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Old 08-02-2021, 09:14 PM   #21
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I’m thinking that the small amount of airspace around and internal combustion engine may be needed to help the engine from overheating. I imagine that if a manufacture could make their engines that much quieter they would be doing it and advertising the lower decibel specs to promote their product.
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Old 08-03-2021, 11:23 AM   #22
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Air works well with plastic bags and a canvas wrap

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for clamping
I'm picturing lining the cowling with a trash bag or similar, and filling with sand (or water).
In my shop, we use heavy plastic bags, filled with pressurized air and the outside wrapped with canvas to hold unusual parts in place while glueing, epoxying, etc. It mates the parts perfectly, especially in confined areas, kinda like vacuum in reverse and costs almost nothing to make the "fixture".
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Old 08-03-2021, 11:59 AM   #23
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A bit late, but I tried thickened epoxy resin and it worked beautifully. I tested it and the material broke apart without any release of the epoxy. I can imagine it would be no fun to remove in the future, but for now it works great as an adhesive.
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Old 08-04-2021, 06:28 AM   #24
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The 2lb MLV is thinner than the factory "insulation" it replaces. The engine is water cooled, and the combustion intake airpath unrestricted.

Noise is not a priority for engine manufacturers, unless its a byproduct of engineering improvements. 4 cycle engines are quieter than 2 cycle engines, and manufacturers call that out as a benefit and feature improvement, but the engineering was not to make the engine quieter.

The manufacturers need to ask themselves, if I make this engine 10 Db quieter, and add $20 to the cost and add 20 lbs to the weight, how many more engines will I sell?

The factory insulation is an acoustic foam noise absorber, it does little to reduce noise transmission through the cowling, weighs nothing, costs very little, and looks like insulation.




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I’m thinking that the small amount of airspace around and internal combustion engine may be needed to help the engine from overheating. I imagine that if a manufacture could make their engines that much quieter they would be doing it and advertising the lower decibel specs to promote their product.
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Old 08-04-2021, 06:59 AM   #25
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Good trick, would work well with water instead of air as well, for those without access to compressed air. Tape to hold the parts in place if necessary



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In my shop, we use heavy plastic bags, filled with pressurized air and the outside wrapped with canvas to hold unusual parts in place while glueing, epoxying, etc. It mates the parts perfectly, especially in confined areas, kinda like vacuum in reverse and costs almost nothing to make the "fixture".
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Old 08-04-2021, 08:24 AM   #26
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The 2lb MLV is thinner than the factory "insulation" it replaces. The engine is water cooled, and the combustion intake airpath unrestricted.

Noise is not a priority for engine manufacturers, unless its a byproduct of engineering improvements. 4 cycle engines are quieter than 2 cycle engines, and manufacturers call that out as a benefit and feature improvement, but the engineering was not to make the engine quieter...

The factory insulation is an acoustic foam noise absorber, it does little to reduce noise transmission through the cowling, weighs nothing, costs very little, and looks like insulation.
Dead-nuts-on, Mr. disease. The 1/2" foam is crap but does a wonderful job of absorbing oil which gets all over your fingers when lifting the cowling off. To address the earlier comment about epoxy - you're probably correct but I much prefer the marine adhesive (either Loctite or 3M) as it has flex and helps with the huge amount of vibration that it will experience, plus will provide some decoupling of the MLV, which is important for its effective performance.

BTW, I still have not had a chance to drop the boat in the water to take dB measurements. Combination of daily afternoon thunderstorms (with lots of pretty lightning) and I'm finishing a paint job on the engine. Looks gorgeous!
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Old 08-04-2021, 11:34 AM   #27
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BTW, I still have not had a chance to drop the boat in the water to take dB measurements. Combination of daily afternoon thunderstorms ...
I'm looking forward to hearing about your results when the storms etc. let up. Would definitely consider doing this, and knowing the benefit in the form of numbers would be really useful. Plus just your impressions.
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Old 08-07-2021, 02:06 PM   #28
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@Frosty, I just returned from the Indian River. Took sound measurements under duplicate conditions: measurements behind windshield to avoid wind noise, 4000 rpm, 30 mph.

