Outboards - How to Reduce Sound?

[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?

[Chotu]

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]

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]

Yes I know expensive but what about four strokes?So quiet you can't hear them.

[S/V Adeline]

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]

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]

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]

Newer 2 cylinder 4 strokes have very little vibration and are quite quiet as well.

[Chotu]

Originally Posted by Sailmonkey

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]

Originally Posted by thomm225

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]

Originally Posted by Chotu

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]

Originally Posted by thomm225

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]

Originally Posted by Chotu

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]

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]

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....