Loud Outboards - How to Reduce Sound?

[Chotu]

Originally Posted by Rapanui

I spent most of my working life on ship noise and vibration reduction, and your problem sounds (sorry) difficult.
The airbourne noise component directly radiated from the engine can be reduced by:

• lining the whole well(s) with a proper acoustic absorption material which would need to be at least 25mm thick but 50mm or better. Given that the material is porous, it will need a thin impermeable layer such as mylar to prevent it wicking water, oil and fuel.
• If the well is open to the sky so that the sound can be directly radiated to say the cockpit, introduce some form of baffling, lined again with acoustic absorption material to prevent direct radiation. Ideally the the well should be closed off completely and other means used to feed fresh air into the enclosure.
• The airbourne noise transmitted through the engine bulkhead can be reduced by lining the inside face of that bulkhead with acoustic transmission loss material which is a material comprising a resilient heavy layer with a springy foam base. The heavy layer should be around 1 lb/sq ft or 5kg/m2. This may not be effective until you address the structureborne noise component first

These 2 things should give a noticeable reduction in the airborne noise above decks and may have some effect reducing the noise in the interior.


The structureborne noise component, which is high frequency engine vibration is more difficult. Once the engine vibration gets into the structure, then there is virtually nothing that will work apart from lining every internal surface with transmission loss material - not good for a performance cat. So you have to isolate the engine at its mounting or improve the effectiveness of the outboards internal engine mounts.

• If the outboard is mounted on a flexible structure i.e. the dynamic stiffness of the structure is too low, the engine mounts will not work effectively. if it possible, some form of added stiffening such as a large section aluminium T or angle behind the aluminium tracks and bolted through. Extend such stiffening to where the bulkhead meets the hull or deck to maximise it.
• You could consider adding vibration mounts either between the aluminium track and the bulkhead, or having a second outboard pad resiliently mounted to the existing one.

Good luck

Thank you very much for lending your experience and thoughts to this thread. These are the general ideas I was heading toward. What I can say is the aluminum tracks and bulkhead the Outboards are mounted on are extremely stiff. They don’t move at all. Not even the slightest bit. It’s all heavy duty stuff. The aluminum tracks are actually 1/4" thick U channel, bolted every 6" to a 1/2" backer/doubler. Absolutely no give at all. They needed to be strong like this in order to support the power from 30 hp outboards.

So, at least I should be in good shape to isolate the motor vibration from this very stiff track.

I should probably add the entire boat is extremely stiff as well. It uses 1 inch foam as a core. Of course there is plenty of glass over that bulkhead as well. And it is supported by steps all around it. So it’s extremely stiff in the area that the Outboard’s connect.

Unfortunately, I can’t close the wells. The airborne noise I may have to tackle a different time. If it still is a problem. These outboards are retractable. They come up out of the water when not in use. So, closing off the top of the well would prevent that from happening. They actually come up out of the well. Which allows you to be able to service them in place. They stay on the track and ride way up.

There have been a lot of good comments on here. Plenty of things for me to try out. Based on your post, I feel like one of the best things I can do to quiet this thing down is to make sure my rig and sails are good in light air. Ha ha.

[wsmurdoch]

While most of this thread has focused on vibrations transmitted through the engine mounts and while I know little about your particular engines, is there a chance that you have an exhaust problem? My outboard motors exhaust into the air below the crowl when at idle and into the water through the center of the propeller or through the lower unit when at speed. Is there a chance that the underwater route is either blocked or that because of the mis-propping is not effective, and your engines are continuing to exhaust into the air at operating speeds? Also, is there a chance that you have a loose joint or a missing gasket in the exhaust system?

[Rapanui]

I understand the constraints you are working with. One point I have is that the dymamic stiffness at say 1 kHz (which is likely to be the highest noise component) is very different to the static stiffness of a structure. At such frequencies, the structure breaks up into a series of resonances. This will be worse for a high performance cat with high strength to weight structure, stronger but not necessarily stiffer at acoustic frequencies. To give you some idea when we were designing a mine hunter, we actually measured the actual dynamic stiffness at acoustic frequencies of every mounting of the 50 or so equipment that were critical, and despite designing to specific acoustic criteria and finite element analysis, a significant proportion had to be reworked to meet the isolation criteria.
What I am trying to say is that despite the existing thick channel and sub structure, you will probably need something with a very high stiffness (moment of inertia) behind the sliding channel, off the top of my head 100mm x 6mm T, angle or channel to allow the existing outboard internal mountings and any additional external mounts to work effectively.

[Sailmonkey]

Would mounting the tracks on a series of mounts like this be a possible solution?

https://www.mcmaster.com/5823k27

The only major problem I can see with the linked mounts would be running the engine astern as the mounts are not rated for anything other than compression and shear.

[a64pilot]

Originally Posted by Chotu

I think this is the plan for the time being. Some vibration isolation. And changing the fundamental vibration frequency of the bulkhead. But yes. I am reaching full RPMs way too early. This is an a normal boat. This is a very high performance catamaran. So I’m at close to 8 kn and hitting the rev limiter. I should be getting somewhere closer to 12 to 14 kn. I used prop prop that the Outboard shop sold me. They didn’t know what they were talking about. They treated me like a pontoon boat. I also didn’t know what I was doing because I had never bought any other Outboard except for a dinghy before. So I didn’t really know how to match the prop to the boat. I figured the low-end power was more important. But since I am under propped I am wasting fuel.

If your hitting the rev limiter, your underpropped for sure.
First you will need a tachometer.
I use this one on my outboard, cost is $22 and it tracks operating time too so I know when to change oil etc. you can’t really prop an engine without a tach.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Then you look up on line to find the operating range for your motor, on my Suzuki it happens to be 5200 to 6200. That is where your engine should max out at full throttle speed stabilized. Boat speed will just be whatever it ends up being, it really doesn’t have anything to do with propping a boat.

