Motor Cowl Soundproofing

[alcam2001]

I have an old noisy-type 2-stroke (1984 V4 jonno) that could do with some silencing ! Currently the glass cowl has a thin (fairly pathetic) foam stuck to the inside - probably does very little.

I am considering removing the foam then doing one or more of:
a) painting car underbody/firewall deadener on the inside
b) gluing vinyl/foam sound absorber/deadener to the inside
c) getting a fitted motor cowl cover
d) all of the above !!!

Does anyone have some specific knowledge of this, or looked into it properly ? Obviously, the above doesn't reduce exhaust noise - I already put on a permatrim - which cuts out some (maybe 20%) of the exhaust noise that gets to the cabin from the exhaust (as well as doing some other very good things).

Specific problems (maybe):
1) fire hazard from any of the materials - foam, underbody paint, glue etc ?
2) sticking vinyl/foam sheets to the underbody paint after coating so they don't fall onto the flywheel (a brand-new cause of on-water motor failure!)
3) motor heat/vibration/oil dislodging/unsticking any of these things
4) not enough sound reduction to be worth the trouble.

Note that voiding warranty is not an issue - probably hasn't been an issue for this motor since about 1986 !!

[Noelm]

not too sure how a permatrin helped with sound, but anyway, I guess all of those things will help a bit, but your cowl is a pretty close fit to the powerhead, so be careful with how much "stuff" you add, fire should not be a problem if you use the proper stuff, and there is a whole host of materials around that will do the job, just not sure how much improvement there will be.

[finding_time]

What about getting an outboard cowling cover made, there quite thick and should deaden the motor noise some what! Most boat trimmers will make them up!

Ian

[Noelm]

Motor covers that can be kept on when running need a bit of thought, most manufacturers have them, you have to be carful of the air vents, which for your Motor are on the rear at the top in what sort of looks like a hand hold to lift spot.

[lippa]

go down to your local E-TEC dealer:-X

im sure they have tonnes of different ideas to silence noisy outboards;D

[FNQCairns]

I did it to my 90 with under cowl foam, works an absolute treat! although it seems to be getting noisy again, due only to the human condition.

Still planing to add a foam filled cover to the outside and will take heed of Noelms advice above, do not be side tracked by claims of increased heat it's entirely a red herring.

http://www.whitworths.com.au/main_itemdetail.asp?item=80200&search123=sound&intAbsolutePage=1

cheers fnq

[bigbrian47]

i bought a cowl cover from OCA for my 90 2str
significantly quieter and fits very well
the mesh over the air intake keeps the wasps out as a bonus
(spraying inox etc does too)
it's been on about 5 yrs and is still in good nic
worth buying
brian

[alcam2001]

Actually the option 3 in my original post was for a fitted cowl - in fact it has arrived from OCA, just not 'fitted' on yet.

Permatrim - the long reaward overhang seems to push the exhaust note further back behind along with some other boat/motor noise - may just be perception or even just optimism - they all count !!!!

Currently the cowl has a layer of open-cell lightweight foam - about 10mm thick - so the new new layer wouldn't extend out any further.

A new issue (since I have been experimenting) is the ability of glue on the foam/vinyl sheets to stick to the layer of underbody paint. Normal spray glue and contact seem to have some problems. I have yet to try using the underbody paint itself as the glue. Or sikaflex (polyurethane) - or even a neoprene-based glue. Then the problem - the glue that sticks, may fail under prolonged heat - dropping a sheet of foam/vinyl into the flywheel...........

BTW, I have just put underbody paint on the back (hidden) side of the various floor and bilge hatches, and inside the motor well.

If you like black, this underbody paint is very neat - it would seem that because it is tar-based, it is likely to fill cracks etc and prevent water trapped behind it - so could be a good corrosion preventative as per car/truck bodies.

[oldboot]

You realy need to think about what your problems are and the various modes of sound transmission and absorbtion.

I don't think undebody paint is a good idea in this instance.....all it will do is add a very small amount dead mass to the cover...... fibreglass is a pretty dead substance to start with and not as light and resonant as thin metal sheet.....so the advantaes will be small.

you do need to get some amount of absorbtion inside the cowl....it will surpress some of the mid and high frequency noise from the motor...... this is what the light foam is suposed to be doing..(and it will to an extent)....but remember the manufactturers are always chasing weight and price.

