Gday all
Does anyone have any knowledge on good and economical engine bay insulation? I am building new engine boxes for my outboards and would like them to be nice and quiet.
cheers
Phil
Whitworths had a shelf of the stuff last time I was there in a few different sizes. , looked about an inch thick.
I lined the engine bay with the Whitworths stuff last year. Reflective layer over foam with glue strips built in. Looks nice, but made no appreciable difference to the noise level.
Sound insulation is like building water containers. If there is a gap, no matter how small, it will leak.
And despite insulation to try and stop air-borne noise from escaping the engine bay, there's the fact that our boats are great big sound-boxes with huge semi-flexible surfaces that could barely be better designed to act like a huge guitar or drum. You'll never stop vibration transmitted through the engine mounts and cutlass from ringing out into the air!
Heaps of the WW stuff was a complete waste of time (on my 28 footer with a 2-cylinder diesel). Proper engine alignment helped a bit (but not all that much). I think the only answer is a bigger boat so you're further from the engine when you're in the cockpit, and/or a multi-cylinder engine which will be inherently smoother. On long motoring legs, I wear my noise-cancelling headphones and/or earplugs for some relief.
Cheers, Graeme
My boats engine bay is lined with dynamat from the usa it is pretty good perhaps stopping 50%+ of the engine noise it is 2 types one is a flat aluminium type sheet self adhesive around the close parts of the motor and then a silver sheet with 8mm black foam backing around the engine bay some good hi fi joints use it in the boot for there sub woofers 
I saw some stuff at the boat show at Sanctuary Cove a couple of years ago. It was fairly heavy but only around a centimetre thick. I asked the bloke how well it works, and he just pulled a sheet of it off the top of an insulated bin full of speakers BLASTING heavy metal music. I had to put my hands over my ears. Before he pulled it off, I didn't even know it was THERE. It was reasonably expensive, but it was the goods.
Peter
My understanding of acoustics is that mass is everything - the best acoustic barriers will be heavy.
This relates to the question .... I once had great success with a colleague drastically cutting noise from a respirator. They had to be super quiet in hospitals.
We did it by relentlessly moving a sound meter around the thing and doing an FFT on the sound received. FFT is a graph showing what frequencies are dominant and which ones not. For example if the crankshaft is doing 2000rpm thats 33 revs per second so if the FFT shows a peak at 33 Hz you know that noise is coming from the crankshaft. Using the sound meter we found the exact holes and gaps and vibrating panels in the respirator and fixed them. They did that on A380 aircraft engines really well.
Sorry to waffle - there are some really good free FFT software packages on the Net. If you are determined to fix the noise It might be worth downloading one and moving its mike around the motor and see exactly where the noise is coming from then you will know exactly what to do to fix it. The engine mounts bolted onto the boat might need to be padded for example.
Heres a mobile phone app that can do it - . End of waffle 
play.google.com/store/apps/details?id=com.xyz.fft&hl=en
Just glue some battens to the flat areas that are going to vibrate and transmit "noise"
Select to expand quoteTrek said..
This relates to the question .... I once had great success with a colleague drastically cutting noise from a respirator. They had to be super quiet in hospitals.
We did it by relentlessly moving a sound meter around the thing and doing an FFT on the sound received. FFT is a graph showing what frequencies are dominant and which ones not. For example if the crankshaft is doing 2000rpm thats 33 revs per second so if the FFT shows a peak at 33 Hz you know that noise is coming from the crankshaft. Using the sound meter we found the exact holes and gaps and vibrating panels in the respirator and fixed them. They did that on A380 aircraft engines really well.
Sorry to waffle - there are some really good free FFT software packages on the Net. If you are determined to fix the noise It might be worth downloading one and moving its mike around the motor and see exactly where the noise is coming from then you will know exactly what to do to fix it. The engine mounts bolted onto the boat might need to be padded for example.
Heres a mobile phone app that can do it - . End of waffle
play.google.com/store/apps/details?id=com.xyz.fft&hl=en
No waffle Trek, all good stuff
Select to expand quotesantanasaga said..
My understanding of acoustics is that mass is everything - the best acoustic barriers will be heavy.
Not everything, but the most important part.
The cheap lightweight acoustic panels do very little. The best acoustic panels used to contain lead, but are now high density vinyl sandwiched between closed cell foam. The heavier the better.
Good quality sound barrier panels are expensive. Another option is using some old lead flashing from a demolition place. You can use the cheaper lightweight panels with your own lead mass sandwiched in between .
Also important is making sure every tiny gap is blocked, as EC31 noted.
You can not close off all the gaps. The engine will need air. The idea is to have the engine air intake come up from as low in the bilge as possible. Make the intake vents for the enclosure so the air has to pass around some corners and of a heavy timber. The intake should be as low as possible to suck in cold air. The exhaust vent for warm air at the top and rear. The idea is to stop any panel vibration and have different internal angles to reflect the sound. Some sound absorbent material would help but probably not worth the trouble. Outboard motors have very little under the covers.
And no matter how well you close the gaps and isolate the air-borne noise, you will still have vibration transmitted mechanically to the hull through the cutlass and engine mounts. In theory to minimise this you would need to either mount the engine and cutlass on very massive lumps of something, or have it wobbling around on very flexible mounts (which obviously is impossible).
Learn to love engine noise, or turn up the radio!
Horrible stuff when it deteriorates. I dont know how old it was on my cav but it would not stick to the panels and left a fine dust of black sht everywhere. Took it out and have not replaced it.
Cav30, my Cavalier is the same. The old foam has disntigrated and is turning to dust. Not much of it left now so I am following this thread before deciding what route to take.
I think I would find where the noise is coming from before trying to fix it Is the noise REALLY coming from the engine mounts vibrating the boat?
ie. Re the respirator (read outboard) the conventional wisdom was to make the plastic case thicker (read foam sound insulation material). But when we did our testing the most noise came from its air intake (read outboard motor air intake). Oops the surmising was wrong. We made a baffle and that noise was fixed. Convention wisdom was that the electric motor in the design was too noisy and several varieties had been tried. We found the whining noise was coming from air turbulence caused by the compressor impellor blades passing its chamber air outlet. Not the electric motor at all. Oops again. Smoothing the blade shape cut the whining heaps. In the end the average noise was 31dBA when before we started it was as much as 43dBA. (dBA is more or less the unit of loudness).
Study and analysis usually beats guessing. Getting a sound meter (or app) and using for 5 minutes would give the answer. I personally think that some sound insulation material is like striped paint
