Engine soundproofing: observations and questions

[RichardS]

As has been said, 2-cylinder engines are the worst for vibration as, in a 4-stroke engine, the pistons move together not in opposite directions. Anyone who ever rode an old BSA or Triumph twin can attest to that.

Beware - Non-Boaty!

My old Speed Twin (5TA) would hit a vibration patch at about 60mph which just happened to resonate with my eyeballs!

Until I was a mph or two through the patch I couldn't see a thing.

Cruising at 58 or 62 mph was fine (although bits of bike did still vibrate off at regular intervals).

I remember a run over to Matlock Bath (the place for bikers in the 60's) following my mate's BSA single cylinder. At one point something with wires shot off his bike and vanished into the fields. When we got to M B we all looked over the bike but couldn't see anything missing. A few days later he tried to use the horn!

Richard

 

[prv]

With modern materials it is possible to make a series of padded bits to wrap nearly all the engine.

That's a good point - I'm sure last time the subject came up, someone pointed to a company making heat-resistant padding specially shaped for common boat engines.

Pete

 

[mjcoon]

As has been said, 2-cylinder engines are the worst for vibration as, in a 4-stroke engine, the pistons move together not in opposite directions.

Could you explain that a bit more, please? Why, given a 4-stroke does not use the crankcase for fuel, cannot the timing allow the pistons to move in opposite directions? Just, in fact, like half a normal 4-cylinder job, though with inferior dynamic balance.

Mike.

 

[earlybird]

Could you explain that a bit more, please? Why, given a 4-stroke does not use the crankcase for fuel, cannot the timing allow the pistons to move in opposite directions? Just, in fact, like half a normal 4-cylinder job, though with inferior dynamic balance.

Mike.

A 180 deg. twin crankshaft would give uneven firing strokes and uneven torque esp at low speeds. Perhaps that's why motorcycle engines, with very light flywheel effect, didn't use them.
However,I think that the Yanmar 2GM20 uses a 180 deg crank, but I'd have to check to be sure.

 

[stuartwineberg]

Linssen

IIR Linssen go to extreme lengths to soundproof their boats - engine in a box and also a sound deadening system based on a sand filled box along the length of the prop shaft - can't remember the details but I am sure I saw it described in a review

 

[Greenheart]

IIR Linssen go to extreme lengths to soundproof their boats - engine in a box and also a sound deadening system based on a sand filled box along the length of the prop shaft - can't remember the details but I am sure I saw it described in a review

Sounds interesting...but the only noisy part of the rotating shaft must be at the bearing, surely? Why the sand-box?

I daresay a 'loose' foundation for the machinery, reducing shunt and shudder, is the idea? In the same way as rubber mountings.

I'm interested in what's been said about saildrives being quieter...or did I misread that? I can't recall the shaft-type, on yachts I've lately been aboard. Is it thundering pressure on the inboard end of the prop shaft, which increases noise aboard shaft-drive vessels?

I spent an hour looking through the Yanmar brochure spec-sheets last evening...

...it made me wonder whether (discounting the enormous cost and dimension/weight considerations) it's reasonable for boatowners to fit much bigger engines than the hull requires, in order that they provide ample power at much lower, quieter revs per minute.

That's a neighbouring point on which I'd like to hear people's points of view: how much noisier does your diesel get, in the top third of the rev-range? I mean, there'd be no point at all, fitting a big lazy lugger, if it's just as deafening at 2000rpm, as at 3000rpm...

Very expensive idea, I realise...but I'm speculating wildly. Also, if you fit a 75hp diesel that peaks around 3000rpm, but you never actually run it at more than 2000rpm (at which speed, let us assume, it delivers sufficient for your needs), doesn't the idle use extend running life?

(I'm thinking of those slow-running 1500rpm diesels which always seem to outlive their owners.)

I think I appreciate that the diesel's highest efficiency is attained at a point high up in its limited rev-range, and that buying a 75hp engine in order to make use only of its bottom 25hp, is insane...I wasn't about to write a cheque, I'm just looking at what's possible.

 

[prv]

I'm interested in what's been said about saildrives being quieter...or did I misread that? I can't recall the shaft-type, on yachts I've lately been aboard. Is it thundering pressure on the inboard end of the prop shaft, which increases noise aboard shaft-drive vessels?

The shaft doesn't generate the noise, but the vibration / noise from the engine travels down it and into the hull. A sail drive, by contrast, makes no contact with the hull whatsoever; it's bolted to the back of the engine and protrudes through a big hole in the hull, sealed with a rubber sheet. If the engine is also on soft rubber mounts, then very little vibration ought to be passed through.