If you speak to a good automotive trim supplier or even jaycar there is a heavy soft foam with adhesive backing ( or without) that is specificlay intended for use in cars.....it will provide both mass(the foam is loaded) and absorbtion.
The better types of foam are flame retardant.

the adhesive is intended to be heat tolerant for firewalls and the like.

If you clean the inside of the cowl with thinners the adhseive should stick pretty damn well.

the peel and stick backed automotive product is what I would be using.....it is made for a similar purpose and is pretty easy to use.

you can cut to shape and try for size the peel the back off and lay it down...... but make sure you get it right first go.....one stuck its stuck.


If you are expecting any adhesive to stick well do not use body deadener or even paint inside the cowl......there are a variety of reasons why the adhesive WILL fail.

Most of the good quality contact cements will work well (outboards don't get all that hot) if heat tolerance is a concern use a high tem contact used for kitchen bench tops.

The big problem with all of the contact type adhesives and that included the peel and sticks... is that they are not all that tolerant of oils and solvents.... but that is less of a problem under a boat cowl that it would be under a bonnet of a car, because most outboards don't vent or spray oil about under the cowl.


I don't think it would be a good idea to go much thicker than 10mm inside a cowl mostly because there wont be the room.


An external cover almost certainly will help.

A very important thing is to make sure the cowl is mechaniclay quiet.... ( and that goes for the rest of the motor and the transom)....make sure that there is nothing rattling and the cowl fits firmly.....just adjusting that latch or replacing the rubber seal can get rid of a noise or two.
dampen or secure anything that rattles or vibrates.

While on the subject of the transom...... have a think about that too....how is the motor attached and is you transom working like a sound board.
Putting a wider and thicker backing plate under the outboard bracket is worth looking at so is damping the transom......quite a lot of low frequency noise will come from your transom particularly if it is an alloy boat.

Sound is my business & I hate noise.

cheers

[alcam2001]

Oldboot, thanks - that helps clarify some of the problems and now probably got the answers I needed.

Thanks everyone for the ideas.

I have tried the Jaycar vinyl/foam for cars - and the problems with glueing to underbody paint seem to be huge - basically the solvents in the underbody paint prevent curing of 2 types of glue I tried. So, my next try will be just using the vinyl/foam onto clean glass cowl as you suggest (after I clean off the underbody paint - fortunately, painting turps onto it seems to totally dissolve it).

A few months ago - with some help from Grand Marlin (Pete) - I put floatation foam (microlen - 0.8 cubic metre) under the floor (except a strip 500mm wide down the keel) and to the gunwhales. all round. Put 100mm thick across most of the transom including the back of the motor well (but not the 'top' of the well). Then carpeted the whole boat (and the foam) up to the gunwhales (including transom). This reduced noise considerably (and looks cushy too!) - can now carry on a conversation at the helm in just a slightly raised voice at 4500rpm !! Used to have to shout loudly, then repeat a few times. Banging in rough bay conditions now vastly reduced - just a distant, quiet thump ocassionally.

My current project is getting the remaining noise down, so I can get kids to sit at the transom seats all day without going deaf !!. A new quieter motor is probably not an option - and the old jonno (found by Mal Leashman) has proven to be extremely good on reliability, starting and performance (not good on noise and fuel). The transom already had a second 10mm backing plate (now covered with foam and carpet) - welded to an extensive underfloor subframe that goes up to the cabin and attaches all main structures in the boat. A very solid tinny (5m quintrex from the 80's).

And, yes, I am confident that the permatrim reduced noise a bit - but too hard to quantify for any skeptics out there. Probably works due to keeping the airated water and exhaust gas submerged for a bit further out, and moving the propogation of the noise rearward and funnelled (due to the shape of the permatrim). Maybe some underbody paint on the TOP surface of the permatrim could deaden another little bit of exhaust noise....... I like this underbody paint stuff!

[oldboot]

Oh you are keen on thie "underbody paint"..... you have to remember with that studd that noise deadning is only a " sold side effect", its primary finction is to hide and cover underbody sins, its second finction is corrosion protection and it does do a little noise damping.

