Loud Outboards - How to Reduce Sound?

[Rapanui]

I should point out that high density styrofoam or any other closed cell foam will not act as an noise absorber and will not do much if anything for noise transmission. There are plenty of acoustic absorption sheet materials used for lining the boat engine bays on the market and that is what you need for lining the outboard well. It will reduce the reverberent noise in the well but the best you can do is reduce it to what it would be radiated by the engine in free space (i.e. no reflecting surfaces). It is important that the open cells are kept dry i.e sealed at the surface to retain its acoustic performance and not impose increased fire risk.

Some form of acoustic 'cap' or baffle over the top of the outboard well be it temporary or not, will reduce the noise radiated significantly used in conjunction with the acoustic lining.
As discussed, stiffening up the slide rails could make a reasonable improvement in internal noise, but you need section modulus, ie beam depth normal to the bulkhead to achieve that with high modulus materials.

[Chotu]

Originally Posted by Rapanui

I should point out that high density styrofoam or any other closed cell foam will not act as an noise absorber and will not do much if anything for noise transmission. There are plenty of acoustic absorption sheet materials used for lining the boat engine bays on the market and that is what you need for lining the outboard well. It will reduce the reverberent noise in the well but the best you can do is reduce it to what it would be radiated by the engine in free space (i.e. no reflecting surfaces). It is important that the open cells are kept dry i.e sealed at the surface to retain its acoustic performance and not impose increased fire risk.

Some form of acoustic 'cap' or baffle over the top of the outboard well be it temporary or not, will reduce the noise radiated significantly used in conjunction with the acoustic lining.
As discussed, stiffening up the slide rails could make a reasonable improvement in internal noise, but you need section modulus, ie beam depth normal to the bulkhead to achieve that with high modulus materials.

Yeah, I didn’t want to start another problem by contradicting someone. But I was going to say the same thing about the foam. Does absolutely nothing. You need mass to stop the transmission of sound. That’s why sound proofing sheets have very heavy materials in them.

I definitely won’t be lining the outboard well with anything. It gets wet also. That’s not a practical solution here.

Putting in the caps on the top of the steps but not the risers will do a Lot for the airborne noise coming up out of the wells. It should direct the sound aft and away from its current path which reflects off of a ceiling to the Helm.

And good thing I took enough physics to know what you were talking about with that last paragraph. Ha ha ha. I’m not certain too many people do.

First step will be isolating the vibration of the Outboard from the already extremely stiff mount and sliding rails.

[a64pilot]

Mass or stiffness, the Mass resists becoming a secondary vibration due to it being heavy and it takes a lot of energy to vibrate something heavy.
But something very stiff will resist vibrating because it’s stiff of course.

[Chotu]

Originally Posted by a64pilot

Mass or stiffness, the Mass resists becoming a secondary vibration due to it being heavy and it takes a lot of energy to vibrate something heavy.
But something very stiff will resist vibrating because it’s stiff of course.

Of course. I’m assuming Stephanie is at this point. That would be a large part of it.

[AndyEss]

Originally Posted by Rapanui

I should point out that high density styrofoam or any other closed cell foam will not act as an noise absorber and will not do much if anything for noise transmission. There are plenty of acoustic absorption sheet materials used for lining the boat engine bays on the market and that is what you need for lining the outboard well. It will reduce the reverberent noise in the well but the best you can do is reduce it to what it would be radiated by the engine in free space (i.e. no reflecting surfaces). It is important that the open cells are kept dry i.e sealed at the surface to retain its acoustic performance and not impose increased fire risk.

Some form of acoustic 'cap' or baffle over the top of the outboard well be it temporary or not, will reduce the noise radiated significantly used in conjunction with the acoustic lining.
As discussed, stiffening up the slide rails could make a reasonable improvement in internal noise, but you need section modulus, ie beam depth normal to the bulkhead to achieve that with high modulus materials.

My thoughts about using the high density styrofoam insulation were to have it act as a reflector, not an absorber.
It's cheap, easily shaped, and easily moved without throwing your back out. Those outboards have to be moved relatively often and require maintenance and fluid checks at regular intervals to ensure good lifespan.
In my engineering, I have often found that empirically trying solutions (especially those that don't require much capital - like bracing a bulkhead with a 2x4) can lead eventually to optimal solutions.
The only way I know how to seal open cell foam from water intrusion is to use closed cell foam. Even then, some water intrusion does occur, especially in submerged or partially submerged states, as found in outboard engine wells.

[KP44]

Originally Posted by thomm225

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

My thought is that a lot of your sound might be prop wash or prop noise rather than the outboards' engines.

[AllEars]

All outboards or internal combustion engines do make noise.
I can't really imagine how the internals of the boat looks like. But I've had good results from dampening or deadening the bulkheads. The bulkheads might transmit the noises very well.

I'd use two different methods simultaneously.
One is the adhere a soft dampening material to the bulkheads. The material will have to be quite dense, and thereby heavy. Good news, you don't have to cover the entire surface, about 1/3 will do in my experience. Main thing is to stop the large surfaces from vibrating.
Make sure the "glue" stays flexible. The glue is just as important as the dampening material.

The second method is to prevent noise from being reflected of the surfaces and into other areas. "Sound traps" can be made from a variety of materials depending on the frequencies you wish to suppress. Youtube has a large number of examples.
When it comes to sound traps the placement is important too, and a bit of experimenting will be well spend.

