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-   -   Loud Outboards - How to Reduce Sound? (https://www.cruisersforum.com/forums/f54/loud-outboards-how-to-reduce-sound-224449.html)

Sailormanbigd 27-09-2019 13:24

Re: Loud Outboards - How to Reduce Sound?
 
REALLY GUYS YOUVVE ALL MISSED THE BOAT. For most effective sound deadening, hoist the friggin sails!!!!!! Fast cat, duh! turn the dam things off already!

ebsail 27-09-2019 18:32

Re: Loud Outboards - How to Reduce Sound?
 
Try taping a towel to each bulkhead and see if it makes any difference. Cheap and easy. If it does, then glueing some foam board or other sound insulation to the bulkheads would work better and be fairly light

waterman46 27-09-2019 19:03

Re: Loud Outboards - How to Reduce Sound?
 
First thing I noticed in your post, you said the plywood mounts "slide up and down in the (aluminum) tracks". If there is enough play there to allow the plywood to slide, that might be your problem. Plywood vibrating against the track. I would at least try to inject some sealant into any gaps you can reach. Better yet if not too much trouble, remove the engine and the plywood then put plywood back in the track with a generous filling of your favorite marine sealant between the track and the plywood.

If that doesn't work, get Honda 4 strokes. I can barely hear our 15hp if I am in the cockpit and it's idling on the dinghy at the stern.

Chotu 28-09-2019 05:26

Re: Loud Outboards - How to Reduce Sound?
 
Thanks for all the great advice. I don’t want to flood the thread by replying to everyone individually. I definitely need to re-prop. And I should probably actually do that after I add the additional weight. It needs a rig still. And some interior items. Or actually, I think I will re-prop now and then re-prop again when the weight is added. A pair of props is not very expensive for Outboards.

I think there are a lot of good points here. Conduction of the sound, making sure there is no rattling in the brackets, isolating the engine itself from the bracket so less is vibrating. These are all good ideas. So is running them at a fairly low rpm.

Doing all of that should probably go a long way. I really hesitate to add any material. I don’t want to weigh this thing down. This is an extremely high performance cruising boat. And every pound does count.

Exploring the problems with the conduction of outboard motor sound into the hull is part of my decision making process on whether to keep this boat or my monohull. I’m definitely leaning toward keeping this boat because it is just so much better of a platform.

Chotu 28-09-2019 05:27

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

Originally Posted by thomm225 (Post 2985678)
Sounds like a lot of boat to deal with and quite lightly built maybe?

Could be dangerous to cruise on.

Maybe you should go with your other boat.

Yeah. Maybe I’ll get a 27 foot Bristol. Lololol

Wow. The wealth of ignorance is just astounding here. I’m reporting you now.

Chotu 28-09-2019 05:33

Re: Loud Outboards - How to Reduce Sound?
 
I think it useful tool on a forum would be a button that allows the original poster to ban someone from the discussion. That would be a nice feature. Could we look at getting that?

That would save the moderators a lot of work. Maybe the original poster would have the right to ban one or two people only. And only in cases of harassment such as we see in this thread.

There is no reason for a thread to be derailed the way it was in this one. This was a thread about noise and noise abatement. In boats. Then this person who has not read anything in his life, has no knowledge, comes on here and ruins the thread. I think the OP should have some control over their own thread. To keep it from going off the rails when it was a pretty good thread overall. Except that one person causing the problems.

a64pilot 28-09-2019 05:45

Re: Loud Outboards - How to Reduce Sound?
 
People should understand that while they may start a thread, itís not ďtheirĒ thread. Itís no ones thread, and people are allowed to comment, even to make silly, uninformed comments not relevant to the conversation, so long as they are polite in doing so. However everyone has as much right to post as anyone else.

There should be an ignore button that you can press that will have you ignore those silly, irrelevant people if you so choose, or just simply ignore the irrelevant posts.

I find it easier to just ignore them myself, in every thread there will be irrelevant posts, often thread drift, sometimes from a well meaning person who doesnít understand.
Not every post will be helpful, itís the nature of a Forum

Sparx 28-09-2019 05:50

Re: Loud Outboards - How to Reduce Sound?
 
Do I read correctly that the bracket carrying the motor is connected directly to the interior bulkhead? Is it possible to build a bracket support system that is connected to the hull independent of the bulkhead? The intent is to transfer vibration into the hull and away from the interior bulkhead. Perhaps one could build some vibration dampening pads into that bracket support structure.

Also putting some reinforcing beams across the bulkhead should dampen it's vibration.