I’d shoot for the lower end of your operating range, as often high RPM means more noise.
Be sure to be running both motors wide open when you try this of course.

You just have to try a higher pitch prop, maybe a shop will let you try different props if they know your buying from them.
I’d try increasing two inches of pitch, that’s quite a bit really, but it will tell you a lot, if for example it drops your RPM by 1,000 then you know that 1 inch means about 500 RPM.
I say two inches since your banging the rev limiter and I assume you want to be at the bottom end of the range.

If you were a planing boat and were looking for max performance, then you would want to be close to max RPM.

Since RPM will determine the vibration frequency, and since your noise may be mostly due to vibration, I’d get the props right first, it may be that is enough to make it tolerable.

Then if it were me and I would plan on motoring at no more than 2/3rds max RPM, so it it turned 6,000, then I’d motor at no more than 4,000. I think your noise will be more tolerable and you motors last longer that way.

[bruceb]

A few thoughts. Part of the problem with outboard's noise issues is the type and pitch of the sound they make. The outboard itself doesn't have enough mass to reduce vibrations very much so it seems noisier than it really is. Outboards have a lot of induction noise that is fairly high pitched, some exhaust noise and of course vibration of the engine.
Each part of the total noise needs to be reduced in different ways. Vibration needs to isolated so the soft pads need to be where the engine fastens to the mount. The less mass you have vibrating the better. Induction noise is usually through the air, so redirecting it and/or absorbing it before it contacts a surface works best. If sound waves have to turn corners the energy is greatly reduced. Maybe a removable lid or blanket over the engine wells? If the thru the hub exhaust is getting uncovered, maybe a larger cavitation plate (like a "Dole" fin) might help direct the sound aft.

B

[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.

[thomm225]

Originally Posted by Dsanduril

Sorry, but I don't know of any 8HP that meet your requirements. The smallest Evinrude makes (that I am aware of) in the DFI market is a 15HP. So you're looking for a different beast.

I've owned and operated two-stroke outboards for something like 50 years. Seagull, Johnson, Evinrude, Mercury, Tohatsu, and a couple of others. Until an Evinrude they were all carbureted, noisy, stinky beasts. I loved their power/weight ratio and hated just about everything else about them. They left a sheen. They were loud. Couldn't always remember if I'd mixed the spare gas can. Four-stroke was so much simpler in so many ways, just heavier.

But modern two-strokes simply aren't your father's Johnson. They are available and sold in the US. They meet all emissions requirements and then some. They are just as fuel efficient as four-strokes. They aren't banned. They're quiet. Can you get them in a 2,4,6,8,9.9 HP? Probably not. But not every outboard is a small engine.

That's good news actually, but my Father never owned an outboard!

I wish they made the lower horse power ones so I could get a larger light weight outboard (maybe 8hp -9.9hp) for my boat even though my 5hp 4 stroke does a fine job. (I refuse to put a heavy 110 lb 9.9 outboard on the back of my boat on a bracket)

My first outboard was a 40 hp Johnson anyway which was mounted on a 16' Chincoteague Scow. I bought the boat, motor, and trailer for $300.00 in 1971 when I was around 16 years old

Over the next 3 months, I went thru three more boats, motors, and trailers being a teenager buying, selling, and trading.

I had an old Mercury 35 at one point that summer with a white case (shows you how old that one was) 1960 I think)

In the end, I wound up with a 25 Evinrude on a 14' Aluminum V bottom boat. It would push that boat to near 25 knots. I took it no further than maybe 15 miles offshore to Tangier Island Chesapeake Bay

Later still I had another 40 HP Johnson. This one said add one quart to 6 gallons of gas on the pop down lid. This in 1980

All these were 2 stroke and I had the one 4 stroke in 1976 and now the new one I bought in 2011. Mercury 5 hp 4 stroke 25" extra long shaft. Very quiet

http://www.sandeace.com/waali/WaaliMotors.html

[morribabes]

I doubt if changing to 4 stroke engines would improve the situation. I think you've got a resonance problem due to the lightweight construction of your boat. I would suggest trying various ways of bracing the engine mounts to reduce the amount of movement. Alternatively, try introducing rubber into the mounts to dampen the vibration.

[a64pilot]

The new 2 strokes are likely built on the orbital engine technology, it was an Aussie adaption that quite a few motors tried before most gave up and went to a four stroke.
A direct injected engine using the orbital engine technology, is not a simple or inexpensive thing.

I believe but am not sure that Mercury’s Optimax engine’s used the Orbital injection system.
http://archive.maas.museum/australia...ticle_id=10041

It has been trying to bring back two strokes for at least 30 years, but for some reason, it just hasn’t happened. Motorcycles would be a big market. Aprillia had I believe a very good two stroke that came out about the time I was Road Racing motorcycles, but for some reason wasn’t successful.
RS 250 maybe? I saw a few on the track, really a good bike, but out of my budget at the time
https://en.m.wikipedia.org/wiki/Aprilia_RS250

I believe modern two strokes are becoming a lot like modern road Diesels, that is they are becoming more and more expensive and complex, gaining weight and meanwhile the spark ignition motor is becoming more and more maintenance free, they are sort of morphing into each other, and I’d expect soon we may even see a gasoline engine that operators at least some of the time in a Diesel cycle.

I wouldn’t expect to see a small orbital injection two stroke, likely more complex and expensive than a four stroke. But. That’s my guess.

[Sailormanbigd]

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]

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]

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]

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]

Originally Posted by thomm225

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.