Why is my boat engine so noisy?

A lot of noise is structure borne, soft engine mounts and flexible couplings in the prop shaft are a good start.
Then look at damping rigid panels
transmission of noise by connections like exhaust, controls, plumbing
break up resonances of rectangular boxes.
silencers on air intake?

As others have said, a lot of NVH time and money went into the Mini.
 
True. I thought of the air intake because of the huge difference it makes on an Eber to silence the combustion air intake. What sort would fit do you think? Currently it's just a mushroom stuck on top.
 
Lazy Kipper said:
True. I thought of the air intake because of the huge difference it makes on an Eber to silence the combustion air intake. What sort would fit do you think? Currently it's just a mushroom stuck on top.
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You could fit an air filter, but you run the risk of restricting the air intake. However, it is unlikely to make much difference - I have one on my Volvo/Perkins and ran for some time without it while saving up to buy a replacement - but could detect no difference for the vast expenditure.

As many have said the source of your excessive noise is partly mechanical, amplified by pressed steel sump and valve cover, partly airborne because unlike the truck it came from the noise cannot escape underneath, and partly because it is not isolated from the structure. Your wheelhouse does not help as it keeps the noise in.

You would really notice the difference with my more modern Volvo which is mechanically quieter, has cast aluminium sump and valve cover to absorb noise and most importantly is mounted on rubber - even better has a saildrive. Result is that it is just about inaudible at the helm (in the open air!), but very noticeable in the aft cabins which are both next to the engine and enclosed.

It is possible to have a quiet installation in a wheelhouse - try a recent Nauticat for example - but you start with an inherently quieter engine and put a lot of effort in to isolating the mechanics from the structure and insulating against airborne noise.
 
One of the little clips that holds on the air filter of my 3YM30 lost it's retaining pin, and the engine suddenly got substantially noisier as the filter housing came loose - noticed it from the cockpit.
 
I think unless you can identify and reduce the biggest noise mechanism, you won't notice changes in the smaller ones.
So you have to attack things in the right order, or just keep trying.
 
jerrytug said:
{pedant hat} surely indirect, that is, injector sprays into pre-combustion chamber? I thought Thorneycrofts were based on BMC designs.
Apart from that, all the suggestions are contributing.
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Yes Thorneycrofts are indirect injection and not true diesels unlike the mini engine and all modern car diesels which are true direct injection compression ignition engines.
In the indirect injection engine combustion takes place in a small side chamber preheated by glow plugs for cold start and kept hot by the use of a ceramic plug covered in little cone shaped spikes.Heat from these plus heat from compression causes ignition in the ignition chamber which fires a jet of combusting gases into the main cylinder.
In a sense it works like the little glow plug engines on model aero engines and almost in a strange way like a spark plug.
Such engines which fire at lower pressure were designed to get rid of the noise plus the necessary heavy build of the origional direct injection diesels.
 
You say the engine room is "almost" lined throughout with absorbent material. I remember reading a long time ago that "almost" isn't good enough - it has to be completely lined, or you might as well not bother. What is the absorbent material? I am given to understand that for best results, it has to have a layer of lead foil in it. The sort of egg-box foam that is fitted in my engine space is probably not doing much!
 
ffiill said:
Yes Thorneycrofts are indirect injection and not true diesels unlike the mini engine and all modern car diesels which are true direct injection compression ignition engines.
In the indirect injection engine combustion takes place in a small side chamber preheated by glow plugs for cold start and kept hot by the use of a ceramic plug covered in little cone shaped spikes.Heat from these plus heat from compression causes ignition in the ignition chamber which fires a jet of combusting gases into the main cylinder.
In a sense it works like the little glow plug engines on model aero engines and almost in a strange way like a spark plug.
Such engines which fire at lower pressure were designed to get rid of the noise plus the necessary heavy build of the origional direct injection diesels.
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Where do you lot dig this rubbish up!

The Thornycroft 230 was based on the Leyland 98 Series engines built at the Leyland plant in Bathgate. 230 comes from the 98mm bore times 125mm stroke giving the four pot motor a displacement of 3.77 liters or 230 cu inches. BMC connection is the fact that the '98' Series engines were built off the transfer lines designed to produce the original BMC 5.1 and 3.4 liter engines, introduced in 1954 to replace the Perkins P4 and P6 and used in BMC Commercial and Nuffield tractor applications. The 5.1 and 3.4 engines were 100mm bore wet liner direct injection motors from the outset. Displacement increased during lifetime to 5.7/3.8 liters with new crank giving grater stroke.

Leyland inherited BMC/Morris commercial and saw the opportunity to slot a new engine in below the 400 Series which was heavy and expensive to produce. Major weakness of the old 100mm bore engines was the wet liner design and Leyland gave the whole engine a complete makeover based around a new liner design. Transfer lines were moved from Midlands to a brand new production plant at Bathgate in 1972 producing trucks, tractors as well as the 98 Series engines.