I've used this method on a number of multihulls, both for racing and cruising :-)

[JeffBurright]

As a vibration dampener, you might consider rubber treadmill mat. It comes as a nice thick sheet if you can find a cast off machine in someone's junk pile.

[CruiserBrad]

I had a similar setup in a previous catamaran.

I would check the mounting system for any play in the sliding mechanism and correct that.
Also rubber bushings where metal is contacting metal.
Also rubber bushings to isolate vibration between the plywood mounting bracket and the hull and between the motor brackets and the sliding mechanism. Hope you find a cure for the excess noise.

Cruiser Brad

[beachhowes]

It sounds to me like you have a resonance problem. The vibration from the outboards is exciting a natural frequency in the hull structure. I don’t have my engineering text books handy but the natural frequency of a structural body is related to its mass and rigidity. If you excite the system with a vibration close to this frequency you get resonance. For most structural systems resonance is bad. If you want an example Google the Tacoma Narrows Bridge.

If resonance is the problem you can attack it 4 ways. You can change the excitation forces (outboard), the mass of the vibrating structure, the rigidity of the structure or add damping. In this case I doubt that damping will be enough.

Your idea of adding the 2x4 is an example of changing the rigidity of the system.
This will raise the natural frequency. Adding mass will lower it. Changing the engine speed will change the excitation frequency which would help but I doubt running a half speed is a viable option.

Many systems are designed to operate above or between natural frequencies. Your car may start to bounce around on a freeway with concrete slabs at just the right spacing or on a washboard dirt road. I think the freeway is probably the natural frequency and the washboard is a multiple of it. Anyway either can get the car bouncing even if it has good shocks

First of all, I would get the right props. The engines have been designed to operate and give the least vibration at specific rpm range.

A relatively cheap and easy next step would be to line the engine room with something like Fat Mat that car geeks use for quieting cars. This adds both mass and damping to the vibrating body. If this helps but not enough you can get specific lead loaded foam or vinyl that take this to the next level.

Finally, and this is a bit of a cop out but there are companies out there that do this for a living. They make their money by selling you a well engineered solution but the price can be pretty high. That said even if you don’t buy from them you might learn about some other potential solutions just by talking to their sales reps.

One other thing I would mention. There are apps for your phone that measure sound level that can be pretty useful for running experiments. They are much better than your ear.

Good luck!

[Buzzman]

I investigated this resonance issue several years ago, in an attempt to reduce noise transmission in stud walls. In a house, not a boat, but might still be somewhat relevant...

The recommended (by a sound engineer) solution was an additional wall, with an air gap between, the second wall lined with sound absorption material of differing densities.

Apparently (if I got this right) differrent levels of sound vibrate at different frequencies, and so different materials (with different densities) are required to mitigate/absord/deflect those frequencies.

I suspect this issue is similarly complex and will require several steps to mitigate effectively. No 'magic bullet'.

On another point, the over-revving of the engine - could it be not just the prop but the gearing as well?

Various manufacturers over the years have built low-geared lower rpm 'pusher' motors for sailboats that provide more thrust at lower rpms.

I recognise these are normally designed for displacement speed monos, rather than high-po cats, but it might be worth considering. Certainly the re-prop idea is worth doing first, as this might change the cruise rpm, reducing the frequency of the vibration and resonance and therefore might require different density of materials to mitigate the sound and/or vibration.

One final query: You mentioned up thread that the outboard well is in the stepped section of the transom, and that (I think) the bulkhead to which it's attached is only as wide as the motor. So is the well itself only anchored to the keel and the deck, or is there some sort of 'stiffening' bulkhead between the sides of the well and the outer skin of the hull shear?

If not, that might be worth stiffening also with foam/glass same as the rest of the boat.

Sorry if this is all wrong but without pics or diagrams, can't really imagine how the 'well' is fixed...

[Dennis.G]

I would think some vibration isolation pad material between the motor and "transom" mount plate, or between the motor mount and boat bulkhead would be first thing to look at. Direct transmission of vibration from motors to the bulkhead, with the bulkhead acting as a sounding board likely the worst offender in noise transmission.

Use of something like these products would be what I would be considering:
https://www.grainger.com/category/ma...-isolator-pads

[Old fella]

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.

Actually I don't think 4 strokes would make stuff all difference, I think your problem is the sound shell they are sitting in, is there any chance you can pull them out of there and hang them on the ass of the boat?.

[gulfislandfred]

You mentioned that you want to get props with more pitch. The more pitch, the lower the RPM for the same thrust, so you might get some relief if you pitch your props as much as the engines can reasonably handle, and run them at lower RPM.


I know that you want to run the engines at the most efficient RPM, and that there is such a thing a too much pitch, but it's something you can mess with.


Also, just run the engines slower, unless you really need the speed. Everything on a boat is a compromise.

[Sputnik]

Not meaning to get involved in the heated discussion re' 4 v 2 stroke, but, could you find a 4 stroke outboard to try? You may find that the lower frequency vibrations of a different magnitude will be naturally managed better by the structure.....saves a lot of time playing with different ideas of sound proofing/absorbing.

4 strokes are usually more fuel efficient also.

Neil