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Reducing engine noise

I have cruiser stern with a barrus shire 50hp, which I have tyred to reduce the sound with stick on foam/rubber/foil insulation. This has reduced the sound quite a lot, but inside the boat the sound is still quite loud. On yachts the engine is insulated very close to the engine, and i was wondering if enclosing the engine within the engine bay would be the next step. I would like to know if it's safe to do regarding engine temperature. I was thinking if it was OK, to vent out off the air intake on the airfillter.The back deck has also been raised by about 150mm.The exhaust is standard, but was told the hospital ones don't make much difference!
many thanks

Asked by: anthony spanswick  | 8.37pm, Monday 5 October

WW says:

Hi Anthony
I believe that you are right about the hospital silencers, they are an expensive way of toning down the exhaust note but do nothing for the diesel clatter which is the lions share of the noise. Enclosing the engine space at deck board level will make a huge difference. My method is to hinge the deck boards and fit budget locks to hold them closed, a foam gasket seals the joint between the boards and the drainage channels. the principle to simple enough, if its air tight its sound tight. There was an article showing how to do it in the July 2014 issue. (Keeping the Noise Down) This can be downloaded from this site. You will need to ensure that the engine can breath through a sound trapped inlet. Best of luck with the project.

Mike Jordan  | 9.07AM, Tuesday 6 October

"Hospital generator" silencers do work quite well- acting as both a silencer and expansion chamber- but the effect on the overall noise levels are fairly minimal. It always amazes me how people spend over £100K on a boat and settle for no noise reduction of their engine!
Yacht engine installations are quite often in compact spaces, with the sound insulation close to the engine, as the engine box is usually kept small to maximise the space within the hull void. There is also one major difference between yacht and narrowboat installations, and that is with the cooling system. The vast majority of yachts use raw or freshwater cooling, where raw water ends up being dumped into the exhaust to cool and silence the engine. This also has the added advantage of reducing the thermal load within the engine space.
On most narrowboats, even a well-lagged dry exhaust system will radiate a considerable amount of heat. Plus, skin cooling tanks not only conduct heat through the hull into the surrounding water, but convect and radiate heat into the air surrounding the engine. This means that the air temperature is a lot higher around most narrowboat installations than in a yacht.
Comprehensive sound insulation in an engine compartment under a sermi-trad or cruiser stern is perfectly feasible. However, you need to provide plenty of air, not just for combustion, but for air cooling. For instance, most people suggest 1 sq inch per horsepower (plus 1 sq ft of skin cooling tank per 4 hp). This then allows the engine to run fairly cool, at full revs on a river, for example, without overheating.
If you box an engine in, you might need to provide mechanical ventilation to keep the engine cool (for example, the cocooned Beta marine "super silent" range have a fan which draws air for cooling through- otherwise the engine would postentially overheat. The Barrus manual advises the amount of ventilation required- any less and the warrenty would potentially be invalid.
On a more practical point, the engine noise can be reduced by good installation, plus applying insulation to every surface, including fuel tanks, weedhatch and bearers- plus bulkhead, etc. There are compounds available which can be poured onto horizontal services to help absorb sound vibrations. You could add perforated barriers around the engine to with additional material on, but unless you are prepared to run calcuations on ventilation, I would strongly suggest that you do not enclose the engine in a very small space. You can also make sound-insulated ducts to allow air in (and out) without allowing sound to travel- like adding a dorade box to vents- or possibly ducting from the cabin bilge (ducting cooling air to enter near the alternators can be greatly beneficial for the longevity of these temperature-sensitive components). Remember that combustion air is only part of the engine needing to breathe- you need to provide routes for air into the engine space, and for hot air to leave. Unfortunately, many narrowboat engine spaces leave a lot to be desired- which leads to problems, such as degrading alternator belts, poor fuel quality (too hot return fuel, etc.)
The quietest modern diesel engines on narrowboats would be freshwater cooled, with a wet exhaust and gas-water seperator, plus proper sound insulation, dual-material deck boards and ducted ventilation. Alas, this would be too much in terms of complexity and maintenance (draining down in winter, for example- and daily checking of mud boxes/fine filters) for most people to consider.
We can give more specific advice if that would help- there have been several articles on the nature of quieter engines over the last few years in the magazine.

Mark Langley  | 8.05PM, Tuesday 6 October

Readers say:

Thankyou very much for your detailed answers.

anthony spanswick  | 9.31PM, Tuesday 6 October

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