Prior to installing the MLV I measured 96db. Installed MLV as mentioned before with partial coverage inside 60% (see earlier photo).

Today measured 80db!Click image for larger version

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I still have about 2-3 sf of MVL to install so will do that one of these days.

Hope you find this thread helpful.
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Old 08-07-2021, 02:34 PM   #29
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Wow, that's a huge drop! I bet you're going to enjoy that. Thanks so much for testing and reporting back
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Old 08-07-2021, 03:44 PM   #30
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I got similar impressive results after installing Soundown Carpet Underlayment on my boat. Soundown makes impressive products for sound and vibration deadening.


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Old 08-07-2021, 06:23 PM   #31
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Mako, that's huge! Congrats, and thanks for sharing this project!
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Old 08-08-2021, 05:54 AM   #32
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Awesome!

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Prior to installing the MLV I measured 96db. Installed MLV as mentioned before with partial coverage inside 60% (see earlier photo).
Today measured 80db!
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Old 08-08-2021, 09:30 AM   #33
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Sure thing guys.

@FlyWright, did you install the Sundown under your carpet or in another area? I was wondering if that could be used on bulkheads to provide some more privacy between cabins, for example.
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Old 08-08-2021, 09:49 AM   #34
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Quote:
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Sure thing guys.

@FlyWright, did you install the Sundown under your carpet or in another area? I was wondering if that could be used on bulkheads to provide some more privacy between cabins, for example.
Only as a carpet underlayment. Click the link in my post above and the thread describes it and includes photos and measurements.
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Old 08-08-2021, 02:35 PM   #35
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The carpet underlayment has a mass loaded vinyl layer bonded to a medium density carpet pad. The MLV layer goes UP. The pad acts as a decoupler and provides a very plush feel underfoot.
For privacy 1lb MLV alone is typically adequate for walls that abut other cabins or passageways, for walls that abut the engine room 2lb would be recommended.
Of course, if you have a bunch of 2lb on hand, more is better




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@FlyWright, did you install the Sundown under your carpet or in another area? I was wondering if that could be used on bulkheads to provide some more privacy between cabins, for example.
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Old 08-29-2021, 09:34 PM   #36
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To followup, I had some remaining material which I applied to the plastic intake box which sits on the carburetors. I assumed that a lot of noise comes through the carbs, which face directly forward to the steering station. Cover the entire box.

Today measured 78db, versus the previous 80db, so it was not much of an improvement. Still rather loud for me, but the only way to get an old 2-stroke quieter is to turn it off!

Ha
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Old 08-29-2021, 10:34 PM   #37
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Thanks for the follow-up to the follow up

Did you ever end up doing the remaining 1/3 of the cowling? Or is it still (just) the forward 2/3 plus the addition to the intake box?

Ha! on the "quiet it down by turning it off." 0 db!
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Old 08-29-2021, 10:45 PM   #38
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IIRC, a 10 dB reduction is a 50% reduction in signal or noise. If that's accurate, you might have more improvement that it appears. 2dB/10dB = 20%...20% of 50% = 10% less noise. That is significant, IMO.

I may be misremembering the details...please correct me if I'm wrong.
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Old 08-30-2021, 04:43 AM   #39
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@Frosty, yes I did finish the small remaining side areas as well.

@Fly, I think your math is probably correct. I know that sound levels are logarithmic, and logarithmic scales always confused me in school.

I think the conclusion of all this is that even for a small dinghy outboard, it is worthwhile to treat the cowling (even 1lb MLV is better than nothing). This is especially true for a 2-stroke but personally I would for a 4 also.

Don’t get me started on my old Gray Marine Jimmies !
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