Chotu 28-09-2019 05:59

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

Originally Posted by Sparx (Post 2985700)
Do I read correctly that the bracket carrying the motor is connected directly to the interior bulkhead? Is it possible to build a bracket support system that is connected to the hull independent of the bulkhead? The intent is to transfer vibration into the hull and away from the interior bulkhead. Perhaps one could build some vibration dampening pads into that bracket support structure.

Also putting some reinforcing beams across the bulkhead should dampen it's vibration.

The bracket is connected to a bulkhead in the engine room. That bulkhead does not run all the way athwart ships in the hull. It does run all the way vertical from deck to keel. There is some complicated geometry here because the boarding stairs of the transom are surrounding this area. Basically, just picture some steps up the back of a catamaran. Now there is a hole in the middle of a couple of the steps. And that hole is my Outboard well going all the way down to the water.

The bulkhead the bracket and track is attached to runs vertically from the waterline to the deck level. However, itís only about as wide as the Outboard. So it takes up approximately the center 1/3 of the steps.

About 3 feet forward of that, there is a watertight bulkhead. This runs deck to keel, port to starboard. It is absolutely watertight and there is no hole in it. This thing seems to vibrate a lot. Or at least conduct the sound better than it should.

Pete7 28-09-2019 06:52

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

Originally Posted by Cheechako (Post 2985190)
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.

My thoughts too. As a cheap experiment buy one of those foam camping mats which are closed cell foam and just tape it in the well area of one engine to see if it makes a difference. If it does then plan on doing a more permanent sound deadening solution.

Pete

Sparx 28-09-2019 08:04

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

Originally Posted by Chotu (Post 2985709)
The bulkhead the bracket and track is attached to runs vertically from the waterline to the deck level. However, itís only about as wide as the Outboard. So it takes up approximately the center 1/3 of the steps.

About 3 feet forward of that, there is a watertight bulkhead. This runs deck to keel, port to starboard. It is absolutely watertight and there is no hole in it. This thing seems to vibrate a lot. Or at least conduct the sound better than it should.

Thanks for the clarification! With this fresh understanding I might try building a foam and glass beam to significantly stiffen the deck where the bracket attaches. Something like a 6 " price of foam epoxies perpendicular to the deck, and glassed in strongly. I would run this beam completely across the width of the hull so that the beam is strongly connected to the underside of the deck and both hull walls. The idea is to change the resonance of the deck, which is translating vibration into the hull. I think a similar transverse beam across the middle of the living quarters bulkhead would reduce the drum effect translating engine compartment noise through that bulkhead.

a64pilot 28-09-2019 08:22

Loud Outboards - How to Reduce Sound?
 
Quote:

Originally Posted by Sparx (Post 2985700)

Also putting some reinforcing beams across the bulkhead should dampen it's vibration.


Once you determine that the bulkhead is part of the problem and Iíd suspicion it is, you can do as he says and done right it can be done with very little weight too.

I think stiffing whatever is acting like a sounding board will go a long way towards fixing the issue, and why I think your 2x4 idea is worth exploring.

Chotu 28-09-2019 08:28

Re: Loud Outboards - How to Reduce Sound?
 
Yeah. I agree with you guys. The first step is to use the 2 x 4 to isolate where the problem is. Jam it in there and see what I can quiet down by stopping vibrations with it. Then I could go ahead and make a more permanent foam and glass vibration damping beam. Incidentally, the whole boat is foam and glass already.

Mad Multi 28-09-2019 10:28

Re: Loud Outboards - How to Reduce Sound?
 
Two strokes are loud and vibrate. Some kind of vibration mitigation between the mount an the hull might help but not likely enough for the money you will spend. 4stroke would help but again expensive. Dont think there is much else to say.

AndyEss 28-09-2019 11:41

Re: Loud Outboards - How to Reduce Sound?
 
For the airborne noise coming from the open well, try a 2” high density styrofoam insulation board to cover the well. Make sure combustion air is available to the motor. The high density styrofoam is easy to remove when you need to lift the motors, but strong enough to take some abuse. It is pretty inexpensive too, compared to most “marine” purchases.
If the sound suppression works, you can cover the styrofoam with cloth (preferably fire resistant) to dress the solution up some.

a64pilot 28-09-2019 11:46

Loud Outboards - How to Reduce Sound?
 
I could easily see how a four stroke may be worse.
They fire half as often and especially parallel twins which many mid size outboards are, are bad for vibrating, if the noise is vibration induced, a four stroke could be louder.
I donít think many use the word smooth to describe their two cylinder four stroke.