In their time the 98 Series were good engines, liners still needed care with water treatment, however remember this engine was launched, they run on advanced timing table as they only had to comply with the old BS au smoke test. Compared with modern engines, 98 Series were built to very 'sloppy' tolerances by modern standards and being mechanical have simple linear timing.

The smaller LDA BMC engines were loosely based on their equivalent gasoline cousins were indirect injection DIESEL engines using Ricardo patent pre-combustion system which requires relatively high compression ratio compared with direction engines, however operated very much on a true Diesel cycle.

The Mini chainsaw LDA diesel engine is also direct injection, however the engine structure is carefully designed to reduce noise, for example deep skirted engine block to reduce noise transfer, but by a long shot common rail injection has a huge number of levers to pull in the noise department. In addition to having a 3D timing table the little Mini diesel engine has some sexy injection noise algorithms. Consider this, throw a bucket of petrol on a bonfire, it busts into flame with a whoomp! Gently tips mall amounts of petrol on the fire and the blaze gets going with no drama, same applies to how the you ignite your compression ignition engine using pilot injection cycles and at the same time applying your three dimensional timing.

Before the advent of electronic diesel engines designing a quiet diesel was akin to designing a quiet hand grenade.............
 
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I'll bet you have gaps and openings in the engine box. If so all the noise simply bypasses the sound proofing. Every minute opening must be sealed including hatch covers. The age of the engine is not the problem it is the sound proofing not being up to scratch and as already said unsuitable engine mounts that don't absorb vibrations.

Indirect injection engines not true diesels, that's the most stupid thing I've read in this forum. and that's saying something!
 
ffiill said:
Yes Thorneycrofts are indirect injection and not true diesels unlike the mini engine and all modern car diesels which are true direct injection compression ignition engines.
In the indirect injection engine combustion takes place in a small side chamber preheated by glow plugs for cold start and kept hot by the use of a ceramic plug covered in little cone shaped spikes.Heat from these plus heat from compressiombn causes ignoition in the ignition chamber which fires a jet of combusting gases into the main cylinder.
In a sense it works like the little glow plug engines on model aero engines and almost in a strange way like a spark plug.
Such engines which fire at lower pressure were designed to get rid of the noise plus the necessary heavy build of the origional direct injection diesels.
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Some confusion here. From memory only the BMC. 1.5 and 1.8 (Captain ) were indirect injection with the 1.5 based on the 1498 cc petrol engine as fitted to the BMC Oxford and Cambridge. These were high compression engines @ 23:1 with ricardo combustion chambers.

Direct Injection engines like the BMC Comodore were much lower @ about. 17:1 (ish) Having rebuilt several 1.5 engines from the crank up, I have never seen any sign of a ceramic glow plug, only the sheath of the electric glow plug.

As to the drop in noise level, my new Peugeot 1.6 diesel is wisper quiet partly due to the fact that each injector fires several times on each power stroke. From memory it is something like a pre injector pulse, a main injector pulse and a post injrector pulse all fired from the electronics.
 
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PCUK said:
I'll bet you have gaps and openings in the engine box. If so all the noise simply bypasses the sound proofing. Every minute opening must be sealed including hatch covers. The age of the engine is not the problem it is the sound proofing not being up to scratch and as already said unsuitable engine mounts that don't absorb vibrations.

Indirect injection engines not true diesels, that's the most stupid thing I've read in this forum. and that's saying something!
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Not quite sure I understand - open up the bonent on the Mini and it's not much louder and there's lots of spaces for noise to get out. On the boat it's way noisier in the engine room so the incomplete soundproofing must be doing something! Just the age of the design then I guess. My guilty secret is that a little bit of me wants it to go pop - I'd love a quiet, frugal, shiny new engine. I just don't want the bill which will be £10k and take me half a lifetime to pay off.
 
smartphone

If you have a smartphone you can download frequency spectrum analysis software for free. This may help you in locating and deadening the source of the noise. A lot of NVH is not simply adding sound proofing materials, but is based on stopping things from resonating by redesigning them or by adding mass in places that change the resonant frequency to one at which it is not being excited.
 
Lazy Kipper said:
Not quite sure I understand - open up the bonent on the Mini and it's not much louder and there's lots of spaces for noise to get out. On the boat it's way noisier in the engine room so the incomplete soundproofing must be doing something! Just the age of the design then I guess. My guilty secret is that a little bit of me wants it to go pop - I'd love a quiet, frugal, shiny new engine. I just don't want the bill which will be £10k and take me half a lifetime to pay off.
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Yes, your Mini engine is inherently much quieter than your boat engine. However, it also uses technology WAY in advance of your boat engine. As various people have already said, for a start, the injector fires multiple times per power stroke (up to 5, I think) so the fuel doesn't explode with a big bang. I think that's the single most important feature in achieving better refinement. After that' it's down to dual mass flywheel and clever engine mounts (AND the positions of the engine mounts). You could duplicate that in a boat if you were prepared to spend a lot of money and put up with the complexity of a Mini engine in a boat, but you'd still be stuck with your noise source being within the enclosed space that is a boat.
 
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