Also, if you fit a 75hp diesel that peaks around 3000rpm, but you never actually run it at more than 2000rpm (at which speed, let us assume, it delivers sufficient for your needs), doesn't the idle use extend running life?

No - quite the opposite!

Pete

 

[jwilson]

Sure - engine compartments on most modern boats are fairly well soundproofed with foil-faced foam with a heavy layer in the middle (used to be lead, don't know if it still is). Engine mounts are generally soft, and saildrives don't make contact with the hull. Designs that still use shaft drives can fit an Aquadrive universal joint to decouple the engine from the shaft, though I don't know if anyone fits these as standard.

I think you're just looking at pictures of old boats.

Pete

Foam or similar soft materials work fairly well to absorb higher frequencies, but there is still little better than suspended lead sheeting to absorb low-frequency sound, so a foam/lead/foam or similar composite works well, but is bulky and expensive.

A lot of the lower frequency sound is however also transferred through the structure, so soundproofing alone is not perfect. Flexible mounts help here.

 

[Greenheart]

The shaft doesn't generate the noise, but the vibration / noise from the engine travels down it and into the hull. A sail drive, by contrast, makes no contact with the hull whatsoever; it's bolted to the back of the engine and protrudes through a big hole in the hull, sealed with a rubber sheet. If the engine is also on soft rubber mounts, then very little vibration ought to be passed through.

Pete

Thanks Pete. I never really understood this aspect of the sail-drive's benefits. I always believed - possibly without reason - that shaft-drive was much more rugged.

I do see that a diesel's 'efficiency-band' - let's say the top 10% of the rev range - is the area at which the engine's manufacturers intend it to be used. So, I infer that keeping the engine quiet must be more a matter of soundproofing, than engine-speed-reduction.

Since I'm in the middle of (several years of) consideration of the exact yacht I'll hopefully one day buy, I'm free to analyse what's ideal and what's possible, aboard any yacht...so...without limitations, p'raps I can ask what contributors believe the ideal compromise is?

I've a fondness for long-keel designs, which tend to wear their shaft-drives with pride...and as was mentioned, presumably the bulk and scale of a substantial motorsailer will do much to absorb engine noise.

My broad view of options led me to wonder if near-silent electric motors could be fuelled not necessarily by batteries, but by hugely-insulated generators, located where their noise isn't as great an obstruction to conversation or sleep, as the average diesel auxilliary.

So you see, I'd consider anything. What I'm asking, is whether there's any widely-accepted ideal solution which can't fail...or, what the favourite is.

 

[prv]

Thanks Pete. I never really understood this aspect of the sail-drive's benefits. I always believed - possibly without reason - that shaft-drive was much more rugged.

We weren't talking about ruggedness. Although I share your belief that a shaft drive is ultimately more reliable and fixable. I do think that saildrives are robust enough, I wouldn't avoid a yacht because it had one.

Pete

 

[fishermantwo]

It is fairly common to insulate the bulkheads around an engine. I came across a material that consists of sheet lead enclosed in thin wadding and an oil-proof fabric, a total of 1/2" thick. It's quite effective. Unfortunately you can't generally insulate the bilge so sound from the engine gets transferred into the structure of the boat.

As has been said, 2-cylinder engines are the worst for vibration as, in a 4-stroke engine, the pistons move together not in opposite directions. Anyone who ever rode an old BSA or Triumph twin can attest to that.

My Volvo has a 180 degree crank as I'm sure most two cylinder diesels have, this coupled with an extremely heavy fly wheel gives just a smooth an engine as a Kubota 3 cylinder!

 

[Greenheart]

Didn't anybody ever try to divide very small cubic capacity, between three, four or more cylinders? Wouldn't that allow a lovely turbine-smoothness in even the smallest yacht auxilliary?

 

[fishermantwo]

Didn't anybody ever try to divide very small cubic capacity, between three, four or more cylinders? Wouldn't that allow a lovely turbine-smoothness in even the smallest yacht auxilliary?

In a petrol engine yes, diesels are noisy and the bits that make noise are going to be more so the greater the number of cylinders. The Mercedes 300d with 5 cylinders is one of the smoothest diesels about. The thread is about absorbing noise though and I still think there is plenty to be learnt from modern diesel cars and how they are so quiet. Custom made covers for the noisy bits like turbo chargers, alternators injection pumps etc.

 

[Dave_Seager]

My Volvo has a 180 degree crank as I'm sure most two cylinder diesels have, this coupled with an extremely heavy fly wheel gives just a smooth an engine as a Kubota 3 cylinder!

