Science of Silence

Exhaust Silencers

Fifteen years ago it was unusual to find a boat fitted with a silencer, these days it's rare to find a leisure craft without one. This is partly due to our increasing desire for a quiet life, and part because modern marine diesel engines rev much higher, with a much more aggressive exhaust note.

lnline Silencer

The simplest type of silencer is the inline muffler-rather similar to a conventional car silencer -see below. This consists of a chamber with specialised internal piping through which the exhaust gas and water must flow. A good inline muffler should reduce exhaust noise by about 40%. The type of muffler offers some protection from back flow by virtue of its increased volume compared with the bare exhaust pipe, but is seldom sufficient on its own.

The best shape for an inline muffler is cylindrical. Oval versions can be easier to accommodate behind furniture and bulkheads while offering the same internal volume, but are more costly.

Water Lift Silencers

More sophisticated than the in line muffler is the vertical lift silencer. These are also referred to as 'water lift' or 'water-trap' silencers. This type of silencer can be positioned well below the waterline, allowing the exhaust run from the engine to have a nice, safe, steep angle. You might think that such a device would rapidly fill with water and eventually block the exhaust completely, but this doesn't happen. In a correctly designed unit, the water level stabilises in the bottom of the chamber, just level with the bottom of the outlet pipe, with pure gas swirling around above it. Water is picked up by the gas on the way out and this further silences and cools the flow. Water-lift silencers should reduce exhaust noise by about 40%, compared with an un-silenced system.

Dual chamber water-lift silencers are even more effective. The two chambers, one above the other, force the gas and water to separate and remix twice, often leading to a total noise reduction in the region of 70%. Naturally, they are more expensive than the single chamber variety.

Water-lift silencers come in all shapes and sizes. Most common is the vertical cylinder variety, which can have top inlets, horizontal or angled side inlets and a wide variety of outlets. However, horizontal lift silencers are also available, and you can get either type with dual inlet stubs for use with V configuration engines. The variations are endless, but the important thing is that the

Internal design should be efficient and the silencer be chosen to exactly match the requirements of the engine and the rest of the exhaust system.

Dual Chamber Silencers

Dual chamber silencers are proven, high noise reduction, low back pressure systems designed for marine engines of all sizes and proven in use over 20 years. Every unit is Lloyds Type Approved and will last indefinitely. A range of standard sizes is manufactured, or alternatively the body can be specifically shaped and the inlet and outlet positions varied. Dual chamber silencers range in diameter from 400mm to 1000mm and are manufactured for exhaust systems from 90mm to 400mm.

The graph shows the noise reduction measured by staff from the Institute of Sound and Vibration Research at Southampton University, who measured exhaust noise output on a 700hp six cylinder marine diesel with a metre exhaust length, and then tested the same engine again when the dual chamber silencer was added to the system. Halyard has over 25,000 Dual Chamber silencers in service on leisure and commercial craft, many of these now 20 years old.

Resonator Silencers

Halyard developed and patented the resonator silencer in 2006, using technology developed in the Helmholtz resonator and adapting this to function in a water-cooled exhaust system. This was achieved over a two year period working with Southampton University. The silencer is designed to suit the engine configuration (straight four- cylinder, straight six, V8 etc.) and the cruising rpm involved. The technology establishes the peak frequency noise output and designs an acoustic “spring” to deal with this. The result is that the noise peak is effectively cancelled and the overall noise level reduces dramatically. A recent independent test conducted on a twin engine 65’ powerboat, working to the EC noise test standard ISO14509, showed an average noise level of 74.6 dB(a) – less than half the noise level maximum at 25 metres required by the European Recreational Craft Directive.

To view the Halyard range of Silencers, please click here.