So you desire a quiet engine....if you encase it in a soundproof box ( with an appropriate air inlet ) does the cooling system
cope with the hot air trapped in the box and does that hot air affect fuel and oil temps ??. Vents to let out hot air also let
the noise out.

Your biggest problem is mechanical transmission of engine vibration to the great big guitar body that is your hull, with all its lovely big thin areas just perfect for sound transmission!

Maybe a sound/vibration expert could chime in, but it seems to me the little bit of air noise would be of no real significance by comparison.

Maybe set it up so the box has a vent under the cabin sole or somewhere else where escaping noise isnt let straight into open space.
the engine will be sucking cool air into the box so it shouldnt get too hot inside anyway.

Wongaga has got it, you have a sound emanating device ( engine ) inside a box and the sound hole for it to escape is the companion way which it just so happens is close to where you probably will be most of the time.
i have had the pleasure of being on a wharf and waiting for my boat to arrive and as she approached I was suprised at how quiet it was !!?

I have a 98hp engine centre mounted, so believe me I've studied acoustic reduction

So 1st, make the engines db output as low as possible, most of the ruckus from an engine in a boat is from the air intake, generally there's between nothing and very little in the way of air filter/ sound dampening on the other hand the exhaust is nicely catered for via the water injection. If you have an air intake shaped like a trumpet, guess what is gunna happen ? Remember the shape of the air intake on the Ol Holdens, Fords, Valiants etc ? A kinda reverse trumpet. Modern vehicles use a soft rubbery ducting that contorts in such a way that it takes up all of the under bonnet space in an attempt to silence the beast !

samsturdy said..
So you desire a quiet engine....if you encase it in a soundproof box ( with an appropriate air inlet ) does the cooling system
cope with the hot air trapped in the box and does that hot air affect fuel and oil temps ??. Vents to let out hot air also let
the noise out.

if it is water cooled (sea water I mean) you could enclose the engine, given thatyou get a fresh air hose to the intake/filter box. You could probably install an exhaust vent as well if you have a room for it.

As others said, you would probably still have some vibration through the hull.

Forget about quiet engines on a yacht. If it is running you will hear it, especially on a 30 odd footer. Why waste your money on trying to sound insulate it??

Also don't forget that all other things being equal, more cylinders = less vibration = lower noise level. Hence Benny 40's whispering their way past my 28 footer with its rumbling 2 cylinder Yanmar.

I recently sailed on a boat that had a thoroughly soundproofed engine compartment, which did a great job of reducing the engine noise.

The engine mounts however did need some improvements to reduce vibration transmission.


In answer to your question of how to deal with the hot air trapped in the compartment, this boat had a forced ventilation setup (separate to the engine intake) which had intake and exhaust ducted to clever vents out the back of the cockpit.
You couldn't hear the fan running unless you were directly behind the vents, and when the engine was running the noise was damped down to something more like a modern diesel sedans engine noise driving around town.
You could hear the engine but it wasn't deafening, could still judge revs by ear, the engine compartment got warm but not roasting

On this point l beg to differ, Cisco!
I only can tell about my old yacht but the twin cylinder Vetus engine never vibrated unduly, never, even before the insulation was really noisy. It had its own GRP cradle which isolated the motor from the hull. It was just a bit noisier to my liking when sleeping in the quarter-berth.
The under-water exhaust was more audible - not noisy by any measure, just burbling nicely - than the motor and the stereo needed to be on half cock only to boot.
Spending a few bucks on insulation might help a lot unless one is blessed with a solid-mounted total dog of a motor in an otherwise empty (sound)box. The rubber mounting - if it is rubber mounted, like mine was - should help a lot keeping the noises as well as the vibration to a civilised level.


Sam, I would not hesitate to modify the engine compartment to make the noises more acceptable especially if there was an 'admiral' around busting my guts.
I guess, it has been modified by someone as l can't believe the manufacturer built the boat with a half covered motor. That would have been unacceptable even in the seventies.

OK...Interesting that you think it's a modification. It's a simple enough task to extend the the casings to reach
the bulkhead so now I'm thinking I'll do that.
Stray...the insulation you are talking about..is that self-adhesive ?. and now I'll be looking for a place to purchase
the insulation, so are you saying the silver backed stuff from Whitworths is useless ?.
And yes, there is an 'admiral' involved although she's no 'princess' and I've noticed we do have to speak louder
in the cockpit when talking to each other, not much louder, but it's noticeable.

I think the stuff from whiworths is ok, its sort of mid range i guess. The bad stuff is just a 1/2 inch of foam with foil on one side.