Loud Outboards - How to Reduce Sound?

[Chotu]

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]

People should understand that while they may start a thread, it’s not “their” thread. It’s 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 doesn’t understand.
Not every post will be helpful, it’s the nature of a Forum

[Sparx]

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]

Originally Posted by Sparx

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, it’s 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]

Originally Posted by Cheechako

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]

Originally Posted by Chotu

The bulkhead the bracket and track is attached to runs vertically from the waterline to the deck level. However, it’s 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]

Originally Posted by Sparx

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 I’d 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]

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]

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]

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]

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 don’t think many use the word smooth to describe their two cylinder four stroke.

[Chotu]

Originally Posted by AndyEss

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.

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]

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]

Originally Posted by Compass790

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. That’s the first step. Next, I will look into changing the frequency of the bulkhead if necessary, and finally, a couple of tread’s 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 won’t be another month before I’m 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]

Originally Posted by Chotu

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 “fair” out my cabin top foredeck so a girlfriend could do her yoga more easily. Not enough flat space without my inserts.