I used to have an Albin 25, fitted with an Albin AD-21, twin-cylinder diesel and this also had the cranks set at 180 degrees. This meant that you got two power strokes in one revolution followed by a second revolution without. The power delivery was smoothed out by a very heavy flywheel. What made it fun was that you also got two exhaust strokes together followed by two with no exhaust. This gave a wonderful "syncopated" exhaust sound, especially at low revolutions. It make me chuckle every time.

 

[penfold]

The key points for retrofitting/noise suppression are;

# Fill gaps in the compartment(with plywood, GRP, whatever the rest of the structure is made of) and seal penetrations with grommets.
# Line the compartment with good quality sound absorption material; manufacturers often recommend using mechanical fastenings as well as adhesive as heat and oil can compromise the bond strength and the insulation is heavy.
# Fit a thrust bearing and a double flexible coupling(or a combined product like the Aquadrive)
# Swap engine mounts for the most compliant available commensurate with the engine not hitting the compartment walls while at extremes of movement(at idle usually).

Where possible attention should be paid to blocking noise paths to the hull or through the bilges; this might extend to fitting a false floor to the compartment or attaching insulation directly to the hull(only advisable if water absorption can be avoided).

I helped fit an Aquadrive to a Halmatic 30 with a VP2002; the difference at idle and at speed was marked. Before fitting your fillings were imperilled when sat in the cockpit with the engine idling, and conversation was strained above 2200 rpm; the owner would not venture over 2500 rpm due to the excessive vibration. Afterward you were aware the engine was idling(with the cover off the engine was dancing the watusi on the soft mounts, but you wouldn't know it in the cockpit), but there was little perceptible vibration; at speed there was no vibration perceived until 3000 rpm was reached.

 

[DaveS]

My Volvo has a 180 degree crank as I'm sure most two cylinder diesels have, this coupled with an extremely heavy fly wheel gives just a smooth an engine as a Kubota 3 cylinder!

The VP 2002 has a 180 degree crank as shown here: http://www.marinepartsexpress.com/V...T/ENGINE/CRANKSHAFT AND RELATED PARTS 24.pdf

I must say I haven't noticed the exhaust beat as unusual. Must listen more closely!

 

[Greenheart]

These are very useful points I'm sure, thank you. I s'pose in truth, there's no reason to suffer tooth-rattling vibration and ear-splitting noise, even from an elderly unit, if some engineroom refitting is included on the list of winter chores.

I remember being aboard a fairly ancient 27' sloop thirty years ago, and although I was fonder of motorboats then, I kept asking the skipper if we could sail without the engine. I think I've been allergic to unnecessary use of power, and noise from engines, ever since!

What are/were those legendary marine engines that could run with almost no perceptible thumping? I'm guessing they were slow-runners and very heavy for their output, which sadly flies in the face of today's obsession with efficiency.

 

[Tranona]

Yes, the 2 cylinder petrol engines were generally much quieter and smoother, particularly 2 strokes such as the Stuart Turner, as were the old 4 cylinder 4 strokes of the 1920s such as the American Kermaths and Greys. Some of these you would have to see the flywheel moving to tell they were running. Unfortunately they seriously lacked power, were often physically large for the meagre power and most of all only worked then they wanted to, not when you commanded them!

The early small diesels were mostly rough and noisy, being derivatives of industrial engines used for dump trucks, cement mixers etc. However, things have moved on a bit and the latest engines are a vast improvement in refinement. Not reached the levels of automotive refinement, although a lot of that is due to designing the structure to isolate the engine from the cabin, and having empty space underneath for the noise to escape. Not so easy with most boats, particularly sailing boats which are passable imitations of drums that magnify the noise and vibration rather than suppress it.

 

[wotayottie]

...but wouldn't the decibel-output reduce very significantly with, for example, a simple, substantial plywood & thick soft foam DIY enclosure, placed over the diesel? (I'm assuming a tubular vent/fan would keep the engine cool and allow air intakes to breath, etc).

(

Dont know where you get the idea that this isnt done. All 3 of my last 3 boats have had just such enclosures. Sure they reduce noise but not to the extent that Tranona suggests for his Bav. In my current case quite a bit comes from the flexible exhaust pipe lying on a grp hull, plus the exhaust itself anfd the vibes and sympathetic vibrations.

 

[penfold]

Engine compartment air inlets are often overlooked as paths of noise; they should be baffled and the baffle lined with absorption material.

The few times my dad's Stuart Turner P55 worked(it was fairly knackered; the crankcase seals were gone, the magneto worked part-time and ST carbs were carp even when new) it was whisper-quiet and vibration-free except at idle; it was bolted straight on a steel bed and had no flexible coupling. 8hp from something the size and weight of a 2GM20.