Sound "proofing" a small outboard?

[km1125]

Picked up another NSF6A2 yesterday and while working on it (needed carb cleaning and impeller), got a wild hair idea about seeing if I could make it quieter. I have a similar motor we use on the dingy and when putting around the marina it's louder than some of the other folks (who are mostly running 2 cylinders).

So I have some sheets of some "dynamat-like" material and started playing with it. Ended up lining the inside of the engine cowling and the inside of the pan under the powerhead. I'll run it on the lake this weekend to see what the results are like. Anyone else try something like this?

 

[iggyw1]

I don't think it would hurt the motor any as it is a water cooled engine. I would not try it on an air cooled motor however (if anyone else has the same idea). Let me know how that works out for you because I may want to try it on my 6 H.P. motor. I would like it to be quieter as I troll with it. Thanks in advance for an update from you.

 

[Scott Danforth]

Wont make much difference

 

[ahicks]

Manf's spend big money quieting their motors. The 2 noisiest parts are the carb intake and the exhaust, with a distant 3rd being the mechanical noise just running.

I think lining the cowl won't hurt a thing, and SHOULD help quiet the carb and mechanical noise, but I wouldn't be looking for night and day difference. It could easily be a noticeable difference though!

 

[km1125]

So I did run it on the lake and it distinctly makes it sound different. I would describe it as sounding like "RRRRRRRRRRRRRRRRRRRRRR" rather than "EEEEEEEEEEEEEEEEEEEE" while I'm up on plane. There is not a huge difference, but it is noticeable. Enough that I'm going to go forward and do this mod to my main engine.

I ran both engines at the dock, both at idle in neutral and at full speed in gear (tried to the dock) and recorded them with a GoPro, then looked at the audio. The engines are propped differently, whereas the 'soundproofed' one has an 8 pitch prop and the 'non-soundproofed' one has the high-thrust 6 pitch prop. The 'non-soundproofed' one also has an integral fuel tank, so the engines are not exactly identical.

Here's a few pics of the measurements. It's easy to see in the first pic when you compare the idle sections. It is measurably lower in volume. The second and third pictures I uploaded the video to a site to do spectrum analysis on the audio. The brighter colors mean louder (yellow and red) and the frequencies are listed on the left.

Not 100% scientific, but enough to validate what I hear with my ears. It adds about 1.5 lbs of weight to the motor, so take that into consideration if you're going to be moving the motor around a lot.

 

[Sea Rider]

Send that info to the manufacturer...LOL !!

Happy Boating

 

[MattFL]

Super cool idea thanks for posting that! I had a 1967 Johnson 6HP 2-cylinder 2-stroke that was so smooth and not that loud. FF to about 2012 and I gave it away and bought a new Tohatsu 6HP 4-stroke single cylinder, I believe it's the same motor as the NSF6A2. The Tohatsu is significantly louder at anything above idle, and one thing I noticed was the Johnson had a solid fiberglass hood and the Tohatsu has a very thin plastic hood. Part of the noise on the Tohatsu was the hood latch, which was loose and would vibrate, so I wrapped the metal part with tape and that made a big difference, but still the regular motor sounds are loud. As mentioned above a significant amount of noise comes from the intake and exhaust. I don't think there's anything that can be done to the exhaust within reason, but adding some sort of snorkel to the air intake might make a significant improvement. I've got one of those super quiet Yamaha generators and you can tell they put a lot of effort into quieting the intake.

 

[km1125]

I'm sure on these small motors that weight and cost are much more important drivers than noise. With the move from 2-strokes to 4 strokes you also get a lot more vibration which resonates though the housing (that can't be dampened much at all). These also don't have isolation between the powerhead and the mounts like larger motors do.

 

[MattFL]

I think that old Johnson may have had rubber dampers between the motor and bracket as well, but as you mentioned not on the little Tohatsu (maybe the higher priced Honda or Yamaha?), The vibrations on this thing are a big cause of noise. It vibrated my little aluminum jon boat so much that I added aluminum L-channel bracing to the sides of the boat to reduce the metal flapping around and that helped a ton with the noise, but everything that is laying around in the boat still rattles around in the boat adding to the noise. But on the up side, it idles happily all day, sips gas and no 2-stroke smoke or oil slick.

 

[ahicks]

I'm sure on these small motors that weight and cost are much more important drivers than noise. With the move from 2-strokes to 4 strokes you also get a lot more vibration which resonates though the housing (that can't be dampened much at all). These also don't have isolation between the powerhead and the mounts like larger motors do.

I'd have to disagree. I think noise level considerations are going to be right up there with cost and weight when it comes to something that wold interest me.

I would not be interested in a noisy motor that transferred vibration to my seat at any price.....or weight for that matter!
I doubt I'd be a buyer of a single cylinder motor either, as getting the vibration out of a single is nearly impossible! You need a twin for that, or better yet a 2 stroke 3 cylinder.

 

[MattFL]

For the general public I think he's right. _Most_ people would prefer to save a few hundred bucks than to have something a little quieter with less vibration. At least at the time they make the purchase, after the first day on the water their minds may change lol. Also the Tohatsu motors are less expensive than Honda and Yamaha, I wonder if those brands are better at vibration and noise.

Edit --> Yes at 0:50 in this video you can see clearly that the Honda powerhead is isolated, probably with rubber, from the tiller handle: https://youtu.be/BjqeEl1RU1s I didn't check the Yamaha but I would expect the same. The Tohatsu is most definitely NOT isolated, but it's a few hundred bucks cheaper too.

Edit 2 --> I have to stop watching those videos, they're making me want to trade my perfectly working Tohatsu for a Honda!
https://youtu.be/_T8WLZv5Cd4

 

[km1125]

On the cost-vs-benefit, there would be a big difference in vibration and noise. Folks might pay to get rid of vibration, but not for just an incremental improvement in noise. These motors are not really that loud, it's just what you compare them to.

It's interesting that the Honda has many parts that look identical to the Tohatsu-manufactured motors. The tiller handle casting looks the same, as does the lower unit and the transom clamp mechanism. However, they do have isolation between the motor and the transom clamp that the Tohatsu's don't have, and it looks like there is also some additional isolation between the powerhead and the tiller handle. The Tohatsus have a rubber isolation on the tiller handle but it doesn't do much.

Just like the Honda, the Tohatsu-manufactured engines have a decompression-start system, where it relieves compression under (I think) 200 revs, so they are pretty easy to pull-start. I'm sure a 10 or 12 year old kid would have no problem getting it started. They also start very easy when things are running properly.

The Tohatsu-based 4, 5 and 6 HP engines are all the same, with the exception of the carb. You can upgrade a 4 or 5 to a 6 just by replacing the carb. On older motors, it would be a preferred method rather than cleaning the old carb. You'd get a carb that works like new, but has 20-50% more power than the old one.