Chotu 28-09-2019 11:58

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

Originally Posted by AndyEss (Post 2985888)
For the airborne noise coming from the open well, try a 2Ē high density styrofoam insulation board to cover the well. Make sure combustion air is available to the motor. The high density styrofoam is easy to remove when you need to lift the motors, but strong enough to take some abuse. It is pretty inexpensive too, compared to most ďmarineĒ purchases.
If the sound suppression works, you can cover the styrofoam with cloth (preferably fire resistant) to dress the solution up some.


You know, this is a pretty good idea. It also would serve as a nice way to close up the "death hole" in my boarding stairs. It would be really nice to have a couple of covers that go over just the top of the steps. Not the riser, but the tread. That would greatly reduce noise I think. Good idea. It would also help make it easier to go up and down the steps.

Compass790 28-09-2019 14:02

Re: Loud Outboards - How to Reduce Sound?
 
What you have in your present setup is akin to bolting a diesel engine to the beds with no motor mounts.
I'd begin with anti-vibration mountings as the first step

Chotu 28-09-2019 14:52

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

Originally Posted by Compass790 (Post 2985965)
What you have in your present setup is akin to bolting a diesel engine to the beds with no motor mounts.
I'd begin with anti-vibration mountings as the first step

Yes, I think this is the plan. Cure the disease rather than treat the symptom. Thatís the first step. Next, I will look into changing the frequency of the bulkhead if necessary, and finally, a couple of treadís over the Outboard well on the steps would work really well I think for practical reasons as well as sound attenuation.

I wish I could report back sooner with the progress. But it wonít be another month before Iím back to the catamaran.

Thank you everyone for your help and for your patience on this thread. I think we have all of the correct answers at this point. A multi faceted approach, starting in the order above. Lots of good ideas here. Thank you.

AndyEss 28-09-2019 20:46

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

Originally Posted by Chotu (Post 2985900)
You know, this is a pretty good idea. It also would serve as a nice way to close up the "death hole" in my boarding stairs. It would be really nice to have a couple of covers that go over just the top of the steps. Not the riser, but the tread. That would greatly reduce noise I think. Good idea. It would also help make it easier to go up and down the steps.

I am not totally clear on whether or not you want to use the high density styrofoam as part of the actual steps. If so, you will probably have to put in some beam elements to carry the bending loads. Depending on the span, the styrofoam board may not hold much.
Still, this would probably be easy to implement with wood or aluminum beam elements. Even fancier would be to fiberglass the load carrying structure.
Regardless, the styrofoam high density insulation is inexpensive, available at big box stores, and easy to experiment with to see how much noise suppression you get. It should work better (in noise suppression) than a thin plywood cover, and it will be lighter and easier to move also.
I used this to ďfairĒ out my cabin top foredeck so a girlfriend could do her yoga more easily. Not enough flat space without my inserts.

Rapanui 29-09-2019 03:43

Re: Loud Outboards - How to Reduce Sound?
 
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 29-09-2019 04:31

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

Originally Posted by Rapanui (Post 2986216)
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 didnít 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. Thatís why sound proofing sheets have very heavy materials in them.

I definitely wonít be lining the outboard well with anything. It gets wet also. Thatís 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. Iím 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 29-09-2019 06:59

Re: Loud Outboards - How to Reduce Sound?
 
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 itís stiff of course.

Chotu 29-09-2019 07:08

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

Originally Posted by a64pilot (Post 2986300)
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 itís stiff of course.

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

AndyEss 29-09-2019 07:57

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

Originally Posted by Rapanui (Post 2986216)
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 29-09-2019 21:36

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.

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

AllEars 04-10-2019 08:47

Re: Loud Outboards - How to Reduce Sound?
 
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 04-10-2019 09:23

Re: Loud Outboards - How to Reduce Sound?
 
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 04-10-2019 11:42

Re: Loud Outboards - How to Reduce Sound?
 
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 04-10-2019 12:14

Re: Loud Outboards - How to Reduce Sound?
 
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 donít 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 donít 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 04-10-2019 17:12

Re: Loud Outboards - How to Reduce Sound?
 
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 04-10-2019 18:09

Re: Loud Outboards - How to Reduce Sound?
 
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 04-10-2019 20:23

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

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

Thatís how it used to be. Currently itís 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 04-10-2019 23:24

Re: Loud Outboards - How to Reduce Sound?
 
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 05-10-2019 06:43

Re: Loud Outboards - How to Reduce Sound?
 
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


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