As I said there is a significant problem with any of the solvent bassed or rubber based ashesives and any paint...... bituminous paints are the very worst.

first the solvent in the glue gets into the paint, then the oils in the paint get into the glue and both end up a complete mess.

Yeh scrub and scrape that gunk off the inside of your cowl, then wash it with turps then thinner till the glass is dry and absolutely clean.... or you will have adhesion problems.

Even spraying paint over bitumen bassed body deadner works very very badly.
They wont even line mark over real fresh bitumen and thats 3/4 rocks and laid hot.

If you must use this sort of gunk, the water bassed stuff does go off and you can coat over it.


All that foam and carpet on the inside of the boat sounds like an acoustic installation........ if you decide its no good as a boat turn it upside down and sell it as a dome of silence.;D

It would be worth taking a noise meter out and measuring actual noise levels in the back seats.....don't want to be sending your kids deaf........I said "deaf";D .

unfortunately there isn't much acoustic design in boats ( not surprised).

But think about this.
My neices husband ( he refuels black hawks and stuff) tells me, the black hawk chopper is designed so that you cant hear them from in front till they are right on top of you... the send all the noise out the back........which is prette clever.

the old hughys you could hear for miles in all directions.

You can do vey clever things with sound with a bit of thaught......and it is less complicated than some would have you believe....you just have to work thru all the issues step by step.... doing the best with each step as you can.

A friend of mine had a huge noise abaitment problem.... a high paid acoustic consultant said the only solution was to brick up the windows and aircondition the place.........I moved the speakers closer to the customers and turned down the amplifier............Of course I was not believed till the same acoustic consultant cane back with his $10 000 noise neasuring equipment and found the same result as my $60 tandy noise meter...........step by step simplest and easiest solution first.

OH have you had the motor tuned and serviced..... that can make a difference too.

cheers

[oldboot]

Oh here is the test for mass based acoustic treatments.......is the cured film or whatever significantly heavy and significnatly dead in comparison to what you are putting it on, or does it significantly stiffen the object?

If not it will do very little.

cheers

[trev1]

A lot of the noise associated with older outboards is actually intake noise, this is why a lot of mid 90's and later carbed outboards had convoluted intake assemblies before the carbs. Intake noise is a very hard noise to silence without affecting performance. As others have said, a later model is thereal way to go for noise reduction as a lot of the baffles/noise reduction techniques are built in to the outboard and very hard to add later.
Intake noise is one of the reasons that the intakes for air are at the rear of the cowl.

[alcam2001]

Thanks olboot and trev.
The acoustic foam from Jaycar has heavy vinyl backing - the vinyl has some heavy substance mixed in it to make it really heavy (kind of like the x-ray apron the dentist puts on before doing xrays). So, if glued firmly, I expect it to deaden - just got to get that underbody paint off first. Pitty, the matt black look inside the cowl was very nice !

However, the air intake is a kind of single chamber/baffle thing at the back of the cowl - since putting acoustic vinyl/foam in there will impede airflow (bad) I will leave the underbody paint inside the chamber since it is only paint thickness, and might keep a bit of deadening on the intake noise.

On a 1999 Camaro that I had back then, the air intake was a very complex convoluted set of baffled chambers about the size of a beer box - the word out there was that it still detracted from performance significantly (estimate on the net were around 3% to 5% of HP) - so I simply removed it - then the noise level increased very slightly, but nothing like as much as the 2 by 3inch Borla exhaust system increased the noise !!

The project after next on the boat is to put in a transom infill, to fill in the big motor and motor-well cutouts on the transom - mostly to improve roughwater freeboard, but also will deflect a bit more motor sound.

Eventually, this will be as quiet in tte boat as if a new ETEC, but for a little bit less $$$$$$$$$$

[oldboot]

Yep as I saud those purpose made sound deadning foam products are "loaded" with something to make them heavy and dense...in the past they used to use lead granules.....don't know what they use thes days....... ground up politicians perhaps?;D

cheers

[Castaway]

...could always buy a cheap pair of ear plugs from the chemist.

[alcam2001]

...could always buy a cheap pair of ear plugs from the chemist.

thanks castaway - yes, sometimes the old ways are the best - we have been using 'finger in the ear' technology, but we wanted to upgrade.